drm/i915: Be wary of data races when reading the active execlists

[linux-2.6-microblaze.git] / drivers / gpu / drm / i915 / i915_request.c

/*
 * Copyright © 2008-2015 Intel Corporation
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice (including the next
 * paragraph) shall be included in all copies or substantial portions of the
 * Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 * IN THE SOFTWARE.
 *
 */

#include <linux/dma-fence-array.h>
#include <linux/dma-fence-chain.h>
#include <linux/irq_work.h>
#include <linux/prefetch.h>
#include <linux/sched.h>
#include <linux/sched/clock.h>
#include <linux/sched/signal.h>

#include "gem/i915_gem_context.h"
#include "gt/intel_breadcrumbs.h"
#include "gt/intel_context.h"
#include "gt/intel_ring.h"
#include "gt/intel_rps.h"

#include "i915_active.h"
#include "i915_drv.h"
#include "i915_globals.h"
#include "i915_trace.h"
#include "intel_pm.h"

struct execute_cb {
        struct irq_work work;
        struct i915_sw_fence *fence;
        void (*hook)(struct i915_request *rq, struct dma_fence *signal);
        struct i915_request *signal;
};

static struct i915_global_request {
        struct i915_global base;
        struct kmem_cache *slab_requests;
        struct kmem_cache *slab_execute_cbs;
} global;

static const char *i915_fence_get_driver_name(struct dma_fence *fence)
{
        return dev_name(to_request(fence)->engine->i915->drm.dev);
}

static const char *i915_fence_get_timeline_name(struct dma_fence *fence)
{
        const struct i915_gem_context *ctx;

        /*
         * The timeline struct (as part of the ppgtt underneath a context)
         * may be freed when the request is no longer in use by the GPU.
         * We could extend the life of a context to beyond that of all
         * fences, possibly keeping the hw resource around indefinitely,
         * or we just give them a false name. Since
         * dma_fence_ops.get_timeline_name is a debug feature, the occasional
         * lie seems justifiable.
         */
        if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
                return "signaled";

        ctx = i915_request_gem_context(to_request(fence));
        if (!ctx)
                return "[" DRIVER_NAME "]";

        return ctx->name;
}

static bool i915_fence_signaled(struct dma_fence *fence)
{
        return i915_request_completed(to_request(fence));
}

static bool i915_fence_enable_signaling(struct dma_fence *fence)
{
        return i915_request_enable_breadcrumb(to_request(fence));
}

static signed long i915_fence_wait(struct dma_fence *fence,
                                   bool interruptible,
                                   signed long timeout)
{
        return i915_request_wait(to_request(fence),
                                 interruptible | I915_WAIT_PRIORITY,
                                 timeout);
}

struct kmem_cache *i915_request_slab_cache(void)
{
        return global.slab_requests;
}

static void i915_fence_release(struct dma_fence *fence)
{
        struct i915_request *rq = to_request(fence);

        /*
         * The request is put onto a RCU freelist (i.e. the address
         * is immediately reused), mark the fences as being freed now.
         * Otherwise the debugobjects for the fences are only marked as
         * freed when the slab cache itself is freed, and so we would get
         * caught trying to reuse dead objects.
         */
        i915_sw_fence_fini(&rq->submit);
        i915_sw_fence_fini(&rq->semaphore);

        /*
         * Keep one request on each engine for reserved use under mempressure
         *
         * We do not hold a reference to the engine here and so have to be
         * very careful in what rq->engine we poke. The virtual engine is
         * referenced via the rq->context and we released that ref during
         * i915_request_retire(), ergo we must not dereference a virtual
         * engine here. Not that we would want to, as the only consumer of
         * the reserved engine->request_pool is the power management parking,
         * which must-not-fail, and that is only run on the physical engines.
         *
         * Since the request must have been executed to be have completed,
         * we know that it will have been processed by the HW and will
         * not be unsubmitted again, so rq->engine and rq->execution_mask
         * at this point is stable. rq->execution_mask will be a single
         * bit if the last and _only_ engine it could execution on was a
         * physical engine, if it's multiple bits then it started on and
         * could still be on a virtual engine. Thus if the mask is not a
         * power-of-two we assume that rq->engine may still be a virtual
         * engine and so a dangling invalid pointer that we cannot dereference
         *
         * For example, consider the flow of a bonded request through a virtual
         * engine. The request is created with a wide engine mask (all engines
         * that we might execute on). On processing the bond, the request mask
         * is reduced to one or more engines. If the request is subsequently
         * bound to a single engine, it will then be constrained to only
         * execute on that engine and never returned to the virtual engine
         * after timeslicing away, see __unwind_incomplete_requests(). Thus we
         * know that if the rq->execution_mask is a single bit, rq->engine
         * can be a physical engine with the exact corresponding mask.
         */
        if (is_power_of_2(rq->execution_mask) &&
            !cmpxchg(&rq->engine->request_pool, NULL, rq))
                return;

        kmem_cache_free(global.slab_requests, rq);
}

const struct dma_fence_ops i915_fence_ops = {
        .get_driver_name = i915_fence_get_driver_name,
        .get_timeline_name = i915_fence_get_timeline_name,
        .enable_signaling = i915_fence_enable_signaling,
        .signaled = i915_fence_signaled,
        .wait = i915_fence_wait,
        .release = i915_fence_release,
};

static void irq_execute_cb(struct irq_work *wrk)
{
        struct execute_cb *cb = container_of(wrk, typeof(*cb), work);

        i915_sw_fence_complete(cb->fence);
        kmem_cache_free(global.slab_execute_cbs, cb);
}

static void irq_execute_cb_hook(struct irq_work *wrk)
{
        struct execute_cb *cb = container_of(wrk, typeof(*cb), work);

        cb->hook(container_of(cb->fence, struct i915_request, submit),
                 &cb->signal->fence);
        i915_request_put(cb->signal);

        irq_execute_cb(wrk);
}

static void __notify_execute_cb(struct i915_request *rq)
{
        struct execute_cb *cb, *cn;

        lockdep_assert_held(&rq->lock);

        GEM_BUG_ON(!i915_request_is_active(rq));
        if (llist_empty(&rq->execute_cb))
                return;

        llist_for_each_entry_safe(cb, cn, rq->execute_cb.first, work.llnode)
                irq_work_queue(&cb->work);

        /*
         * XXX Rollback on __i915_request_unsubmit()
         *
         * In the future, perhaps when we have an active time-slicing scheduler,
         * it will be interesting to unsubmit parallel execution and remove
         * busywaits from the GPU until their master is restarted. This is
         * quite hairy, we have to carefully rollback the fence and do a
         * preempt-to-idle cycle on the target engine, all the while the
         * master execute_cb may refire.
         */
        init_llist_head(&rq->execute_cb);
}

static void free_capture_list(struct i915_request *request)
{
        struct i915_capture_list *capture;

        capture = fetch_and_zero(&request->capture_list);
        while (capture) {
                struct i915_capture_list *next = capture->next;

                kfree(capture);
                capture = next;
        }
}

static void __i915_request_fill(struct i915_request *rq, u8 val)
{
        void *vaddr = rq->ring->vaddr;
        u32 head;

        head = rq->infix;
        if (rq->postfix < head) {
                memset(vaddr + head, val, rq->ring->size - head);
                head = 0;
        }
        memset(vaddr + head, val, rq->postfix - head);
}

static void remove_from_engine(struct i915_request *rq)
{
        struct intel_engine_cs *engine, *locked;

        /*
         * Virtual engines complicate acquiring the engine timeline lock,
         * as their rq->engine pointer is not stable until under that
         * engine lock. The simple ploy we use is to take the lock then
         * check that the rq still belongs to the newly locked engine.
         */
        locked = READ_ONCE(rq->engine);
        spin_lock_irq(&locked->active.lock);
        while (unlikely(locked != (engine = READ_ONCE(rq->engine)))) {
                spin_unlock(&locked->active.lock);
                spin_lock(&engine->active.lock);
                locked = engine;
        }
        list_del_init(&rq->sched.link);
        clear_bit(I915_FENCE_FLAG_PQUEUE, &rq->fence.flags);
        clear_bit(I915_FENCE_FLAG_HOLD, &rq->fence.flags);
        spin_unlock_irq(&locked->active.lock);
}

bool i915_request_retire(struct i915_request *rq)
{
        if (!i915_request_completed(rq))
                return false;

        RQ_TRACE(rq, "\n");

        GEM_BUG_ON(!i915_sw_fence_signaled(&rq->submit));
        trace_i915_request_retire(rq);

        /*
         * We know the GPU must have read the request to have
         * sent us the seqno + interrupt, so use the position
         * of tail of the request to update the last known position
         * of the GPU head.
         *
         * Note this requires that we are always called in request
         * completion order.
         */
        GEM_BUG_ON(!list_is_first(&rq->link,
                                  &i915_request_timeline(rq)->requests));
        if (IS_ENABLED(CONFIG_DRM_I915_DEBUG_GEM))
                /* Poison before we release our space in the ring */
                __i915_request_fill(rq, POISON_FREE);
        rq->ring->head = rq->postfix;

        /*
         * We only loosely track inflight requests across preemption,
         * and so we may find ourselves attempting to retire a _completed_
         * request that we have removed from the HW and put back on a run
         * queue.
         */
        remove_from_engine(rq);

        i915_request_mark_complete(rq);
        if (!i915_request_signaled(rq)) {
                spin_lock_irq(&rq->lock);
                dma_fence_signal_locked(&rq->fence);
                spin_unlock_irq(&rq->lock);
        }

        if (i915_request_has_waitboost(rq)) {
                GEM_BUG_ON(!atomic_read(&rq->engine->gt->rps.num_waiters));
                atomic_dec(&rq->engine->gt->rps.num_waiters);
        }

        /*
         * We only loosely track inflight requests across preemption,
         * and so we may find ourselves attempting to retire a _completed_
         * request that we have removed from the HW and put back on a run
         * queue.
         *
         * As we set I915_FENCE_FLAG_ACTIVE on the request, this should be
         * after removing the breadcrumb and signaling it, so that we do not
         * inadvertently attach the breadcrumb to a completed request.
         */
        remove_from_engine(rq);
        GEM_BUG_ON(!llist_empty(&rq->execute_cb));
        spin_unlock_irq(&rq->lock);

        __list_del_entry(&rq->link); /* poison neither prev/next (RCU walks) */

        intel_context_exit(rq->context);
        intel_context_unpin(rq->context);

        free_capture_list(rq);
        i915_sched_node_fini(&rq->sched);
        i915_request_put(rq);

        return true;
}

void i915_request_retire_upto(struct i915_request *rq)
{
        struct intel_timeline * const tl = i915_request_timeline(rq);
        struct i915_request *tmp;

        RQ_TRACE(rq, "\n");

        GEM_BUG_ON(!i915_request_completed(rq));

        do {
                tmp = list_first_entry(&tl->requests, typeof(*tmp), link);
        } while (i915_request_retire(tmp) && tmp != rq);
}

static void __llist_add(struct llist_node *node, struct llist_head *head)
{
        node->next = head->first;
        head->first = node;
}

static struct i915_request * const *
__engine_active(struct intel_engine_cs *engine)
{
        return READ_ONCE(engine->execlists.active);
}

static bool __request_in_flight(const struct i915_request *signal)
{
        struct i915_request * const *port, *rq;
        bool inflight = false;

        if (!i915_request_is_ready(signal))
                return false;

        /*
         * Even if we have unwound the request, it may still be on
         * the GPU (preempt-to-busy). If that request is inside an
         * unpreemptible critical section, it will not be removed. Some
         * GPU functions may even be stuck waiting for the paired request
         * (__await_execution) to be submitted and cannot be preempted
         * until the bond is executing.
         *
         * As we know that there are always preemption points between
         * requests, we know that only the currently executing request
         * may be still active even though we have cleared the flag.
         * However, we can't rely on our tracking of ELSP[0] to know
         * which request is currently active and so maybe stuck, as
         * the tracking maybe an event behind. Instead assume that
         * if the context is still inflight, then it is still active
         * even if the active flag has been cleared.
         *
         * To further complicate matters, if there a pending promotion, the HW
         * may either perform a context switch to the second inflight execlists,
         * or it may switch to the pending set of execlists. In the case of the
         * latter, it may send the ACK and we process the event copying the
         * pending[] over top of inflight[], _overwriting_ our *active. Since
         * this implies the HW is arbitrating and not struck in *active, we do
         * not worry about complete accuracy, but we do require no read/write
         * tearing of the pointer [the read of the pointer must be valid, even
         * as the array is being overwritten, for which we require the writes
         * to avoid tearing.]
         *
         * Note that the read of *execlists->active may race with the promotion
         * of execlists->pending[] to execlists->inflight[], overwritting
         * the value at *execlists->active. This is fine. The promotion implies
         * that we received an ACK from the HW, and so the context is not
         * stuck -- if we do not see ourselves in *active, the inflight status
         * is valid. If instead we see ourselves being copied into *active,
         * we are inflight and may signal the callback.
         */
        if (!intel_context_inflight(signal->context))
                return false;

        rcu_read_lock();
        for (port = __engine_active(signal->engine);
             (rq = READ_ONCE(*port)); /* may race with promotion of pending[] */
             port++) {
                if (rq->context == signal->context) {
                        inflight = i915_seqno_passed(rq->fence.seqno,
                                                     signal->fence.seqno);
                        break;
                }
        }
        rcu_read_unlock();

        return inflight;
}

static int
__await_execution(struct i915_request *rq,
                  struct i915_request *signal,
                  void (*hook)(struct i915_request *rq,
                               struct dma_fence *signal),
                  gfp_t gfp)
{
        struct execute_cb *cb;

        if (i915_request_is_active(signal)) {
                if (hook)
                        hook(rq, &signal->fence);
                return 0;
        }

        cb = kmem_cache_alloc(global.slab_execute_cbs, gfp);
        if (!cb)
                return -ENOMEM;

        cb->fence = &rq->submit;
        i915_sw_fence_await(cb->fence);
        init_irq_work(&cb->work, irq_execute_cb);

        if (hook) {
                cb->hook = hook;
                cb->signal = i915_request_get(signal);
                cb->work.func = irq_execute_cb_hook;
        }

        spin_lock_irq(&signal->lock);
        if (i915_request_is_active(signal) || __request_in_flight(signal)) {
                if (hook) {
                        hook(rq, &signal->fence);
                        i915_request_put(signal);
                }
                i915_sw_fence_complete(cb->fence);
                kmem_cache_free(global.slab_execute_cbs, cb);
        } else {
                __llist_add(&cb->work.llnode, &signal->execute_cb);
        }
        spin_unlock_irq(&signal->lock);

        return 0;
}

static bool fatal_error(int error)
{
        switch (error) {
        case 0: /* not an error! */
        case -EAGAIN: /* innocent victim of a GT reset (__i915_request_reset) */
        case -ETIMEDOUT: /* waiting for Godot (timer_i915_sw_fence_wake) */
                return false;
        default:
                return true;
        }
}

void __i915_request_skip(struct i915_request *rq)
{
        GEM_BUG_ON(!fatal_error(rq->fence.error));

        if (rq->infix == rq->postfix)
                return;

        /*
         * As this request likely depends on state from the lost
         * context, clear out all the user operations leaving the
         * breadcrumb at the end (so we get the fence notifications).
         */
        __i915_request_fill(rq, 0);
        rq->infix = rq->postfix;
}

void i915_request_set_error_once(struct i915_request *rq, int error)
{
        int old;

        GEM_BUG_ON(!IS_ERR_VALUE((long)error));

        if (i915_request_signaled(rq))
                return;

        old = READ_ONCE(rq->fence.error);
        do {
                if (fatal_error(old))
                        return;
        } while (!try_cmpxchg(&rq->fence.error, &old, error));
}

bool __i915_request_submit(struct i915_request *request)
{
        struct intel_engine_cs *engine = request->engine;
        bool result = false;

        RQ_TRACE(request, "\n");

        GEM_BUG_ON(!irqs_disabled());
        lockdep_assert_held(&engine->active.lock);

        /*
         * With the advent of preempt-to-busy, we frequently encounter
         * requests that we have unsubmitted from HW, but left running
         * until the next ack and so have completed in the meantime. On
         * resubmission of that completed request, we can skip
         * updating the payload, and execlists can even skip submitting
         * the request.
         *
         * We must remove the request from the caller's priority queue,
         * and the caller must only call us when the request is in their
         * priority queue, under the active.lock. This ensures that the
         * request has *not* yet been retired and we can safely move
         * the request into the engine->active.list where it will be
         * dropped upon retiring. (Otherwise if resubmit a *retired*
         * request, this would be a horrible use-after-free.)
         */
        if (i915_request_completed(request))
                goto xfer;

        if (unlikely(intel_context_is_banned(request->context)))
                i915_request_set_error_once(request, -EIO);
        if (unlikely(fatal_error(request->fence.error)))
                __i915_request_skip(request);

        /*
         * Are we using semaphores when the gpu is already saturated?
         *
         * Using semaphores incurs a cost in having the GPU poll a
         * memory location, busywaiting for it to change. The continual
         * memory reads can have a noticeable impact on the rest of the
         * system with the extra bus traffic, stalling the cpu as it too
         * tries to access memory across the bus (perf stat -e bus-cycles).
         *
         * If we installed a semaphore on this request and we only submit
         * the request after the signaler completed, that indicates the
         * system is overloaded and using semaphores at this time only
         * increases the amount of work we are doing. If so, we disable
         * further use of semaphores until we are idle again, whence we
         * optimistically try again.
         */
        if (request->sched.semaphores &&
            i915_sw_fence_signaled(&request->semaphore))
                engine->saturated |= request->sched.semaphores;

        engine->emit_fini_breadcrumb(request,
                                     request->ring->vaddr + request->postfix);

        trace_i915_request_execute(request);
        engine->serial++;
        result = true;

xfer:
        if (!test_and_set_bit(I915_FENCE_FLAG_ACTIVE, &request->fence.flags)) {
                list_move_tail(&request->sched.link, &engine->active.requests);
                clear_bit(I915_FENCE_FLAG_PQUEUE, &request->fence.flags);
        }

        /*
         * XXX Rollback bonded-execution on __i915_request_unsubmit()?
         *
         * In the future, perhaps when we have an active time-slicing scheduler,
         * it will be interesting to unsubmit parallel execution and remove
         * busywaits from the GPU until their master is restarted. This is
         * quite hairy, we have to carefully rollback the fence and do a
         * preempt-to-idle cycle on the target engine, all the while the
         * master execute_cb may refire.
         */
        __notify_execute_cb(request);

        if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &request->fence.flags))
                i915_request_enable_breadcrumb(request);

        return result;
}

void i915_request_submit(struct i915_request *request)
{
        struct intel_engine_cs *engine = request->engine;
        unsigned long flags;

        /* Will be called from irq-context when using foreign fences. */
        spin_lock_irqsave(&engine->active.lock, flags);

        __i915_request_submit(request);

        spin_unlock_irqrestore(&engine->active.lock, flags);
}

void __i915_request_unsubmit(struct i915_request *request)
{
        struct intel_engine_cs *engine = request->engine;

        /*
         * Only unwind in reverse order, required so that the per-context list
         * is kept in seqno/ring order.
         */
        RQ_TRACE(request, "\n");

        GEM_BUG_ON(!irqs_disabled());
        lockdep_assert_held(&engine->active.lock);

        /*
         * Before we remove this breadcrumb from the signal list, we have
         * to ensure that a concurrent dma_fence_enable_signaling() does not
         * attach itself. We first mark the request as no longer active and
         * make sure that is visible to other cores, and then remove the
         * breadcrumb if attached.
         */
        GEM_BUG_ON(!test_bit(I915_FENCE_FLAG_ACTIVE, &request->fence.flags));
        clear_bit_unlock(I915_FENCE_FLAG_ACTIVE, &request->fence.flags);
        if (test_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &request->fence.flags))
                i915_request_cancel_breadcrumb(request);

        /* We've already spun, don't charge on resubmitting. */
        if (request->sched.semaphores && i915_request_started(request))
                request->sched.semaphores = 0;

        /*
         * We don't need to wake_up any waiters on request->execute, they
         * will get woken by any other event or us re-adding this request
         * to the engine timeline (__i915_request_submit()). The waiters
         * should be quite adapt at finding that the request now has a new
         * global_seqno to the one they went to sleep on.
         */
}

void i915_request_unsubmit(struct i915_request *request)
{
        struct intel_engine_cs *engine = request->engine;
        unsigned long flags;

        /* Will be called from irq-context when using foreign fences. */
        spin_lock_irqsave(&engine->active.lock, flags);

        __i915_request_unsubmit(request);

        spin_unlock_irqrestore(&engine->active.lock, flags);
}

static int __i915_sw_fence_call
submit_notify(struct i915_sw_fence *fence, enum i915_sw_fence_notify state)
{
        struct i915_request *request =
                container_of(fence, typeof(*request), submit);

        switch (state) {
        case FENCE_COMPLETE:
                trace_i915_request_submit(request);

                if (unlikely(fence->error))
                        i915_request_set_error_once(request, fence->error);

                /*
                 * We need to serialize use of the submit_request() callback
                 * with its hotplugging performed during an emergency
                 * i915_gem_set_wedged().  We use the RCU mechanism to mark the
                 * critical section in order to force i915_gem_set_wedged() to
                 * wait until the submit_request() is completed before
                 * proceeding.
                 */
                rcu_read_lock();
                request->engine->submit_request(request);
                rcu_read_unlock();
                break;

        case FENCE_FREE:
                i915_request_put(request);
                break;
        }

        return NOTIFY_DONE;
}

static int __i915_sw_fence_call
semaphore_notify(struct i915_sw_fence *fence, enum i915_sw_fence_notify state)
{
        struct i915_request *rq = container_of(fence, typeof(*rq), semaphore);

        switch (state) {
        case FENCE_COMPLETE:
                break;

        case FENCE_FREE:
                i915_request_put(rq);
                break;
        }

        return NOTIFY_DONE;
}

static void retire_requests(struct intel_timeline *tl)
{
        struct i915_request *rq, *rn;

        list_for_each_entry_safe(rq, rn, &tl->requests, link)
                if (!i915_request_retire(rq))
                        break;
}

static noinline struct i915_request *
request_alloc_slow(struct intel_timeline *tl,
                   struct i915_request **rsvd,
                   gfp_t gfp)
{
        struct i915_request *rq;

        /* If we cannot wait, dip into our reserves */
        if (!gfpflags_allow_blocking(gfp)) {
                rq = xchg(rsvd, NULL);
                if (!rq) /* Use the normal failure path for one final WARN */
                        goto out;

                return rq;
        }

        if (list_empty(&tl->requests))
                goto out;

        /* Move our oldest request to the slab-cache (if not in use!) */
        rq = list_first_entry(&tl->requests, typeof(*rq), link);
        i915_request_retire(rq);

        rq = kmem_cache_alloc(global.slab_requests,
                              gfp | __GFP_RETRY_MAYFAIL | __GFP_NOWARN);
        if (rq)
                return rq;

        /* Ratelimit ourselves to prevent oom from malicious clients */
        rq = list_last_entry(&tl->requests, typeof(*rq), link);
        cond_synchronize_rcu(rq->rcustate);

        /* Retire our old requests in the hope that we free some */
        retire_requests(tl);

out:
        return kmem_cache_alloc(global.slab_requests, gfp);
}

static void __i915_request_ctor(void *arg)
{
        struct i915_request *rq = arg;

        spin_lock_init(&rq->lock);
        i915_sched_node_init(&rq->sched);
        i915_sw_fence_init(&rq->submit, submit_notify);
        i915_sw_fence_init(&rq->semaphore, semaphore_notify);

        dma_fence_init(&rq->fence, &i915_fence_ops, &rq->lock, 0, 0);

        rq->capture_list = NULL;

        init_llist_head(&rq->execute_cb);
}

struct i915_request *
__i915_request_create(struct intel_context *ce, gfp_t gfp)
{
        struct intel_timeline *tl = ce->timeline;
        struct i915_request *rq;
        u32 seqno;
        int ret;

        might_sleep_if(gfpflags_allow_blocking(gfp));

        /* Check that the caller provided an already pinned context */
        __intel_context_pin(ce);

        /*
         * Beware: Dragons be flying overhead.
         *
         * We use RCU to look up requests in flight. The lookups may
         * race with the request being allocated from the slab freelist.
         * That is the request we are writing to here, may be in the process
         * of being read by __i915_active_request_get_rcu(). As such,
         * we have to be very careful when overwriting the contents. During
         * the RCU lookup, we change chase the request->engine pointer,
         * read the request->global_seqno and increment the reference count.
         *
         * The reference count is incremented atomically. If it is zero,
         * the lookup knows the request is unallocated and complete. Otherwise,
         * it is either still in use, or has been reallocated and reset
         * with dma_fence_init(). This increment is safe for release as we
         * check that the request we have a reference to and matches the active
         * request.
         *
         * Before we increment the refcount, we chase the request->engine
         * pointer. We must not call kmem_cache_zalloc() or else we set
         * that pointer to NULL and cause a crash during the lookup. If
         * we see the request is completed (based on the value of the
         * old engine and seqno), the lookup is complete and reports NULL.
         * If we decide the request is not completed (new engine or seqno),
         * then we grab a reference and double check that it is still the
         * active request - which it won't be and restart the lookup.
         *
         * Do not use kmem_cache_zalloc() here!
         */
        rq = kmem_cache_alloc(global.slab_requests,
                              gfp | __GFP_RETRY_MAYFAIL | __GFP_NOWARN);
        if (unlikely(!rq)) {
                rq = request_alloc_slow(tl, &ce->engine->request_pool, gfp);
                if (!rq) {
                        ret = -ENOMEM;
                        goto err_unreserve;
                }
        }

        rq->context = ce;
        rq->engine = ce->engine;
        rq->ring = ce->ring;
        rq->execution_mask = ce->engine->mask;

        kref_init(&rq->fence.refcount);
        rq->fence.flags = 0;
        rq->fence.error = 0;
        INIT_LIST_HEAD(&rq->fence.cb_list);

        ret = intel_timeline_get_seqno(tl, rq, &seqno);
        if (ret)
                goto err_free;

        rq->fence.context = tl->fence_context;
        rq->fence.seqno = seqno;

        RCU_INIT_POINTER(rq->timeline, tl);
        RCU_INIT_POINTER(rq->hwsp_cacheline, tl->hwsp_cacheline);
        rq->hwsp_seqno = tl->hwsp_seqno;
        GEM_BUG_ON(i915_request_completed(rq));

        rq->rcustate = get_state_synchronize_rcu(); /* acts as smp_mb() */

        /* We bump the ref for the fence chain */
        i915_sw_fence_reinit(&i915_request_get(rq)->submit);
        i915_sw_fence_reinit(&i915_request_get(rq)->semaphore);

        i915_sched_node_reinit(&rq->sched);

        /* No zalloc, everything must be cleared after use */
        rq->batch = NULL;
        GEM_BUG_ON(rq->capture_list);
        GEM_BUG_ON(!llist_empty(&rq->execute_cb));

        /*
         * Reserve space in the ring buffer for all the commands required to
         * eventually emit this request. This is to guarantee that the
         * i915_request_add() call can't fail. Note that the reserve may need
         * to be redone if the request is not actually submitted straight
         * away, e.g. because a GPU scheduler has deferred it.
         *
         * Note that due to how we add reserved_space to intel_ring_begin()
         * we need to double our request to ensure that if we need to wrap
         * around inside i915_request_add() there is sufficient space at
         * the beginning of the ring as well.
         */
        rq->reserved_space =
                2 * rq->engine->emit_fini_breadcrumb_dw * sizeof(u32);

        /*
         * Record the position of the start of the request so that
         * should we detect the updated seqno part-way through the
         * GPU processing the request, we never over-estimate the
         * position of the head.
         */
        rq->head = rq->ring->emit;

        ret = rq->engine->request_alloc(rq);
        if (ret)
                goto err_unwind;

        rq->infix = rq->ring->emit; /* end of header; start of user payload */

        intel_context_mark_active(ce);
        list_add_tail_rcu(&rq->link, &tl->requests);

        return rq;

err_unwind:
        ce->ring->emit = rq->head;

        /* Make sure we didn't add ourselves to external state before freeing */
        GEM_BUG_ON(!list_empty(&rq->sched.signalers_list));
        GEM_BUG_ON(!list_empty(&rq->sched.waiters_list));

err_free:
        kmem_cache_free(global.slab_requests, rq);
err_unreserve:
        intel_context_unpin(ce);
        return ERR_PTR(ret);
}

struct i915_request *
i915_request_create(struct intel_context *ce)
{
        struct i915_request *rq;
        struct intel_timeline *tl;

        tl = intel_context_timeline_lock(ce);
        if (IS_ERR(tl))
                return ERR_CAST(tl);

        /* Move our oldest request to the slab-cache (if not in use!) */
        rq = list_first_entry(&tl->requests, typeof(*rq), link);
        if (!list_is_last(&rq->link, &tl->requests))
                i915_request_retire(rq);

        intel_context_enter(ce);
        rq = __i915_request_create(ce, GFP_KERNEL);
        intel_context_exit(ce); /* active reference transferred to request */
        if (IS_ERR(rq))
                goto err_unlock;

        /* Check that we do not interrupt ourselves with a new request */
        rq->cookie = lockdep_pin_lock(&tl->mutex);

        return rq;

err_unlock:
        intel_context_timeline_unlock(tl);
        return rq;
}

static int
i915_request_await_start(struct i915_request *rq, struct i915_request *signal)
{
        struct dma_fence *fence;
        int err;

        if (i915_request_timeline(rq) == rcu_access_pointer(signal->timeline))
                return 0;

        if (i915_request_started(signal))
                return 0;

        fence = NULL;
        rcu_read_lock();
        spin_lock_irq(&signal->lock);
        do {
                struct list_head *pos = READ_ONCE(signal->link.prev);
                struct i915_request *prev;

                /* Confirm signal has not been retired, the link is valid */
                if (unlikely(i915_request_started(signal)))
                        break;

                /* Is signal the earliest request on its timeline? */
                if (pos == &rcu_dereference(signal->timeline)->requests)
                        break;

                /*
                 * Peek at the request before us in the timeline. That
                 * request will only be valid before it is retired, so
                 * after acquiring a reference to it, confirm that it is
                 * still part of the signaler's timeline.
                 */
                prev = list_entry(pos, typeof(*prev), link);
                if (!i915_request_get_rcu(prev))
                        break;

                /* After the strong barrier, confirm prev is still attached */
                if (unlikely(READ_ONCE(prev->link.next) != &signal->link)) {
                        i915_request_put(prev);
                        break;
                }

                fence = &prev->fence;
        } while (0);
        spin_unlock_irq(&signal->lock);
        rcu_read_unlock();
        if (!fence)
                return 0;

        err = 0;
        if (!intel_timeline_sync_is_later(i915_request_timeline(rq), fence))
                err = i915_sw_fence_await_dma_fence(&rq->submit,
                                                    fence, 0,
                                                    I915_FENCE_GFP);
        dma_fence_put(fence);

        return err;
}

static intel_engine_mask_t
already_busywaiting(struct i915_request *rq)
{
        /*
         * Polling a semaphore causes bus traffic, delaying other users of
         * both the GPU and CPU. We want to limit the impact on others,
         * while taking advantage of early submission to reduce GPU
         * latency. Therefore we restrict ourselves to not using more
         * than one semaphore from each source, and not using a semaphore
         * if we have detected the engine is saturated (i.e. would not be
         * submitted early and cause bus traffic reading an already passed
         * semaphore).
         *
         * See the are-we-too-late? check in __i915_request_submit().
         */
        return rq->sched.semaphores | READ_ONCE(rq->engine->saturated);
}

static int
__emit_semaphore_wait(struct i915_request *to,
                      struct i915_request *from,
                      u32 seqno)
{
        const int has_token = INTEL_GEN(to->engine->i915) >= 12;
        u32 hwsp_offset;
        int len, err;
        u32 *cs;

        GEM_BUG_ON(INTEL_GEN(to->engine->i915) < 8);
        GEM_BUG_ON(i915_request_has_initial_breadcrumb(to));

        /* We need to pin the signaler's HWSP until we are finished reading. */
        err = intel_timeline_read_hwsp(from, to, &hwsp_offset);
        if (err)
                return err;

        len = 4;
        if (has_token)
                len += 2;

        cs = intel_ring_begin(to, len);
        if (IS_ERR(cs))
                return PTR_ERR(cs);

        /*
         * Using greater-than-or-equal here means we have to worry
         * about seqno wraparound. To side step that issue, we swap
         * the timeline HWSP upon wrapping, so that everyone listening
         * for the old (pre-wrap) values do not see the much smaller
         * (post-wrap) values than they were expecting (and so wait
         * forever).
         */
        *cs++ = (MI_SEMAPHORE_WAIT |
                 MI_SEMAPHORE_GLOBAL_GTT |
                 MI_SEMAPHORE_POLL |
                 MI_SEMAPHORE_SAD_GTE_SDD) +
                has_token;
        *cs++ = seqno;
        *cs++ = hwsp_offset;
        *cs++ = 0;
        if (has_token) {
                *cs++ = 0;
                *cs++ = MI_NOOP;
        }

        intel_ring_advance(to, cs);
        return 0;
}

static int
emit_semaphore_wait(struct i915_request *to,
                    struct i915_request *from,
                    gfp_t gfp)
{
        const intel_engine_mask_t mask = READ_ONCE(from->engine)->mask;
        struct i915_sw_fence *wait = &to->submit;

        if (!intel_context_use_semaphores(to->context))
                goto await_fence;

        if (i915_request_has_initial_breadcrumb(to))
                goto await_fence;

        if (!rcu_access_pointer(from->hwsp_cacheline))
                goto await_fence;

        /*
         * If this or its dependents are waiting on an external fence
         * that may fail catastrophically, then we want to avoid using
         * sempahores as they bypass the fence signaling metadata, and we
         * lose the fence->error propagation.
         */
        if (from->sched.flags & I915_SCHED_HAS_EXTERNAL_CHAIN)
                goto await_fence;

        /* Just emit the first semaphore we see as request space is limited. */
        if (already_busywaiting(to) & mask)
                goto await_fence;

        if (i915_request_await_start(to, from) < 0)
                goto await_fence;

        /* Only submit our spinner after the signaler is running! */
        if (__await_execution(to, from, NULL, gfp))
                goto await_fence;

        if (__emit_semaphore_wait(to, from, from->fence.seqno))
                goto await_fence;

        to->sched.semaphores |= mask;
        wait = &to->semaphore;

await_fence:
        return i915_sw_fence_await_dma_fence(wait,
                                             &from->fence, 0,
                                             I915_FENCE_GFP);
}

static bool intel_timeline_sync_has_start(struct intel_timeline *tl,
                                          struct dma_fence *fence)
{
        return __intel_timeline_sync_is_later(tl,
                                              fence->context,
                                              fence->seqno - 1);
}

static int intel_timeline_sync_set_start(struct intel_timeline *tl,
                                         const struct dma_fence *fence)
{
        return __intel_timeline_sync_set(tl, fence->context, fence->seqno - 1);
}

static int
__i915_request_await_execution(struct i915_request *to,
                               struct i915_request *from,
                               void (*hook)(struct i915_request *rq,
                                            struct dma_fence *signal))
{
        int err;

        GEM_BUG_ON(intel_context_is_barrier(from->context));

        /* Submit both requests at the same time */
        err = __await_execution(to, from, hook, I915_FENCE_GFP);
        if (err)
                return err;

        /* Squash repeated depenendices to the same timelines */
        if (intel_timeline_sync_has_start(i915_request_timeline(to),
                                          &from->fence))
                return 0;

        /*
         * Wait until the start of this request.
         *
         * The execution cb fires when we submit the request to HW. But in
         * many cases this may be long before the request itself is ready to
         * run (consider that we submit 2 requests for the same context, where
         * the request of interest is behind an indefinite spinner). So we hook
         * up to both to reduce our queues and keep the execution lag minimised
         * in the worst case, though we hope that the await_start is elided.
         */
        err = i915_request_await_start(to, from);
        if (err < 0)
                return err;

        /*
         * Ensure both start together [after all semaphores in signal]
         *
         * Now that we are queued to the HW at roughly the same time (thanks
         * to the execute cb) and are ready to run at roughly the same time
         * (thanks to the await start), our signaler may still be indefinitely
         * delayed by waiting on a semaphore from a remote engine. If our
         * signaler depends on a semaphore, so indirectly do we, and we do not
         * want to start our payload until our signaler also starts theirs.
         * So we wait.
         *
         * However, there is also a second condition for which we need to wait
         * for the precise start of the signaler. Consider that the signaler
         * was submitted in a chain of requests following another context
         * (with just an ordinary intra-engine fence dependency between the
         * two). In this case the signaler is queued to HW, but not for
         * immediate execution, and so we must wait until it reaches the
         * active slot.
         */
        if (intel_engine_has_semaphores(to->engine) &&
            !i915_request_has_initial_breadcrumb(to)) {
                err = __emit_semaphore_wait(to, from, from->fence.seqno - 1);
                if (err < 0)
                        return err;
        }

        /* Couple the dependency tree for PI on this exposed to->fence */
        if (to->engine->schedule) {
                err = i915_sched_node_add_dependency(&to->sched,
                                                     &from->sched,
                                                     I915_DEPENDENCY_WEAK);
                if (err < 0)
                        return err;
        }

        return intel_timeline_sync_set_start(i915_request_timeline(to),
                                             &from->fence);
}

static void mark_external(struct i915_request *rq)
{
        /*
         * The downside of using semaphores is that we lose metadata passing
         * along the signaling chain. This is particularly nasty when we
         * need to pass along a fatal error such as EFAULT or EDEADLK. For
         * fatal errors we want to scrub the request before it is executed,
         * which means that we cannot preload the request onto HW and have
         * it wait upon a semaphore.
         */
        rq->sched.flags |= I915_SCHED_HAS_EXTERNAL_CHAIN;
}

static int
__i915_request_await_external(struct i915_request *rq, struct dma_fence *fence)
{
        mark_external(rq);
        return i915_sw_fence_await_dma_fence(&rq->submit, fence,
                                             i915_fence_context_timeout(rq->engine->i915,
                                                                        fence->context),
                                             I915_FENCE_GFP);
}

static int
i915_request_await_external(struct i915_request *rq, struct dma_fence *fence)
{
        struct dma_fence *iter;
        int err = 0;

        if (!to_dma_fence_chain(fence))
                return __i915_request_await_external(rq, fence);

        dma_fence_chain_for_each(iter, fence) {
                struct dma_fence_chain *chain = to_dma_fence_chain(iter);

                if (!dma_fence_is_i915(chain->fence)) {
                        err = __i915_request_await_external(rq, iter);
                        break;
                }

                err = i915_request_await_dma_fence(rq, chain->fence);
                if (err < 0)
                        break;
        }

        dma_fence_put(iter);
        return err;
}

int
i915_request_await_execution(struct i915_request *rq,
                             struct dma_fence *fence,
                             void (*hook)(struct i915_request *rq,
                                          struct dma_fence *signal))
{
        struct dma_fence **child = &fence;
        unsigned int nchild = 1;
        int ret;

        if (dma_fence_is_array(fence)) {
                struct dma_fence_array *array = to_dma_fence_array(fence);

                /* XXX Error for signal-on-any fence arrays */

                child = array->fences;
                nchild = array->num_fences;
                GEM_BUG_ON(!nchild);
        }

        do {
                fence = *child++;
                if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) {
                        i915_sw_fence_set_error_once(&rq->submit, fence->error);
                        continue;
                }

                if (fence->context == rq->fence.context)
                        continue;

                /*
                 * We don't squash repeated fence dependencies here as we
                 * want to run our callback in all cases.
                 */

                if (dma_fence_is_i915(fence))
                        ret = __i915_request_await_execution(rq,
                                                             to_request(fence),
                                                             hook);
                else
                        ret = i915_request_await_external(rq, fence);
                if (ret < 0)
                        return ret;
        } while (--nchild);

        return 0;
}

static int
await_request_submit(struct i915_request *to, struct i915_request *from)
{
        /*
         * If we are waiting on a virtual engine, then it may be
         * constrained to execute on a single engine *prior* to submission.
         * When it is submitted, it will be first submitted to the virtual
         * engine and then passed to the physical engine. We cannot allow
         * the waiter to be submitted immediately to the physical engine
         * as it may then bypass the virtual request.
         */
        if (to->engine == READ_ONCE(from->engine))
                return i915_sw_fence_await_sw_fence_gfp(&to->submit,
                                                        &from->submit,
                                                        I915_FENCE_GFP);
        else
                return __i915_request_await_execution(to, from, NULL);
}

static int
i915_request_await_request(struct i915_request *to, struct i915_request *from)
{
        int ret;

        GEM_BUG_ON(to == from);
        GEM_BUG_ON(to->timeline == from->timeline);

        if (i915_request_completed(from)) {
                i915_sw_fence_set_error_once(&to->submit, from->fence.error);
                return 0;
        }

        if (to->engine->schedule) {
                ret = i915_sched_node_add_dependency(&to->sched,
                                                     &from->sched,
                                                     I915_DEPENDENCY_EXTERNAL);
                if (ret < 0)
                        return ret;
        }

        if (is_power_of_2(to->execution_mask | READ_ONCE(from->execution_mask)))
                ret = await_request_submit(to, from);
        else
                ret = emit_semaphore_wait(to, from, I915_FENCE_GFP);
        if (ret < 0)
                return ret;

        return 0;
}

int
i915_request_await_dma_fence(struct i915_request *rq, struct dma_fence *fence)
{
        struct dma_fence **child = &fence;
        unsigned int nchild = 1;
        int ret;

        /*
         * Note that if the fence-array was created in signal-on-any mode,
         * we should *not* decompose it into its individual fences. However,
         * we don't currently store which mode the fence-array is operating
         * in. Fortunately, the only user of signal-on-any is private to
         * amdgpu and we should not see any incoming fence-array from
         * sync-file being in signal-on-any mode.
         */
        if (dma_fence_is_array(fence)) {
                struct dma_fence_array *array = to_dma_fence_array(fence);

                child = array->fences;
                nchild = array->num_fences;
                GEM_BUG_ON(!nchild);
        }

        do {
                fence = *child++;
                if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) {
                        i915_sw_fence_set_error_once(&rq->submit, fence->error);
                        continue;
                }

                /*
                 * Requests on the same timeline are explicitly ordered, along
                 * with their dependencies, by i915_request_add() which ensures
                 * that requests are submitted in-order through each ring.
                 */
                if (fence->context == rq->fence.context)
                        continue;

                /* Squash repeated waits to the same timelines */
                if (fence->context &&
                    intel_timeline_sync_is_later(i915_request_timeline(rq),
                                                 fence))
                        continue;

                if (dma_fence_is_i915(fence))
                        ret = i915_request_await_request(rq, to_request(fence));
                else
                        ret = i915_request_await_external(rq, fence);
                if (ret < 0)
                        return ret;

                /* Record the latest fence used against each timeline */
                if (fence->context)
                        intel_timeline_sync_set(i915_request_timeline(rq),
                                                fence);
        } while (--nchild);

        return 0;
}

/**
 * i915_request_await_object - set this request to (async) wait upon a bo
 * @to: request we are wishing to use
 * @obj: object which may be in use on another ring.
 * @write: whether the wait is on behalf of a writer
 *
 * This code is meant to abstract object synchronization with the GPU.
 * Conceptually we serialise writes between engines inside the GPU.
 * We only allow one engine to write into a buffer at any time, but
 * multiple readers. To ensure each has a coherent view of memory, we must:
 *
 * - If there is an outstanding write request to the object, the new
 *   request must wait for it to complete (either CPU or in hw, requests
 *   on the same ring will be naturally ordered).
 *
 * - If we are a write request (pending_write_domain is set), the new
 *   request must wait for outstanding read requests to complete.
 *
 * Returns 0 if successful, else propagates up the lower layer error.
 */
int
i915_request_await_object(struct i915_request *to,
                          struct drm_i915_gem_object *obj,
                          bool write)
{
        struct dma_fence *excl;
        int ret = 0;

        if (write) {
                struct dma_fence **shared;
                unsigned int count, i;

                ret = dma_resv_get_fences_rcu(obj->base.resv,
                                                        &excl, &count, &shared);
                if (ret)
                        return ret;

                for (i = 0; i < count; i++) {
                        ret = i915_request_await_dma_fence(to, shared[i]);
                        if (ret)
                                break;

                        dma_fence_put(shared[i]);
                }

                for (; i < count; i++)
                        dma_fence_put(shared[i]);
                kfree(shared);
        } else {
                excl = dma_resv_get_excl_rcu(obj->base.resv);
        }

        if (excl) {
                if (ret == 0)
                        ret = i915_request_await_dma_fence(to, excl);

                dma_fence_put(excl);
        }

        return ret;
}

static struct i915_request *
__i915_request_add_to_timeline(struct i915_request *rq)
{
        struct intel_timeline *timeline = i915_request_timeline(rq);
        struct i915_request *prev;

        /*
         * Dependency tracking and request ordering along the timeline
         * is special cased so that we can eliminate redundant ordering
         * operations while building the request (we know that the timeline
         * itself is ordered, and here we guarantee it).
         *
         * As we know we will need to emit tracking along the timeline,
         * we embed the hooks into our request struct -- at the cost of
         * having to have specialised no-allocation interfaces (which will
         * be beneficial elsewhere).
         *
         * A second benefit to open-coding i915_request_await_request is
         * that we can apply a slight variant of the rules specialised
         * for timelines that jump between engines (such as virtual engines).
         * If we consider the case of virtual engine, we must emit a dma-fence
         * to prevent scheduling of the second request until the first is
         * complete (to maximise our greedy late load balancing) and this
         * precludes optimising to use semaphores serialisation of a single
         * timeline across engines.
         */
        prev = to_request(__i915_active_fence_set(&timeline->last_request,
                                                  &rq->fence));
        if (prev && !i915_request_completed(prev)) {
                /*
                 * The requests are supposed to be kept in order. However,
                 * we need to be wary in case the timeline->last_request
                 * is used as a barrier for external modification to this
                 * context.
                 */
                GEM_BUG_ON(prev->context == rq->context &&
                           i915_seqno_passed(prev->fence.seqno,
                                             rq->fence.seqno));

                if (is_power_of_2(READ_ONCE(prev->engine)->mask | rq->engine->mask))
                        i915_sw_fence_await_sw_fence(&rq->submit,
                                                     &prev->submit,
                                                     &rq->submitq);
                else
                        __i915_sw_fence_await_dma_fence(&rq->submit,
                                                        &prev->fence,
                                                        &rq->dmaq);
                if (rq->engine->schedule)
                        __i915_sched_node_add_dependency(&rq->sched,
                                                         &prev->sched,
                                                         &rq->dep,
                                                         0);
        }

        /*
         * Make sure that no request gazumped us - if it was allocated after
         * our i915_request_alloc() and called __i915_request_add() before
         * us, the timeline will hold its seqno which is later than ours.
         */
        GEM_BUG_ON(timeline->seqno != rq->fence.seqno);

        return prev;
}

/*
 * NB: This function is not allowed to fail. Doing so would mean the the
 * request is not being tracked for completion but the work itself is
 * going to happen on the hardware. This would be a Bad Thing(tm).
 */
struct i915_request *__i915_request_commit(struct i915_request *rq)
{
        struct intel_engine_cs *engine = rq->engine;
        struct intel_ring *ring = rq->ring;
        u32 *cs;

        RQ_TRACE(rq, "\n");

        /*
         * To ensure that this call will not fail, space for its emissions
         * should already have been reserved in the ring buffer. Let the ring
         * know that it is time to use that space up.
         */
        GEM_BUG_ON(rq->reserved_space > ring->space);
        rq->reserved_space = 0;
        rq->emitted_jiffies = jiffies;

        /*
         * Record the position of the start of the breadcrumb so that
         * should we detect the updated seqno part-way through the
         * GPU processing the request, we never over-estimate the
         * position of the ring's HEAD.
         */
        cs = intel_ring_begin(rq, engine->emit_fini_breadcrumb_dw);
        GEM_BUG_ON(IS_ERR(cs));
        rq->postfix = intel_ring_offset(rq, cs);

        return __i915_request_add_to_timeline(rq);
}

void __i915_request_queue(struct i915_request *rq,
                          const struct i915_sched_attr *attr)
{
        /*
         * Let the backend know a new request has arrived that may need
         * to adjust the existing execution schedule due to a high priority
         * request - i.e. we may want to preempt the current request in order
         * to run a high priority dependency chain *before* we can execute this
         * request.
         *
         * This is called before the request is ready to run so that we can
         * decide whether to preempt the entire chain so that it is ready to
         * run at the earliest possible convenience.
         */
        if (attr && rq->engine->schedule)
                rq->engine->schedule(rq, attr);
        i915_sw_fence_commit(&rq->semaphore);
        i915_sw_fence_commit(&rq->submit);
}

void i915_request_add(struct i915_request *rq)
{
        struct intel_timeline * const tl = i915_request_timeline(rq);
        struct i915_sched_attr attr = {};
        struct i915_gem_context *ctx;

        lockdep_assert_held(&tl->mutex);
        lockdep_unpin_lock(&tl->mutex, rq->cookie);

        trace_i915_request_add(rq);
        __i915_request_commit(rq);

        /* XXX placeholder for selftests */
        rcu_read_lock();
        ctx = rcu_dereference(rq->context->gem_context);
        if (ctx)
                attr = ctx->sched;
        rcu_read_unlock();

        __i915_request_queue(rq, &attr);

        mutex_unlock(&tl->mutex);
}

static unsigned long local_clock_ns(unsigned int *cpu)
{
        unsigned long t;

        /*
         * Cheaply and approximately convert from nanoseconds to microseconds.
         * The result and subsequent calculations are also defined in the same
         * approximate microseconds units. The principal source of timing
         * error here is from the simple truncation.
         *
         * Note that local_clock() is only defined wrt to the current CPU;
         * the comparisons are no longer valid if we switch CPUs. Instead of
         * blocking preemption for the entire busywait, we can detect the CPU
         * switch and use that as indicator of system load and a reason to
         * stop busywaiting, see busywait_stop().
         */
        *cpu = get_cpu();
        t = local_clock();
        put_cpu();

        return t;
}

static bool busywait_stop(unsigned long timeout, unsigned int cpu)
{
        unsigned int this_cpu;

        if (time_after(local_clock_ns(&this_cpu), timeout))
                return true;

        return this_cpu != cpu;
}

static bool __i915_spin_request(struct i915_request * const rq, int state)
{
        unsigned long timeout_ns;
        unsigned int cpu;

        /*
         * Only wait for the request if we know it is likely to complete.
         *
         * We don't track the timestamps around requests, nor the average
         * request length, so we do not have a good indicator that this
         * request will complete within the timeout. What we do know is the
         * order in which requests are executed by the context and so we can
         * tell if the request has been started. If the request is not even
         * running yet, it is a fair assumption that it will not complete
         * within our relatively short timeout.
         */
        if (!i915_request_is_running(rq))
                return false;

        /*
         * When waiting for high frequency requests, e.g. during synchronous
         * rendering split between the CPU and GPU, the finite amount of time
         * required to set up the irq and wait upon it limits the response
         * rate. By busywaiting on the request completion for a short while we
         * can service the high frequency waits as quick as possible. However,
         * if it is a slow request, we want to sleep as quickly as possible.
         * The tradeoff between waiting and sleeping is roughly the time it
         * takes to sleep on a request, on the order of a microsecond.
         */

        timeout_ns = READ_ONCE(rq->engine->props.max_busywait_duration_ns);
        timeout_ns += local_clock_ns(&cpu);
        do {
                if (dma_fence_is_signaled(&rq->fence))
                        return true;

                if (signal_pending_state(state, current))
                        break;

                if (busywait_stop(timeout_ns, cpu))
                        break;

                cpu_relax();
        } while (!need_resched());

        return false;
}

struct request_wait {
        struct dma_fence_cb cb;
        struct task_struct *tsk;
};

static void request_wait_wake(struct dma_fence *fence, struct dma_fence_cb *cb)
{
        struct request_wait *wait = container_of(cb, typeof(*wait), cb);

        wake_up_process(fetch_and_zero(&wait->tsk));
}

/**
 * i915_request_wait - wait until execution of request has finished
 * @rq: the request to wait upon
 * @flags: how to wait
 * @timeout: how long to wait in jiffies
 *
 * i915_request_wait() waits for the request to be completed, for a
 * maximum of @timeout jiffies (with MAX_SCHEDULE_TIMEOUT implying an
 * unbounded wait).
 *
 * Returns the remaining time (in jiffies) if the request completed, which may
 * be zero or -ETIME if the request is unfinished after the timeout expires.
 * May return -EINTR is called with I915_WAIT_INTERRUPTIBLE and a signal is
 * pending before the request completes.
 */
long i915_request_wait(struct i915_request *rq,
                       unsigned int flags,
                       long timeout)
{
        const int state = flags & I915_WAIT_INTERRUPTIBLE ?
                TASK_INTERRUPTIBLE : TASK_UNINTERRUPTIBLE;
        struct request_wait wait;

        might_sleep();
        GEM_BUG_ON(timeout < 0);

        if (dma_fence_is_signaled(&rq->fence))
                return timeout;

        if (!timeout)
                return -ETIME;

        trace_i915_request_wait_begin(rq, flags);

        /*
         * We must never wait on the GPU while holding a lock as we
         * may need to perform a GPU reset. So while we don't need to
         * serialise wait/reset with an explicit lock, we do want
         * lockdep to detect potential dependency cycles.
         */
        mutex_acquire(&rq->engine->gt->reset.mutex.dep_map, 0, 0, _THIS_IP_);

        /*
         * Optimistic spin before touching IRQs.
         *
         * We may use a rather large value here to offset the penalty of
         * switching away from the active task. Frequently, the client will
         * wait upon an old swapbuffer to throttle itself to remain within a
         * frame of the gpu. If the client is running in lockstep with the gpu,
         * then it should not be waiting long at all, and a sleep now will incur
         * extra scheduler latency in producing the next frame. To try to
         * avoid adding the cost of enabling/disabling the interrupt to the
         * short wait, we first spin to see if the request would have completed
         * in the time taken to setup the interrupt.
         *
         * We need upto 5us to enable the irq, and upto 20us to hide the
         * scheduler latency of a context switch, ignoring the secondary
         * impacts from a context switch such as cache eviction.
         *
         * The scheme used for low-latency IO is called "hybrid interrupt
         * polling". The suggestion there is to sleep until just before you
         * expect to be woken by the device interrupt and then poll for its
         * completion. That requires having a good predictor for the request
         * duration, which we currently lack.
         */
        if (IS_ACTIVE(CONFIG_DRM_I915_MAX_REQUEST_BUSYWAIT) &&
            __i915_spin_request(rq, state))
                goto out;

        /*
         * This client is about to stall waiting for the GPU. In many cases
         * this is undesirable and limits the throughput of the system, as
         * many clients cannot continue processing user input/output whilst
         * blocked. RPS autotuning may take tens of milliseconds to respond
         * to the GPU load and thus incurs additional latency for the client.
         * We can circumvent that by promoting the GPU frequency to maximum
         * before we sleep. This makes the GPU throttle up much more quickly
         * (good for benchmarks and user experience, e.g. window animations),
         * but at a cost of spending more power processing the workload
         * (bad for battery).
         */
        if (flags & I915_WAIT_PRIORITY && !i915_request_started(rq))
                intel_rps_boost(rq);

        wait.tsk = current;
        if (dma_fence_add_callback(&rq->fence, &wait.cb, request_wait_wake))
                goto out;

        /*
         * Flush the submission tasklet, but only if it may help this request.
         *
         * We sometimes experience some latency between the HW interrupts and
         * tasklet execution (mostly due to ksoftirqd latency, but it can also
         * be due to lazy CS events), so lets run the tasklet manually if there
         * is a chance it may submit this request. If the request is not ready
         * to run, as it is waiting for other fences to be signaled, flushing
         * the tasklet is busy work without any advantage for this client.
         *
         * If the HW is being lazy, this is the last chance before we go to
         * sleep to catch any pending events. We will check periodically in
         * the heartbeat to flush the submission tasklets as a last resort
         * for unhappy HW.
         */
        if (i915_request_is_ready(rq))
                intel_engine_flush_submission(rq->engine);

        for (;;) {
                set_current_state(state);

                if (dma_fence_is_signaled(&rq->fence))
                        break;

                if (signal_pending_state(state, current)) {
                        timeout = -ERESTARTSYS;
                        break;
                }

                if (!timeout) {
                        timeout = -ETIME;
                        break;
                }

                timeout = io_schedule_timeout(timeout);
        }
        __set_current_state(TASK_RUNNING);

        if (READ_ONCE(wait.tsk))
                dma_fence_remove_callback(&rq->fence, &wait.cb);
        GEM_BUG_ON(!list_empty(&wait.cb.node));

out:
        mutex_release(&rq->engine->gt->reset.mutex.dep_map, _THIS_IP_);
        trace_i915_request_wait_end(rq);
        return timeout;
}

#if IS_ENABLED(CONFIG_DRM_I915_SELFTEST)
#include "selftests/mock_request.c"
#include "selftests/i915_request.c"
#endif

static void i915_global_request_shrink(void)
{
        kmem_cache_shrink(global.slab_execute_cbs);
        kmem_cache_shrink(global.slab_requests);
}

static void i915_global_request_exit(void)
{
        kmem_cache_destroy(global.slab_execute_cbs);
        kmem_cache_destroy(global.slab_requests);
}

static struct i915_global_request global = { {
        .shrink = i915_global_request_shrink,
        .exit = i915_global_request_exit,
} };

int __init i915_global_request_init(void)
{
        global.slab_requests =
                kmem_cache_create("i915_request",
                                  sizeof(struct i915_request),
                                  __alignof__(struct i915_request),
                                  SLAB_HWCACHE_ALIGN |
                                  SLAB_RECLAIM_ACCOUNT |
                                  SLAB_TYPESAFE_BY_RCU,
                                  __i915_request_ctor);
        if (!global.slab_requests)
                return -ENOMEM;

        global.slab_execute_cbs = KMEM_CACHE(execute_cb,
                                             SLAB_HWCACHE_ALIGN |
                                             SLAB_RECLAIM_ACCOUNT |
                                             SLAB_TYPESAFE_BY_RCU);
        if (!global.slab_execute_cbs)
                goto err_requests;

        i915_global_register(&global.base);
        return 0;

err_requests:
        kmem_cache_destroy(global.slab_requests);
        return -ENOMEM;
}