drm: Add a prefetching memcpy_from_wc

Reading out of write-combining mapped memory is typically very slow
since the CPU doesn't prefetch. However some archs have special
instructions to do this.

So add a best-effort memcpy_from_wc taking dma-buf-map pointer
arguments that attempts to use a fast prefetching memcpy and
otherwise falls back to ordinary memcopies, taking the iomem tagging
into account.

The code is largely copied from i915_memcpy_from_wc.

Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Christian König <christian.koenig@amd.com>
Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Christian König <christian.koenig@amd.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Link: https://lore.kernel.org/r/20210602083818.241793-5-thomas.hellstrom@linux.intel.com
Link: https://patchwork.freedesktop.org/patch/msgid/20210602083818.241793-5-thomas.hellstrom@linux.intel.com

diff --git a/Documentation/gpu/drm-mm.rst b/Documentation/gpu/drm-mm.rst
index 21be6de..d5a73fa 100644 (file)
--- a/Documentation/gpu/drm-mm.rst
+++ b/Documentation/gpu/drm-mm.rst
@@ -469,8 +469,8 @@ DRM MM Range Allocator Function References
 .. kernel-doc:: drivers/gpu/drm/drm_mm.c
    :export:
 
-DRM Cache Handling
-==================
+DRM Cache Handling and Fast WC memcpy()
+=======================================
 
 .. kernel-doc:: drivers/gpu/drm/drm_cache.c
    :export:

diff --git a/drivers/gpu/drm/drm_cache.c b/drivers/gpu/drm/drm_cache.c
index 79a50ef..546599f 100644 (file)
--- a/drivers/gpu/drm/drm_cache.c
+++ b/drivers/gpu/drm/drm_cache.c
@@ -28,6 +28,7 @@
  * Authors: Thomas Hellström <thomas-at-tungstengraphics-dot-com>
  */
 
+#include <linux/dma-buf-map.h>
 #include <linux/export.h>
 #include <linux/highmem.h>
 #include <linux/mem_encrypt.h>
@@ -35,6 +36,9 @@
 
 #include <drm/drm_cache.h>
 
+/* A small bounce buffer that fits on the stack. */
+#define MEMCPY_BOUNCE_SIZE 128
+
 #if defined(CONFIG_X86)
 #include <asm/smp.h>
 
@@ -209,3 +213,147 @@ bool drm_need_swiotlb(int dma_bits)
        return max_iomem > ((u64)1 << dma_bits);
 }
 EXPORT_SYMBOL(drm_need_swiotlb);
+
+static void memcpy_fallback(struct dma_buf_map *dst,
+                           const struct dma_buf_map *src,
+                           unsigned long len)
+{
+       if (!dst->is_iomem && !src->is_iomem) {
+               memcpy(dst->vaddr, src->vaddr, len);
+       } else if (!src->is_iomem) {
+               dma_buf_map_memcpy_to(dst, src->vaddr, len);
+       } else if (!dst->is_iomem) {
+               memcpy_fromio(dst->vaddr, src->vaddr_iomem, len);
+       } else {
+               /*
+                * Bounce size is not performance tuned, but using a
+                * bounce buffer like this is significantly faster than
+                * resorting to ioreadxx() + iowritexx().
+                */
+               char bounce[MEMCPY_BOUNCE_SIZE];
+               void __iomem *_src = src->vaddr_iomem;
+               void __iomem *_dst = dst->vaddr_iomem;
+
+               while (len >= MEMCPY_BOUNCE_SIZE) {
+                       memcpy_fromio(bounce, _src, MEMCPY_BOUNCE_SIZE);
+                       memcpy_toio(_dst, bounce, MEMCPY_BOUNCE_SIZE);
+                       _src += MEMCPY_BOUNCE_SIZE;
+                       _dst += MEMCPY_BOUNCE_SIZE;
+                       len -= MEMCPY_BOUNCE_SIZE;
+               }
+               if (len) {
+                       memcpy_fromio(bounce, _src, MEMCPY_BOUNCE_SIZE);
+                       memcpy_toio(_dst, bounce, MEMCPY_BOUNCE_SIZE);
+               }
+       }
+}
+
+#ifdef CONFIG_X86
+
+static DEFINE_STATIC_KEY_FALSE(has_movntdqa);
+
+static void __memcpy_ntdqa(void *dst, const void *src, unsigned long len)
+{
+       kernel_fpu_begin();
+
+       while (len >= 4) {
+               asm("movntdqa   (%0), %%xmm0\n"
+                   "movntdqa 16(%0), %%xmm1\n"
+                   "movntdqa 32(%0), %%xmm2\n"
+                   "movntdqa 48(%0), %%xmm3\n"
+                   "movaps %%xmm0,   (%1)\n"
+                   "movaps %%xmm1, 16(%1)\n"
+                   "movaps %%xmm2, 32(%1)\n"
+                   "movaps %%xmm3, 48(%1)\n"
+                   :: "r" (src), "r" (dst) : "memory");
+               src += 64;
+               dst += 64;
+               len -= 4;
+       }
+       while (len--) {
+               asm("movntdqa (%0), %%xmm0\n"
+                   "movaps %%xmm0, (%1)\n"
+                   :: "r" (src), "r" (dst) : "memory");
+               src += 16;
+               dst += 16;
+       }
+
+       kernel_fpu_end();
+}
+
+/*
+ * __drm_memcpy_from_wc copies @len bytes from @src to @dst using
+ * non-temporal instructions where available. Note that all arguments
+ * (@src, @dst) must be aligned to 16 bytes and @len must be a multiple
+ * of 16.
+ */
+static void __drm_memcpy_from_wc(void *dst, const void *src, unsigned long len)
+{
+       if (unlikely(((unsigned long)dst | (unsigned long)src | len) & 15))
+               memcpy(dst, src, len);
+       else if (likely(len))
+               __memcpy_ntdqa(dst, src, len >> 4);
+}
+
+/**
+ * drm_memcpy_from_wc - Perform the fastest available memcpy from a source
+ * that may be WC.
+ * @dst: The destination pointer
+ * @src: The source pointer
+ * @len: The size of the area o transfer in bytes
+ *
+ * Tries an arch optimized memcpy for prefetching reading out of a WC region,
+ * and if no such beast is available, falls back to a normal memcpy.
+ */
+void drm_memcpy_from_wc(struct dma_buf_map *dst,
+                       const struct dma_buf_map *src,
+                       unsigned long len)
+{
+       if (WARN_ON(in_interrupt())) {
+               memcpy_fallback(dst, src, len);
+               return;
+       }
+
+       if (static_branch_likely(&has_movntdqa)) {
+               __drm_memcpy_from_wc(dst->is_iomem ?
+                                    (void __force *)dst->vaddr_iomem :
+                                    dst->vaddr,
+                                    src->is_iomem ?
+                                    (void const __force *)src->vaddr_iomem :
+                                    src->vaddr,
+                                    len);
+               return;
+       }
+
+       memcpy_fallback(dst, src, len);
+}
+EXPORT_SYMBOL(drm_memcpy_from_wc);
+
+/*
+ * drm_memcpy_init_early - One time initialization of the WC memcpy code
+ */
+void drm_memcpy_init_early(void)
+{
+       /*
+        * Some hypervisors (e.g. KVM) don't support VEX-prefix instructions
+        * emulation. So don't enable movntdqa in hypervisor guest.
+        */
+       if (static_cpu_has(X86_FEATURE_XMM4_1) &&
+           !boot_cpu_has(X86_FEATURE_HYPERVISOR))
+               static_branch_enable(&has_movntdqa);
+}
+#else
+void drm_memcpy_from_wc(struct dma_buf_map *dst,
+                       const struct dma_buf_map *src,
+                       unsigned long len)
+{
+       WARN_ON(in_interrupt());
+
+       memcpy_fallback(dst, src, len);
+}
+EXPORT_SYMBOL(drm_memcpy_from_wc);
+
+void drm_memcpy_init_early(void)
+{
+}
+#endif /* CONFIG_X86 */

diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
index 3d8d68a..8804ec7 100644 (file)
--- a/drivers/gpu/drm/drm_drv.c
+++ b/drivers/gpu/drm/drm_drv.c
@@ -35,6 +35,7 @@
 #include <linux/slab.h>
 #include <linux/srcu.h>
 
+#include <drm/drm_cache.h>
 #include <drm/drm_client.h>
 #include <drm/drm_color_mgmt.h>
 #include <drm/drm_drv.h>
@@ -1041,6 +1042,7 @@ static int __init drm_core_init(void)
 
        drm_connector_ida_init();
        idr_init(&drm_minors_idr);
+       drm_memcpy_init_early();
 
        ret = drm_sysfs_init();
        if (ret < 0) {

diff --git a/include/drm/drm_cache.h b/include/drm/drm_cache.h
index e9ad486..cc9de16 100644 (file)
--- a/include/drm/drm_cache.h
+++ b/include/drm/drm_cache.h
@@ -35,6 +35,8 @@
 
 #include <linux/scatterlist.h>
 
+struct dma_buf_map;
+
 void drm_clflush_pages(struct page *pages[], unsigned long num_pages);
 void drm_clflush_sg(struct sg_table *st);
 void drm_clflush_virt_range(void *addr, unsigned long length);
@@ -70,4 +72,9 @@ static inline bool drm_arch_can_wc_memory(void)
 #endif
 }
 
+void drm_memcpy_init_early(void);
+
+void drm_memcpy_from_wc(struct dma_buf_map *dst,
+                       const struct dma_buf_map *src,
+                       unsigned long len);
 #endif