drivers/gpu/drm/ttm/ttm_bo_util.c

   1 /* SPDX-License-Identifier: GPL-2.0 OR MIT */
   2 /**************************************************************************
   3  *
   4  * Copyright (c) 2007-2009 VMware, Inc., Palo Alto, CA., USA
   5  * All Rights Reserved.
   6  *
   7  * Permission is hereby granted, free of charge, to any person obtaining a
   8  * copy of this software and associated documentation files (the
   9  * "Software"), to deal in the Software without restriction, including
  10  * without limitation the rights to use, copy, modify, merge, publish,
  11  * distribute, sub license, and/or sell copies of the Software, and to
  12  * permit persons to whom the Software is furnished to do so, subject to
  13  * the following conditions:
  14  *
  15  * The above copyright notice and this permission notice (including the
  16  * next paragraph) shall be included in all copies or substantial portions
  17  * of the Software.
  18  *
  19  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  20  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  21  * FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT. IN NO EVENT SHALL
  22  * THE COPYRIGHT HOLDERS, AUTHORS AND/OR ITS SUPPLIERS BE LIABLE FOR ANY CLAIM,
  23  * DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
  24  * OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
  25  * USE OR OTHER DEALINGS IN THE SOFTWARE.
  26  *
  27  **************************************************************************/
  28 /*
  29  * Authors: Thomas Hellstrom <thellstrom-at-vmware-dot-com>
  30  */
  31
  32 #include <drm/ttm/ttm_bo_driver.h>
  33 #include <drm/ttm/ttm_placement.h>
  34 #include <drm/drm_vma_manager.h>
  35 #include <linux/dma-buf-map.h>
  36 #include <linux/io.h>
  37 #include <linux/highmem.h>
  38 #include <linux/wait.h>
  39 #include <linux/slab.h>
  40 #include <linux/vmalloc.h>
  41 #include <linux/module.h>
  42 #include <linux/dma-resv.h>
  43
  44 struct ttm_transfer_obj {
  45         struct ttm_buffer_object base;
  46         struct ttm_buffer_object *bo;
  47 };
  48
  49 int ttm_mem_io_reserve(struct ttm_bo_device *bdev,
  50                        struct ttm_resource *mem)
  51 {
  52         if (mem->bus.offset || mem->bus.addr)
  53                 return 0;
  54
  55         mem->bus.is_iomem = false;
  56         if (!bdev->driver->io_mem_reserve)
  57                 return 0;
  58
  59         return bdev->driver->io_mem_reserve(bdev, mem);
  60 }
  61
  62 void ttm_mem_io_free(struct ttm_bo_device *bdev,
  63                      struct ttm_resource *mem)
  64 {
  65         if (!mem->bus.offset && !mem->bus.addr)
  66                 return;
  67
  68         if (bdev->driver->io_mem_free)
  69                 bdev->driver->io_mem_free(bdev, mem);
  70
  71         mem->bus.offset = 0;
  72         mem->bus.addr = NULL;
  73 }
  74
  75 static int ttm_resource_ioremap(struct ttm_bo_device *bdev,
  76                                struct ttm_resource *mem,
  77                                void **virtual)
  78 {
  79         int ret;
  80         void *addr;
  81
  82         *virtual = NULL;
  83         ret = ttm_mem_io_reserve(bdev, mem);
  84         if (ret || !mem->bus.is_iomem)
  85                 return ret;
  86
  87         if (mem->bus.addr) {
  88                 addr = mem->bus.addr;
  89         } else {
  90                 size_t bus_size = (size_t)mem->num_pages << PAGE_SHIFT;
  91
  92                 if (mem->bus.caching == ttm_write_combined)
  93                         addr = ioremap_wc(mem->bus.offset, bus_size);
  94                 else
  95                         addr = ioremap(mem->bus.offset, bus_size);
  96                 if (!addr) {
  97                         ttm_mem_io_free(bdev, mem);
  98                         return -ENOMEM;
  99                 }
 100         }
 101         *virtual = addr;
 102         return 0;
 103 }
 104
 105 static void ttm_resource_iounmap(struct ttm_bo_device *bdev,
 106                                 struct ttm_resource *mem,
 107                                 void *virtual)
 108 {
 109         if (virtual && mem->bus.addr == NULL)
 110                 iounmap(virtual);
 111         ttm_mem_io_free(bdev, mem);
 112 }
 113
 114 static int ttm_copy_io_page(void *dst, void *src, unsigned long page)
 115 {
 116         uint32_t *dstP =
 117             (uint32_t *) ((unsigned long)dst + (page << PAGE_SHIFT));
 118         uint32_t *srcP =
 119             (uint32_t *) ((unsigned long)src + (page << PAGE_SHIFT));
 120
 121         int i;
 122         for (i = 0; i < PAGE_SIZE / sizeof(uint32_t); ++i)
 123                 iowrite32(ioread32(srcP++), dstP++);
 124         return 0;
 125 }
 126
 127 static int ttm_copy_io_ttm_page(struct ttm_tt *ttm, void *src,
 128                                 unsigned long page,
 129                                 pgprot_t prot)
 130 {
 131         struct page *d = ttm->pages[page];
 132         void *dst;
 133
 134         if (!d)
 135                 return -ENOMEM;
 136
 137         src = (void *)((unsigned long)src + (page << PAGE_SHIFT));
 138         dst = kmap_atomic_prot(d, prot);
 139         if (!dst)
 140                 return -ENOMEM;
 141
 142         memcpy_fromio(dst, src, PAGE_SIZE);
 143
 144         kunmap_atomic(dst);
 145
 146         return 0;
 147 }
 148
 149 static int ttm_copy_ttm_io_page(struct ttm_tt *ttm, void *dst,
 150                                 unsigned long page,
 151                                 pgprot_t prot)
 152 {
 153         struct page *s = ttm->pages[page];
 154         void *src;
 155
 156         if (!s)
 157                 return -ENOMEM;
 158
 159         dst = (void *)((unsigned long)dst + (page << PAGE_SHIFT));
 160         src = kmap_atomic_prot(s, prot);
 161         if (!src)
 162                 return -ENOMEM;
 163
 164         memcpy_toio(dst, src, PAGE_SIZE);
 165
 166         kunmap_atomic(src);
 167
 168         return 0;
 169 }
 170
 171 int ttm_bo_move_memcpy(struct ttm_buffer_object *bo,
 172                        struct ttm_operation_ctx *ctx,
 173                        struct ttm_resource *new_mem)
 174 {
 175         struct ttm_bo_device *bdev = bo->bdev;
 176         struct ttm_resource_manager *man = ttm_manager_type(bdev, new_mem->mem_type);
 177         struct ttm_tt *ttm = bo->ttm;
 178         struct ttm_resource *old_mem = &bo->mem;
 179         struct ttm_resource old_copy = *old_mem;
 180         void *old_iomap;
 181         void *new_iomap;
 182         int ret;
 183         unsigned long i;
 184
 185         ret = ttm_bo_wait_ctx(bo, ctx);
 186         if (ret)
 187                 return ret;
 188
 189         ret = ttm_resource_ioremap(bdev, old_mem, &old_iomap);
 190         if (ret)
 191                 return ret;
 192         ret = ttm_resource_ioremap(bdev, new_mem, &new_iomap);
 193         if (ret)
 194                 goto out;
 195
 196         /*
 197          * Single TTM move. NOP.
 198          */
 199         if (old_iomap == NULL && new_iomap == NULL)
 200                 goto out2;
 201
 202         /*
 203          * Don't move nonexistent data. Clear destination instead.
 204          */
 205         if (old_iomap == NULL &&
 206             (ttm == NULL || (!ttm_tt_is_populated(ttm) &&
 207                              !(ttm->page_flags & TTM_PAGE_FLAG_SWAPPED)))) {
 208                 memset_io(new_iomap, 0, new_mem->num_pages*PAGE_SIZE);
 209                 goto out2;
 210         }
 211
 212         /*
 213          * TTM might be null for moves within the same region.
 214          */
 215         if (ttm) {
 216                 ret = ttm_tt_populate(bdev, ttm, ctx);
 217                 if (ret)
 218                         goto out1;
 219         }
 220
 221         for (i = 0; i < new_mem->num_pages; ++i) {
 222                 if (old_iomap == NULL) {
 223                         pgprot_t prot = ttm_io_prot(bo, old_mem, PAGE_KERNEL);
 224                         ret = ttm_copy_ttm_io_page(ttm, new_iomap, i,
 225                                                    prot);
 226                 } else if (new_iomap == NULL) {
 227                         pgprot_t prot = ttm_io_prot(bo, new_mem, PAGE_KERNEL);
 228                         ret = ttm_copy_io_ttm_page(ttm, old_iomap, i,
 229                                                    prot);
 230                 } else {
 231                         ret = ttm_copy_io_page(new_iomap, old_iomap, i);
 232                 }
 233                 if (ret)
 234                         goto out1;
 235         }
 236         mb();
 237 out2:
 238         old_copy = *old_mem;
 239
 240         ttm_bo_assign_mem(bo, new_mem);
 241
 242         if (!man->use_tt)
 243                 ttm_bo_tt_destroy(bo);
 244
 245 out1:
 246         ttm_resource_iounmap(bdev, old_mem, new_iomap);
 247 out:
 248         ttm_resource_iounmap(bdev, &old_copy, old_iomap);
 249
 250         /*
 251          * On error, keep the mm node!
 252          */
 253         if (!ret)
 254                 ttm_resource_free(bo, &old_copy);
 255         return ret;
 256 }
 257 EXPORT_SYMBOL(ttm_bo_move_memcpy);
 258
 259 static void ttm_transfered_destroy(struct ttm_buffer_object *bo)
 260 {
 261         struct ttm_transfer_obj *fbo;
 262
 263         fbo = container_of(bo, struct ttm_transfer_obj, base);
 264         ttm_bo_put(fbo->bo);
 265         kfree(fbo);
 266 }
 267
 268 /**
 269  * ttm_buffer_object_transfer
 270  *
 271  * @bo: A pointer to a struct ttm_buffer_object.
 272  * @new_obj: A pointer to a pointer to a newly created ttm_buffer_object,
 273  * holding the data of @bo with the old placement.
 274  *
 275  * This is a utility function that may be called after an accelerated move
 276  * has been scheduled. A new buffer object is created as a placeholder for
 277  * the old data while it's being copied. When that buffer object is idle,
 278  * it can be destroyed, releasing the space of the old placement.
 279  * Returns:
 280  * !0: Failure.
 281  */
 282
 283 static int ttm_buffer_object_transfer(struct ttm_buffer_object *bo,
 284                                       struct ttm_buffer_object **new_obj)
 285 {
 286         struct ttm_transfer_obj *fbo;
 287         int ret;
 288
 289         fbo = kmalloc(sizeof(*fbo), GFP_KERNEL);
 290         if (!fbo)
 291                 return -ENOMEM;
 292
 293         fbo->base = *bo;
 294
 295         ttm_bo_get(bo);
 296         fbo->bo = bo;
 297
 298         /**
 299          * Fix up members that we shouldn't copy directly:
 300          * TODO: Explicit member copy would probably be better here.
 301          */
 302
 303         atomic_inc(&ttm_bo_glob.bo_count);
 304         INIT_LIST_HEAD(&fbo->base.ddestroy);
 305         INIT_LIST_HEAD(&fbo->base.lru);
 306         INIT_LIST_HEAD(&fbo->base.swap);
 307         fbo->base.moving = NULL;
 308         drm_vma_node_reset(&fbo->base.base.vma_node);
 309
 310         kref_init(&fbo->base.kref);
 311         fbo->base.destroy = &ttm_transfered_destroy;
 312         fbo->base.acc_size = 0;
 313         fbo->base.pin_count = 1;
 314         if (bo->type != ttm_bo_type_sg)
 315                 fbo->base.base.resv = &fbo->base.base._resv;
 316
 317         dma_resv_init(&fbo->base.base._resv);
 318         fbo->base.base.dev = NULL;
 319         ret = dma_resv_trylock(&fbo->base.base._resv);
 320         WARN_ON(!ret);
 321
 322         *new_obj = &fbo->base;
 323         return 0;
 324 }
 325
 326 pgprot_t ttm_io_prot(struct ttm_buffer_object *bo, struct ttm_resource *res,
 327                      pgprot_t tmp)
 328 {
 329         struct ttm_resource_manager *man;
 330         enum ttm_caching caching;
 331
 332         man = ttm_manager_type(bo->bdev, res->mem_type);
 333         caching = man->use_tt ? bo->ttm->caching : res->bus.caching;
 334
 335         /* Cached mappings need no adjustment */
 336         if (caching == ttm_cached)
 337                 return tmp;
 338
 339 #if defined(__i386__) || defined(__x86_64__)
 340         if (caching == ttm_write_combined)
 341                 tmp = pgprot_writecombine(tmp);
 342         else if (boot_cpu_data.x86 > 3)
 343                 tmp = pgprot_noncached(tmp);
 344 #endif
 345 #if defined(__ia64__) || defined(__arm__) || defined(__aarch64__) || \
 346     defined(__powerpc__) || defined(__mips__)
 347         if (caching == ttm_write_combined)
 348                 tmp = pgprot_writecombine(tmp);
 349         else
 350                 tmp = pgprot_noncached(tmp);
 351 #endif
 352 #if defined(__sparc__)
 353         tmp = pgprot_noncached(tmp);
 354 #endif
 355         return tmp;
 356 }
 357 EXPORT_SYMBOL(ttm_io_prot);
 358
 359 static int ttm_bo_ioremap(struct ttm_buffer_object *bo,
 360                           unsigned long offset,
 361                           unsigned long size,
 362                           struct ttm_bo_kmap_obj *map)
 363 {
 364         struct ttm_resource *mem = &bo->mem;
 365
 366         if (bo->mem.bus.addr) {
 367                 map->bo_kmap_type = ttm_bo_map_premapped;
 368                 map->virtual = (void *)(((u8 *)bo->mem.bus.addr) + offset);
 369         } else {
 370                 map->bo_kmap_type = ttm_bo_map_iomap;
 371                 if (mem->bus.caching == ttm_write_combined)
 372                         map->virtual = ioremap_wc(bo->mem.bus.offset + offset,
 373                                                   size);
 374                 else
 375                         map->virtual = ioremap(bo->mem.bus.offset + offset,
 376                                                size);
 377         }
 378         return (!map->virtual) ? -ENOMEM : 0;
 379 }
 380
 381 static int ttm_bo_kmap_ttm(struct ttm_buffer_object *bo,
 382                            unsigned long start_page,
 383                            unsigned long num_pages,
 384                            struct ttm_bo_kmap_obj *map)
 385 {
 386         struct ttm_resource *mem = &bo->mem;
 387         struct ttm_operation_ctx ctx = {
 388                 .interruptible = false,
 389                 .no_wait_gpu = false
 390         };
 391         struct ttm_tt *ttm = bo->ttm;
 392         pgprot_t prot;
 393         int ret;
 394
 395         BUG_ON(!ttm);
 396
 397         ret = ttm_tt_populate(bo->bdev, ttm, &ctx);
 398         if (ret)
 399                 return ret;
 400
 401         if (num_pages == 1 && ttm->caching == ttm_cached) {
 402                 /*
 403                  * We're mapping a single page, and the desired
 404                  * page protection is consistent with the bo.
 405                  */
 406
 407                 map->bo_kmap_type = ttm_bo_map_kmap;
 408                 map->page = ttm->pages[start_page];
 409                 map->virtual = kmap(map->page);
 410         } else {
 411                 /*
 412                  * We need to use vmap to get the desired page protection
 413                  * or to make the buffer object look contiguous.
 414                  */
 415                 prot = ttm_io_prot(bo, mem, PAGE_KERNEL);
 416                 map->bo_kmap_type = ttm_bo_map_vmap;
 417                 map->virtual = vmap(ttm->pages + start_page, num_pages,
 418                                     0, prot);
 419         }
 420         return (!map->virtual) ? -ENOMEM : 0;
 421 }
 422
 423 int ttm_bo_kmap(struct ttm_buffer_object *bo,
 424                 unsigned long start_page, unsigned long num_pages,
 425                 struct ttm_bo_kmap_obj *map)
 426 {
 427         unsigned long offset, size;
 428         int ret;
 429
 430         map->virtual = NULL;
 431         map->bo = bo;
 432         if (num_pages > bo->num_pages)
 433                 return -EINVAL;
 434         if (start_page > bo->num_pages)
 435                 return -EINVAL;
 436
 437         ret = ttm_mem_io_reserve(bo->bdev, &bo->mem);
 438         if (ret)
 439                 return ret;
 440         if (!bo->mem.bus.is_iomem) {
 441                 return ttm_bo_kmap_ttm(bo, start_page, num_pages, map);
 442         } else {
 443                 offset = start_page << PAGE_SHIFT;
 444                 size = num_pages << PAGE_SHIFT;
 445                 return ttm_bo_ioremap(bo, offset, size, map);
 446         }
 447 }
 448 EXPORT_SYMBOL(ttm_bo_kmap);
 449
 450 void ttm_bo_kunmap(struct ttm_bo_kmap_obj *map)
 451 {
 452         if (!map->virtual)
 453                 return;
 454         switch (map->bo_kmap_type) {
 455         case ttm_bo_map_iomap:
 456                 iounmap(map->virtual);
 457                 break;
 458         case ttm_bo_map_vmap:
 459                 vunmap(map->virtual);
 460                 break;
 461         case ttm_bo_map_kmap:
 462                 kunmap(map->page);
 463                 break;
 464         case ttm_bo_map_premapped:
 465                 break;
 466         default:
 467                 BUG();
 468         }
 469         ttm_mem_io_free(map->bo->bdev, &map->bo->mem);
 470         map->virtual = NULL;
 471         map->page = NULL;
 472 }
 473 EXPORT_SYMBOL(ttm_bo_kunmap);
 474
 475 int ttm_bo_vmap(struct ttm_buffer_object *bo, struct dma_buf_map *map)
 476 {
 477         struct ttm_resource *mem = &bo->mem;
 478         int ret;
 479
 480         ret = ttm_mem_io_reserve(bo->bdev, mem);
 481         if (ret)
 482                 return ret;
 483
 484         if (mem->bus.is_iomem) {
 485                 void __iomem *vaddr_iomem;
 486                 size_t size = bo->num_pages << PAGE_SHIFT;
 487
 488                 if (mem->bus.addr)
 489                         vaddr_iomem = (void __iomem *)mem->bus.addr;
 490                 else if (mem->bus.caching == ttm_write_combined)
 491                         vaddr_iomem = ioremap_wc(mem->bus.offset, size);
 492                 else
 493                         vaddr_iomem = ioremap(mem->bus.offset, size);
 494
 495                 if (!vaddr_iomem)
 496                         return -ENOMEM;
 497
 498                 dma_buf_map_set_vaddr_iomem(map, vaddr_iomem);
 499
 500         } else {
 501                 struct ttm_operation_ctx ctx = {
 502                         .interruptible = false,
 503                         .no_wait_gpu = false
 504                 };
 505                 struct ttm_tt *ttm = bo->ttm;
 506                 pgprot_t prot;
 507                 void *vaddr;
 508
 509                 ret = ttm_tt_populate(bo->bdev, ttm, &ctx);
 510                 if (ret)
 511                         return ret;
 512
 513                 /*
 514                  * We need to use vmap to get the desired page protection
 515                  * or to make the buffer object look contiguous.
 516                  */
 517                 prot = ttm_io_prot(bo, mem, PAGE_KERNEL);
 518                 vaddr = vmap(ttm->pages, bo->num_pages, 0, prot);
 519                 if (!vaddr)
 520                         return -ENOMEM;
 521
 522                 dma_buf_map_set_vaddr(map, vaddr);
 523         }
 524
 525         return 0;
 526 }
 527 EXPORT_SYMBOL(ttm_bo_vmap);
 528
 529 void ttm_bo_vunmap(struct ttm_buffer_object *bo, struct dma_buf_map *map)
 530 {
 531         struct ttm_resource *mem = &bo->mem;
 532
 533         if (dma_buf_map_is_null(map))
 534                 return;
 535
 536         if (!map->is_iomem)
 537                 vunmap(map->vaddr);
 538         else if (!mem->bus.addr)
 539                 iounmap(map->vaddr_iomem);
 540         dma_buf_map_clear(map);
 541
 542         ttm_mem_io_free(bo->bdev, &bo->mem);
 543 }
 544 EXPORT_SYMBOL(ttm_bo_vunmap);
 545
 546 static int ttm_bo_wait_free_node(struct ttm_buffer_object *bo,
 547                                  bool dst_use_tt)
 548 {
 549         int ret;
 550         ret = ttm_bo_wait(bo, false, false);
 551         if (ret)
 552                 return ret;
 553
 554         if (!dst_use_tt)
 555                 ttm_bo_tt_destroy(bo);
 556         ttm_resource_free(bo, &bo->mem);
 557         return 0;
 558 }
 559
 560 static int ttm_bo_move_to_ghost(struct ttm_buffer_object *bo,
 561                                 struct dma_fence *fence,
 562                                 bool dst_use_tt)
 563 {
 564         struct ttm_buffer_object *ghost_obj;
 565         int ret;
 566
 567         /**
 568          * This should help pipeline ordinary buffer moves.
 569          *
 570          * Hang old buffer memory on a new buffer object,
 571          * and leave it to be released when the GPU
 572          * operation has completed.
 573          */
 574
 575         dma_fence_put(bo->moving);
 576         bo->moving = dma_fence_get(fence);
 577
 578         ret = ttm_buffer_object_transfer(bo, &ghost_obj);
 579         if (ret)
 580                 return ret;
 581
 582         dma_resv_add_excl_fence(&ghost_obj->base._resv, fence);
 583
 584         /**
 585          * If we're not moving to fixed memory, the TTM object
 586          * needs to stay alive. Otherwhise hang it on the ghost
 587          * bo to be unbound and destroyed.
 588          */
 589
 590         if (dst_use_tt)
 591                 ghost_obj->ttm = NULL;
 592         else
 593                 bo->ttm = NULL;
 594
 595         dma_resv_unlock(&ghost_obj->base._resv);
 596         ttm_bo_put(ghost_obj);
 597         return 0;
 598 }
 599
 600 static void ttm_bo_move_pipeline_evict(struct ttm_buffer_object *bo,
 601                                        struct dma_fence *fence)
 602 {
 603         struct ttm_bo_device *bdev = bo->bdev;
 604         struct ttm_resource_manager *from = ttm_manager_type(bdev, bo->mem.mem_type);
 605
 606         /**
 607          * BO doesn't have a TTM we need to bind/unbind. Just remember
 608          * this eviction and free up the allocation
 609          */
 610         spin_lock(&from->move_lock);
 611         if (!from->move || dma_fence_is_later(fence, from->move)) {
 612                 dma_fence_put(from->move);
 613                 from->move = dma_fence_get(fence);
 614         }
 615         spin_unlock(&from->move_lock);
 616
 617         ttm_resource_free(bo, &bo->mem);
 618
 619         dma_fence_put(bo->moving);
 620         bo->moving = dma_fence_get(fence);
 621 }
 622
 623 int ttm_bo_move_accel_cleanup(struct ttm_buffer_object *bo,
 624                               struct dma_fence *fence,
 625                               bool evict,
 626                               bool pipeline,
 627                               struct ttm_resource *new_mem)
 628 {
 629         struct ttm_bo_device *bdev = bo->bdev;
 630         struct ttm_resource_manager *from = ttm_manager_type(bdev, bo->mem.mem_type);
 631         struct ttm_resource_manager *man = ttm_manager_type(bdev, new_mem->mem_type);
 632         int ret = 0;
 633
 634         dma_resv_add_excl_fence(bo->base.resv, fence);
 635         if (!evict)
 636                 ret = ttm_bo_move_to_ghost(bo, fence, man->use_tt);
 637         else if (!from->use_tt && pipeline)
 638                 ttm_bo_move_pipeline_evict(bo, fence);
 639         else
 640                 ret = ttm_bo_wait_free_node(bo, man->use_tt);
 641
 642         if (ret)
 643                 return ret;
 644
 645         ttm_bo_assign_mem(bo, new_mem);
 646
 647         return 0;
 648 }
 649 EXPORT_SYMBOL(ttm_bo_move_accel_cleanup);
 650
 651 int ttm_bo_pipeline_gutting(struct ttm_buffer_object *bo)
 652 {
 653         struct ttm_buffer_object *ghost;
 654         int ret;
 655
 656         ret = ttm_buffer_object_transfer(bo, &ghost);
 657         if (ret)
 658                 return ret;
 659
 660         ret = dma_resv_copy_fences(&ghost->base._resv, bo->base.resv);
 661         /* Last resort, wait for the BO to be idle when we are OOM */
 662         if (ret)
 663                 ttm_bo_wait(bo, false, false);
 664
 665         memset(&bo->mem, 0, sizeof(bo->mem));
 666         bo->mem.mem_type = TTM_PL_SYSTEM;
 667         bo->ttm = NULL;
 668
 669         dma_resv_unlock(&ghost->base._resv);
 670         ttm_bo_put(ghost);
 671
 672         return 0;
 673 }