drm/amd/amdgpu: Code comments for the amdgpu_ttm.c driver. (v2)

NFC just comments.

(v2):  Updated based on feedback from Alex Deucher.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

1 files changed, 341 insertions, 7 deletions

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
index 69a2b25b3696..e93a0a237dc3 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ttm.c
@@ -63,16 +63,44 @@ static void amdgpu_ttm_debugfs_fini(struct amdgpu_device *adev);
 /*
  * Global memory.
  */
+
+/**
+ * amdgpu_ttm_mem_global_init - Initialize and acquire reference to
+ * memory object
+ *
+ * @ref: Object for initialization.
+ *
+ * This is called by drm_global_item_ref() when an object is being
+ * initialized.
+ */
 static int amdgpu_ttm_mem_global_init(struct drm_global_reference *ref)
 {
         return ttm_mem_global_init(ref->object);
 }
 
+/**
+ * amdgpu_ttm_mem_global_release - Drop reference to a memory object
+ *
+ * @ref: Object being removed
+ *
+ * This is called by drm_global_item_unref() when an object is being
+ * released.
+ */
 static void amdgpu_ttm_mem_global_release(struct drm_global_reference *ref)
 {
         ttm_mem_global_release(ref->object);
 }
 
+/**
+ * amdgpu_ttm_global_init - Initialize global TTM memory reference
+ *                                                      structures.
+ *
+ * @adev:       AMDGPU device for which the global structures need to be
+ *                      registered.
+ *
+ * This is called as part of the AMDGPU ttm init from amdgpu_ttm_init()
+ * during bring up.
+ */
 static int amdgpu_ttm_global_init(struct amdgpu_device *adev)
 {
         struct drm_global_reference *global_ref;
@@ -80,7 +108,9 @@ static int amdgpu_ttm_global_init(struct amdgpu_device *adev)
         struct drm_sched_rq *rq;
         int r;
 
+        /* ensure reference is false in case init fails */
         adev->mman.mem_global_referenced = false;
+
         global_ref = &adev->mman.mem_global_ref;
         global_ref->global_type = DRM_GLOBAL_TTM_MEM;
         global_ref->size = sizeof(struct ttm_mem_global);
@@ -146,6 +176,18 @@ static int amdgpu_invalidate_caches(struct ttm_bo_device *bdev, uint32_t flags)
         return 0;
 }
 
+/**
+ * amdgpu_init_mem_type -       Initialize a memory manager for a specific
+ *                                                      type of memory request.
+ *
+ * @bdev:       The TTM BO device object (contains a reference to
+ *                      amdgpu_device)
+ * @type:       The type of memory requested
+ * @man:
+ *
+ * This is called by ttm_bo_init_mm() when a buffer object is being
+ * initialized.
+ */
 static int amdgpu_init_mem_type(struct ttm_bo_device *bdev, uint32_t type,
                                 struct ttm_mem_type_manager *man)
 {
@@ -161,6 +203,7 @@ static int amdgpu_init_mem_type(struct ttm_bo_device *bdev, uint32_t type,
                 man->default_caching = TTM_PL_FLAG_CACHED;
                 break;
         case TTM_PL_TT:
+                /* GTT memory  */
                 man->func = &amdgpu_gtt_mgr_func;
                 man->gpu_offset = adev->gmc.gart_start;
                 man->available_caching = TTM_PL_MASK_CACHING;
@@ -193,6 +236,14 @@ static int amdgpu_init_mem_type(struct ttm_bo_device *bdev, uint32_t type,
         return 0;
 }
 
+/**
+ * amdgpu_evict_flags - Compute placement flags
+ *
+ * @bo: The buffer object to evict
+ * @placement: Possible destination(s) for evicted BO
+ *
+ * Fill in placement data when ttm_bo_evict() is called
+ */
 static void amdgpu_evict_flags(struct ttm_buffer_object *bo,
                                 struct ttm_placement *placement)
 {
@@ -204,12 +255,14 @@ static void amdgpu_evict_flags(struct ttm_buffer_object *bo,
                 .flags = TTM_PL_MASK_CACHING | TTM_PL_FLAG_SYSTEM
         };
 
+        /* Don't handle scatter gather BOs */
         if (bo->type == ttm_bo_type_sg) {
                 placement->num_placement = 0;
                 placement->num_busy_placement = 0;
                 return;
         }
 
+        /* Object isn't an AMDGPU object so ignore */
         if (!amdgpu_ttm_bo_is_amdgpu_bo(bo)) {
                 placement->placement = &placements;
                 placement->busy_placement = &placements;
@@ -217,10 +270,12 @@ static void amdgpu_evict_flags(struct ttm_buffer_object *bo,
                 placement->num_busy_placement = 1;
                 return;
         }
+
         abo = ttm_to_amdgpu_bo(bo);
         switch (bo->mem.mem_type) {
         case TTM_PL_VRAM:
                 if (!adev->mman.buffer_funcs_enabled) {
+                        /* Move to system memory */
                         amdgpu_ttm_placement_from_domain(abo, AMDGPU_GEM_DOMAIN_CPU);
                 } else if (adev->gmc.visible_vram_size < adev->gmc.real_vram_size &&
                            !(abo->flags & AMDGPU_GEM_CREATE_CPU_ACCESS_REQUIRED) &&
@@ -238,6 +293,7 @@ static void amdgpu_evict_flags(struct ttm_buffer_object *bo,
                         abo->placement.busy_placement = &abo->placements[1];
                         abo->placement.num_busy_placement = 1;
                 } else {
+                        /* Move to GTT memory */
                         amdgpu_ttm_placement_from_domain(abo, AMDGPU_GEM_DOMAIN_GTT);
                 }
                 break;
@@ -248,6 +304,15 @@ static void amdgpu_evict_flags(struct ttm_buffer_object *bo,
         *placement = abo->placement;
 }
 
+/**
+ * amdgpu_verify_access - Verify access for a mmap call
+ *
+ * @bo:         The buffer object to map
+ * @filp:       The file pointer from the process performing the mmap
+ *
+ * This is called by ttm_bo_mmap() to verify whether a process
+ * has the right to mmap a BO to their process space.
+ */
 static int amdgpu_verify_access(struct ttm_buffer_object *bo, struct file *filp)
 {
         struct amdgpu_bo *abo = ttm_to_amdgpu_bo(bo);
@@ -265,6 +330,15 @@ static int amdgpu_verify_access(struct ttm_buffer_object *bo, struct file *filp)
                                           filp->private_data);
 }
 
+/**
+ * amdgpu_move_null - Register memory for a buffer object
+ *
+ * @bo:                 The bo to assign the memory to
+ * @new_mem:    The memory to be assigned.
+ *
+ * Assign the memory from new_mem to the memory of the buffer object
+ * bo.
+ */
 static void amdgpu_move_null(struct ttm_buffer_object *bo,
                              struct ttm_mem_reg *new_mem)
 {
@@ -275,6 +349,10 @@ static void amdgpu_move_null(struct ttm_buffer_object *bo,
         new_mem->mm_node = NULL;
 }
 
+/**
+ * amdgpu_mm_node_addr -        Compute the GPU relative offset of a GTT
+ *                                                      buffer.
+ */
 static uint64_t amdgpu_mm_node_addr(struct ttm_buffer_object *bo,
                                     struct drm_mm_node *mm_node,
                                     struct ttm_mem_reg *mem)
@@ -289,9 +367,10 @@ static uint64_t amdgpu_mm_node_addr(struct ttm_buffer_object *bo,
 }
 
 /**
- * amdgpu_find_mm_node - Helper function finds the drm_mm_node
- *  corresponding to @offset. It also modifies the offset to be
- *  within the drm_mm_node returned
+ * amdgpu_find_mm_node -        Helper function finds the drm_mm_node
+ *                                              corresponding to @offset. It also modifies
+ *                                                      the offset to be within the drm_mm_node
+ *                                                      returned
  */
 static struct drm_mm_node *amdgpu_find_mm_node(struct ttm_mem_reg *mem,
                                                unsigned long *offset)
@@ -430,7 +509,12 @@ error:
         return r;
 }
 
-
+/**
+ * amdgpu_move_blit - Copy an entire buffer to another buffer
+ *
+ * This is a helper called by amdgpu_bo_move() and
+ * amdgpu_move_vram_ram() to help move buffers to and from VRAM.
+ */
 static int amdgpu_move_blit(struct ttm_buffer_object *bo,
                             bool evict, bool no_wait_gpu,
                             struct ttm_mem_reg *new_mem,
@@ -465,6 +549,11 @@ error:
         return r;
 }
 
+/**
+ * amdgpu_move_vram_ram - Copy VRAM buffer to RAM buffer
+ *
+ * Called by amdgpu_bo_move().
+ */
 static int amdgpu_move_vram_ram(struct ttm_buffer_object *bo, bool evict,
                                 struct ttm_operation_ctx *ctx,
                                 struct ttm_mem_reg *new_mem)
@@ -477,6 +566,8 @@ static int amdgpu_move_vram_ram(struct ttm_buffer_object *bo, bool evict,
         int r;
 
         adev = amdgpu_ttm_adev(bo->bdev);
+
+        /* create space/pages for new_mem in GTT space */
         tmp_mem = *new_mem;
         tmp_mem.mm_node = NULL;
         placement.num_placement = 1;
@@ -491,25 +582,36 @@ static int amdgpu_move_vram_ram(struct ttm_buffer_object *bo, bool evict,
                 return r;
         }
 
+        /* set caching flags */
         r = ttm_tt_set_placement_caching(bo->ttm, tmp_mem.placement);
         if (unlikely(r)) {
                 goto out_cleanup;
         }
 
+        /* Bind the memory to the GTT space */
         r = ttm_tt_bind(bo->ttm, &tmp_mem, ctx);
         if (unlikely(r)) {
                 goto out_cleanup;
         }
+
+        /* blit VRAM to GTT */
         r = amdgpu_move_blit(bo, true, ctx->no_wait_gpu, &tmp_mem, old_mem);
         if (unlikely(r)) {
                 goto out_cleanup;
         }
+
+        /* move BO (in tmp_mem) to new_mem */
         r = ttm_bo_move_ttm(bo, ctx, new_mem);
 out_cleanup:
         ttm_bo_mem_put(bo, &tmp_mem);
         return r;
 }
 
+/**
+ * amdgpu_move_ram_vram - Copy buffer from RAM to VRAM
+ *
+ * Called by amdgpu_bo_move().
+ */
 static int amdgpu_move_ram_vram(struct ttm_buffer_object *bo, bool evict,
                                 struct ttm_operation_ctx *ctx,
                                 struct ttm_mem_reg *new_mem)
@@ -522,6 +624,8 @@ static int amdgpu_move_ram_vram(struct ttm_buffer_object *bo, bool evict,
         int r;
 
         adev = amdgpu_ttm_adev(bo->bdev);
+
+        /* make space in GTT for old_mem buffer */
         tmp_mem = *new_mem;
         tmp_mem.mm_node = NULL;
         placement.num_placement = 1;
@@ -535,10 +639,14 @@ static int amdgpu_move_ram_vram(struct ttm_buffer_object *bo, bool evict,
         if (unlikely(r)) {
                 return r;
         }
+
+        /* move/bind old memory to GTT space */
         r = ttm_bo_move_ttm(bo, ctx, &tmp_mem);
         if (unlikely(r)) {
                 goto out_cleanup;
         }
+
+        /* copy to VRAM */
         r = amdgpu_move_blit(bo, true, ctx->no_wait_gpu, new_mem, old_mem);
         if (unlikely(r)) {
                 goto out_cleanup;
@@ -548,6 +656,11 @@ out_cleanup:
         return r;
 }
 
+/**
+ * amdgpu_bo_move - Move a buffer object to a new memory location
+ *
+ * Called by ttm_bo_handle_move_mem()
+ */
 static int amdgpu_bo_move(struct ttm_buffer_object *bo, bool evict,
                           struct ttm_operation_ctx *ctx,
                           struct ttm_mem_reg *new_mem)
@@ -613,6 +726,11 @@ memcpy:
         return 0;
 }
 
+/**
+ * amdgpu_ttm_io_mem_reserve - Reserve a block of memory during a fault
+ *
+ * Called by ttm_mem_io_reserve() ultimately via ttm_bo_vm_fault()
+ */
 static int amdgpu_ttm_io_mem_reserve(struct ttm_bo_device *bdev, struct ttm_mem_reg *mem)
 {
         struct ttm_mem_type_manager *man = &bdev->man[mem->mem_type];
@@ -690,6 +808,14 @@ struct amdgpu_ttm_tt {
         uint32_t                last_set_pages;
 };
 
+/**
+ * amdgpu_ttm_tt_get_user_pages -       Pin pages of memory pointed to
+ *                                                                      by a USERPTR pointer to memory
+ *
+ * Called by amdgpu_gem_userptr_ioctl() and amdgpu_cs_parser_bos().
+ * This provides a wrapper around the get_user_pages() call to provide
+ * device accessible pages that back user memory.
+ */
 int amdgpu_ttm_tt_get_user_pages(struct ttm_tt *ttm, struct page **pages)
 {
         struct amdgpu_ttm_tt *gtt = (void *)ttm;
@@ -719,6 +845,7 @@ int amdgpu_ttm_tt_get_user_pages(struct ttm_tt *ttm, struct page **pages)
                 }
         }
 
+        /* loop enough times using contiguous pages of memory */
         do {
                 unsigned num_pages = ttm->num_pages - pinned;
                 uint64_t userptr = gtt->userptr + pinned * PAGE_SIZE;
@@ -757,6 +884,14 @@ release_pages:
         return r;
 }
 
+/**
+ * amdgpu_ttm_tt_set_user_pages -       Copy pages in, putting old pages
+ *                                                                      as necessary.
+ *
+ * Called by amdgpu_cs_list_validate().  This creates the page list
+ * that backs user memory and will ultimately be mapped into the device
+ * address space.
+ */
 void amdgpu_ttm_tt_set_user_pages(struct ttm_tt *ttm, struct page **pages)
 {
         struct amdgpu_ttm_tt *gtt = (void *)ttm;
@@ -771,6 +906,11 @@ void amdgpu_ttm_tt_set_user_pages(struct ttm_tt *ttm, struct page **pages)
         }
 }
 
+/**
+ * amdgpu_ttm_tt_mark_user_page - Mark pages as dirty
+ *
+ * Called while unpinning userptr pages
+ */
 void amdgpu_ttm_tt_mark_user_pages(struct ttm_tt *ttm)
 {
         struct amdgpu_ttm_tt *gtt = (void *)ttm;
@@ -789,7 +929,12 @@ void amdgpu_ttm_tt_mark_user_pages(struct ttm_tt *ttm)
         }
 }
 
-/* prepare the sg table with the user pages */
+/**
+ * amdgpu_ttm_tt_pin_userptr -  prepare the sg table with the
+ *                                                              user pages
+ *
+ * Called by amdgpu_ttm_backend_bind()
+ **/
 static int amdgpu_ttm_tt_pin_userptr(struct ttm_tt *ttm)
 {
         struct amdgpu_device *adev = amdgpu_ttm_adev(ttm->bdev);
@@ -801,17 +946,20 @@ static int amdgpu_ttm_tt_pin_userptr(struct ttm_tt *ttm)
         enum dma_data_direction direction = write ?
                 DMA_BIDIRECTIONAL : DMA_TO_DEVICE;
 
+        /* Allocate an SG array and squash pages into it */
         r = sg_alloc_table_from_pages(ttm->sg, ttm->pages, ttm->num_pages, 0,
                                       ttm->num_pages << PAGE_SHIFT,
                                       GFP_KERNEL);
         if (r)
                 goto release_sg;
 
+        /* Map SG to device */
         r = -ENOMEM;
         nents = dma_map_sg(adev->dev, ttm->sg->sgl, ttm->sg->nents, direction);
         if (nents != ttm->sg->nents)
                 goto release_sg;
 
+        /* convert SG to linear array of pages and dma addresses */
         drm_prime_sg_to_page_addr_arrays(ttm->sg, ttm->pages,
                                          gtt->ttm.dma_address, ttm->num_pages);
 
@@ -822,6 +970,9 @@ release_sg:
         return r;
 }
 
+/**
+ * amdgpu_ttm_tt_unpin_userptr - Unpin and unmap userptr pages
+ */
 static void amdgpu_ttm_tt_unpin_userptr(struct ttm_tt *ttm)
 {
         struct amdgpu_device *adev = amdgpu_ttm_adev(ttm->bdev);
@@ -835,9 +986,10 @@ static void amdgpu_ttm_tt_unpin_userptr(struct ttm_tt *ttm)
         if (!ttm->sg->sgl)
                 return;
 
-        /* free the sg table and pages again */
+        /* unmap the pages mapped to the device */
         dma_unmap_sg(adev->dev, ttm->sg->sgl, ttm->sg->nents, direction);
 
+        /* mark the pages as dirty */
         amdgpu_ttm_tt_mark_user_pages(ttm);
 
         sg_free_table(ttm->sg);
@@ -882,6 +1034,12 @@ gart_bind_fail:
         return r;
 }
 
+/**
+ * amdgpu_ttm_backend_bind - Bind GTT memory
+ *
+ * Called by ttm_tt_bind() on behalf of ttm_bo_handle_move_mem().
+ * This handles binding GTT memory to the device address space.
+ */
 static int amdgpu_ttm_backend_bind(struct ttm_tt *ttm,
                                    struct ttm_mem_reg *bo_mem)
 {
@@ -912,7 +1070,10 @@ static int amdgpu_ttm_backend_bind(struct ttm_tt *ttm,
                 return 0;
         }
 
+        /* compute PTE flags relevant to this BO memory */
         flags = amdgpu_ttm_tt_pte_flags(adev, ttm, bo_mem);
+
+        /* bind pages into GART page tables */
         gtt->offset = (u64)bo_mem->start << PAGE_SHIFT;
         r = amdgpu_gart_bind(adev, gtt->offset, ttm->num_pages,
                 ttm->pages, gtt->ttm.dma_address, flags);
@@ -923,6 +1084,9 @@ static int amdgpu_ttm_backend_bind(struct ttm_tt *ttm,
         return r;
 }
 
+/**
+ * amdgpu_ttm_alloc_gart - Allocate GART memory for buffer object
+ */
 int amdgpu_ttm_alloc_gart(struct ttm_buffer_object *bo)
 {
         struct amdgpu_device *adev = amdgpu_ttm_adev(bo->bdev);
@@ -938,6 +1102,7 @@ int amdgpu_ttm_alloc_gart(struct ttm_buffer_object *bo)
             amdgpu_gtt_mgr_has_gart_addr(&bo->mem))
                 return 0;
 
+        /* allocate GTT space */
         tmp = bo->mem;
         tmp.mm_node = NULL;
         placement.num_placement = 1;
@@ -953,7 +1118,10 @@ int amdgpu_ttm_alloc_gart(struct ttm_buffer_object *bo)
         if (unlikely(r))
                 return r;
 
+        /* compute PTE flags for this buffer object */
         flags = amdgpu_ttm_tt_pte_flags(adev, bo->ttm, &tmp);
+
+        /* Bind pages */
         gtt->offset = (u64)tmp.start << PAGE_SHIFT;
         r = amdgpu_ttm_gart_bind(adev, bo, flags);
         if (unlikely(r)) {
@@ -969,6 +1137,12 @@ int amdgpu_ttm_alloc_gart(struct ttm_buffer_object *bo)
         return 0;
 }
 
+/**
+ * amdgpu_ttm_recover_gart - Rebind GTT pages
+ *
+ * Called by amdgpu_gtt_mgr_recover() from amdgpu_device_reset() to
+ * rebind GTT pages during a GPU reset.
+ */
 int amdgpu_ttm_recover_gart(struct ttm_buffer_object *tbo)
 {
         struct amdgpu_device *adev = amdgpu_ttm_adev(tbo->bdev);
@@ -984,12 +1158,19 @@ int amdgpu_ttm_recover_gart(struct ttm_buffer_object *tbo)
         return r;
 }
 
+/**
+ * amdgpu_ttm_backend_unbind - Unbind GTT mapped pages
+ *
+ * Called by ttm_tt_unbind() on behalf of ttm_bo_move_ttm() and
+ * ttm_tt_destroy().
+ */
 static int amdgpu_ttm_backend_unbind(struct ttm_tt *ttm)
 {
         struct amdgpu_device *adev = amdgpu_ttm_adev(ttm->bdev);
         struct amdgpu_ttm_tt *gtt = (void *)ttm;
         int r;
 
+        /* if the pages have userptr pinning then clear that first */
         if (gtt->userptr)
                 amdgpu_ttm_tt_unpin_userptr(ttm);
 
@@ -1021,6 +1202,13 @@ static struct ttm_backend_func amdgpu_backend_func = {
         .destroy = &amdgpu_ttm_backend_destroy,
 };
 
+/**
+ * amdgpu_ttm_tt_create - Create a ttm_tt object for a given BO
+ *
+ * @bo: The buffer object to create a GTT ttm_tt object around
+ *
+ * Called by ttm_tt_create().
+ */
 static struct ttm_tt *amdgpu_ttm_tt_create(struct ttm_buffer_object *bo,
                                            uint32_t page_flags)
 {
@@ -1034,6 +1222,8 @@ static struct ttm_tt *amdgpu_ttm_tt_create(struct ttm_buffer_object *bo,
                 return NULL;
         }
         gtt->ttm.ttm.func = &amdgpu_backend_func;
+
+        /* allocate space for the uninitialized page entries */
         if (ttm_sg_tt_init(&gtt->ttm, bo, page_flags)) {
                 kfree(gtt);
                 return NULL;
@@ -1041,6 +1231,12 @@ static struct ttm_tt *amdgpu_ttm_tt_create(struct ttm_buffer_object *bo,
         return &gtt->ttm.ttm;
 }
 
+/**
+ * amdgpu_ttm_tt_populate - Map GTT pages visible to the device
+ *
+ * Map the pages of a ttm_tt object to an address space visible
+ * to the underlying device.
+ */
 static int amdgpu_ttm_tt_populate(struct ttm_tt *ttm,
                         struct ttm_operation_ctx *ctx)
 {
@@ -1048,6 +1244,7 @@ static int amdgpu_ttm_tt_populate(struct ttm_tt *ttm,
         struct amdgpu_ttm_tt *gtt = (void *)ttm;
         bool slave = !!(ttm->page_flags & TTM_PAGE_FLAG_SG);
 
+        /* user pages are bound by amdgpu_ttm_tt_pin_userptr() */
         if (gtt && gtt->userptr) {
                 ttm->sg = kzalloc(sizeof(struct sg_table), GFP_KERNEL);
                 if (!ttm->sg)
@@ -1072,9 +1269,17 @@ static int amdgpu_ttm_tt_populate(struct ttm_tt *ttm,
         }
 #endif
 
+        /* fall back to generic helper to populate the page array
+         * and map them to the device */
         return ttm_populate_and_map_pages(adev->dev, &gtt->ttm, ctx);
 }
 
+/**
+ * amdgpu_ttm_tt_unpopulate - unmap GTT pages and unpopulate page arrays
+ *
+ * Unmaps pages of a ttm_tt object from the device address space and
+ * unpopulates the page array backing it.
+ */
 static void amdgpu_ttm_tt_unpopulate(struct ttm_tt *ttm)
 {
         struct amdgpu_device *adev;
@@ -1100,9 +1305,21 @@ static void amdgpu_ttm_tt_unpopulate(struct ttm_tt *ttm)
         }
 #endif
 
+        /* fall back to generic helper to unmap and unpopulate array */
         ttm_unmap_and_unpopulate_pages(adev->dev, &gtt->ttm);
 }
 
+/**
+ * amdgpu_ttm_tt_set_userptr -  Initialize userptr GTT ttm_tt
+ *                                                              for the current task
+ *
+ * @ttm: The ttm_tt object to bind this userptr object to
+ * @addr:  The address in the current tasks VM space to use
+ * @flags: Requirements of userptr object.
+ *
+ * Called by amdgpu_gem_userptr_ioctl() to bind userptr pages
+ * to current task
+ */
 int amdgpu_ttm_tt_set_userptr(struct ttm_tt *ttm, uint64_t addr,
                               uint32_t flags)
 {
@@ -1127,6 +1344,9 @@ int amdgpu_ttm_tt_set_userptr(struct ttm_tt *ttm, uint64_t addr,
         return 0;
 }
 
+/**
+ * amdgpu_ttm_tt_get_usermm - Return memory manager for ttm_tt object
+ */
 struct mm_struct *amdgpu_ttm_tt_get_usermm(struct ttm_tt *ttm)
 {
         struct amdgpu_ttm_tt *gtt = (void *)ttm;
@@ -1140,6 +1360,12 @@ struct mm_struct *amdgpu_ttm_tt_get_usermm(struct ttm_tt *ttm)
         return gtt->usertask->mm;
 }
 
+/**
+ * amdgpu_ttm_tt_affect_userptr -       Determine if a ttm_tt object lays
+ *                                                                      inside an address range for the
+ *                                                                      current task.
+ *
+ */
 bool amdgpu_ttm_tt_affect_userptr(struct ttm_tt *ttm, unsigned long start,
                                   unsigned long end)
 {
@@ -1150,10 +1376,16 @@ bool amdgpu_ttm_tt_affect_userptr(struct ttm_tt *ttm, unsigned long start,
         if (gtt == NULL || !gtt->userptr)
                 return false;
 
+        /* Return false if no part of the ttm_tt object lies within
+         * the range
+         */
         size = (unsigned long)gtt->ttm.ttm.num_pages * PAGE_SIZE;
         if (gtt->userptr > end || gtt->userptr + size <= start)
                 return false;
 
+        /* Search the lists of tasks that hold this mapping and see
+         * if current is one of them.  If it is return false.
+         */
         spin_lock(&gtt->guptasklock);
         list_for_each_entry(entry, &gtt->guptasks, list) {
                 if (entry->task == current) {
@@ -1168,6 +1400,10 @@ bool amdgpu_ttm_tt_affect_userptr(struct ttm_tt *ttm, unsigned long start,
         return true;
 }
 
+/**
+ * amdgpu_ttm_tt_userptr_invalidated -  Has the ttm_tt object been
+ *                                                                              invalidated?
+ */
 bool amdgpu_ttm_tt_userptr_invalidated(struct ttm_tt *ttm,
                                        int *last_invalidated)
 {
@@ -1178,6 +1414,12 @@ bool amdgpu_ttm_tt_userptr_invalidated(struct ttm_tt *ttm,
         return prev_invalidated != *last_invalidated;
 }
 
+/**
+ * amdgpu_ttm_tt_userptr_needs_pages -  Have the pages backing this
+ *                                                                              ttm_tt object been invalidated
+ *                                                                              since the last time they've
+ *                                                                              been set?
+ */
 bool amdgpu_ttm_tt_userptr_needs_pages(struct ttm_tt *ttm)
 {
         struct amdgpu_ttm_tt *gtt = (void *)ttm;
@@ -1188,6 +1430,9 @@ bool amdgpu_ttm_tt_userptr_needs_pages(struct ttm_tt *ttm)
         return atomic_read(&gtt->mmu_invalidations) != gtt->last_set_pages;
 }
 
+/**
+ * amdgpu_ttm_tt_is_readonly - Is the ttm_tt object read only?
+ */
 bool amdgpu_ttm_tt_is_readonly(struct ttm_tt *ttm)
 {
         struct amdgpu_ttm_tt *gtt = (void *)ttm;
@@ -1198,6 +1443,12 @@ bool amdgpu_ttm_tt_is_readonly(struct ttm_tt *ttm)
         return !!(gtt->userflags & AMDGPU_GEM_USERPTR_READONLY);
 }
 
+/**
+ * amdgpu_ttm_tt_pte_flags - Compute PTE flags for ttm_tt object
+ *
+ * @ttm: The ttm_tt object to compute the flags for
+ * @mem: The memory registry backing this ttm_tt object
+ */
 uint64_t amdgpu_ttm_tt_pte_flags(struct amdgpu_device *adev, struct ttm_tt *ttm,
                                  struct ttm_mem_reg *mem)
 {
@@ -1222,6 +1473,16 @@ uint64_t amdgpu_ttm_tt_pte_flags(struct amdgpu_device *adev, struct ttm_tt *ttm,
         return flags;
 }
 
+/**
+ * amdgpu_ttm_bo_eviction_valuable -    Check to see if we can evict
+ *                                                                              a buffer object.
+ *
+ * Return true if eviction is sensible.  Called by
+ * ttm_mem_evict_first() on behalf of ttm_bo_mem_force_space()
+ * which tries to evict buffer objects until it can find space
+ * for a new object and by ttm_bo_force_list_clean() which is
+ * used to clean out a memory space.
+ */
 static bool amdgpu_ttm_bo_eviction_valuable(struct ttm_buffer_object *bo,
                                             const struct ttm_place *place)
 {
@@ -1268,6 +1529,19 @@ static bool amdgpu_ttm_bo_eviction_valuable(struct ttm_buffer_object *bo,
         return ttm_bo_eviction_valuable(bo, place);
 }
 
+/**
+ * amdgpu_ttm_access_memory -   Read or Write memory that backs a
+ *                                                              buffer object.
+ *
+ * @bo:  The buffer object to read/write
+ * @offset:  Offset into buffer object
+ * @buf:  Secondary buffer to write/read from
+ * @len: Length in bytes of access
+ * @write:  true if writing
+ *
+ * This is used to access VRAM that backs a buffer object via MMIO
+ * access for debugging purposes.
+ */
 static int amdgpu_ttm_access_memory(struct ttm_buffer_object *bo,
                                     unsigned long offset,
                                     void *buf, int len, int write)
@@ -1444,13 +1718,22 @@ error_create:
         adev->fw_vram_usage.reserved_bo = NULL;
         return r;
 }
-
+/**
+ * amdgpu_ttm_init -    Init the memory management (ttm) as well as
+ *                                              various gtt/vram related fields.
+ *
+ * This initializes all of the memory space pools that the TTM layer
+ * will need such as the GTT space (system memory mapped to the device),
+ * VRAM (on-board memory), and on-chip memories (GDS, GWS, OA) which
+ * can be mapped per VMID.
+ */
 int amdgpu_ttm_init(struct amdgpu_device *adev)
 {
         uint64_t gtt_size;
         int r;
         u64 vis_vram_limit;
 
+        /* initialize global references for vram/gtt */
         r = amdgpu_ttm_global_init(adev);
         if (r) {
                 return r;
@@ -1471,6 +1754,7 @@ int amdgpu_ttm_init(struct amdgpu_device *adev)
         /* We opt to avoid OOM on system pages allocations */
         adev->mman.bdev.no_retry = true;
 
+        /* Initialize VRAM pool with all of VRAM divided into pages */
         r = ttm_bo_init_mm(&adev->mman.bdev, TTM_PL_VRAM,
                                 adev->gmc.real_vram_size >> PAGE_SHIFT);
         if (r) {
@@ -1500,6 +1784,10 @@ int amdgpu_ttm_init(struct amdgpu_device *adev)
                 return r;
         }
 
+        /* allocate memory as required for VGA
+         * This is used for VGA emulation and pre-OS scanout buffers to
+         * avoid display artifacts while transitioning between pre-OS
+         * and driver.  */
         if (adev->gmc.stolen_size) {
                 r = amdgpu_bo_create_kernel(adev, adev->gmc.stolen_size, PAGE_SIZE,
                                             AMDGPU_GEM_DOMAIN_VRAM,
@@ -1511,6 +1799,8 @@ int amdgpu_ttm_init(struct amdgpu_device *adev)
         DRM_INFO("amdgpu: %uM of VRAM memory ready\n",
                  (unsigned) (adev->gmc.real_vram_size / (1024 * 1024)));
 
+        /* Compute GTT size, either bsaed on 3/4th the size of RAM size
+         * or whatever the user passed on module init */
         if (amdgpu_gtt_size == -1) {
                 struct sysinfo si;
 
@@ -1521,6 +1811,8 @@ int amdgpu_ttm_init(struct amdgpu_device *adev)
         }
         else
                 gtt_size = (uint64_t)amdgpu_gtt_size << 20;
+
+        /* Initialize GTT memory pool */
         r = ttm_bo_init_mm(&adev->mman.bdev, TTM_PL_TT, gtt_size >> PAGE_SHIFT);
         if (r) {
                 DRM_ERROR("Failed initializing GTT heap.\n");
@@ -1529,6 +1821,7 @@ int amdgpu_ttm_init(struct amdgpu_device *adev)
         DRM_INFO("amdgpu: %uM of GTT memory ready.\n",
                  (unsigned)(gtt_size / (1024 * 1024)));
 
+        /* Initialize various on-chip memory pools */
         adev->gds.mem.total_size = adev->gds.mem.total_size << AMDGPU_GDS_SHIFT;
         adev->gds.mem.gfx_partition_size = adev->gds.mem.gfx_partition_size << AMDGPU_GDS_SHIFT;
         adev->gds.mem.cs_partition_size = adev->gds.mem.cs_partition_size << AMDGPU_GDS_SHIFT;
@@ -1568,6 +1861,7 @@ int amdgpu_ttm_init(struct amdgpu_device *adev)
                 }
         }
 
+        /* Register debugfs entries for amdgpu_ttm */
         r = amdgpu_ttm_debugfs_init(adev);
         if (r) {
                 DRM_ERROR("Failed to init debugfs\n");
@@ -1576,11 +1870,19 @@ int amdgpu_ttm_init(struct amdgpu_device *adev)
         return 0;
 }
 
+/**
+ * amdgpu_ttm_late_init -       Handle any late initialization for
+ *                                                      amdgpu_ttm
+ */
 void amdgpu_ttm_late_init(struct amdgpu_device *adev)
 {
+        /* return the VGA stolen memory (if any) back to VRAM */
         amdgpu_bo_free_kernel(&adev->stolen_vga_memory, NULL, NULL);
 }
 
+/**
+ * amdgpu_ttm_fini - De-initialize the TTM memory pools
+ */
 void amdgpu_ttm_fini(struct amdgpu_device *adev)
 {
         if (!adev->mman.initialized)
@@ -1908,6 +2210,11 @@ static const struct drm_info_list amdgpu_ttm_debugfs_list[] = {
 #endif
 };
 
+/**
+ * amdgpu_ttm_vram_read - Linear read access to VRAM
+ *
+ * Accesses VRAM via MMIO for debugging purposes.
+ */
 static ssize_t amdgpu_ttm_vram_read(struct file *f, char __user *buf,
                                     size_t size, loff_t *pos)
 {
@@ -1947,6 +2254,11 @@ static ssize_t amdgpu_ttm_vram_read(struct file *f, char __user *buf,
         return result;
 }
 
+/**
+ * amdgpu_ttm_vram_write - Linear write access to VRAM
+ *
+ * Accesses VRAM via MMIO for debugging purposes.
+ */
 static ssize_t amdgpu_ttm_vram_write(struct file *f, const char __user *buf,
                                     size_t size, loff_t *pos)
 {
@@ -1995,6 +2307,9 @@ static const struct file_operations amdgpu_ttm_vram_fops = {
 
 #ifdef CONFIG_DRM_AMDGPU_GART_DEBUGFS
 
+/**
+ * amdgpu_ttm_gtt_read - Linear read access to GTT memory
+ */
 static ssize_t amdgpu_ttm_gtt_read(struct file *f, char __user *buf,
                                    size_t size, loff_t *pos)
 {
@@ -2042,6 +2357,13 @@ static const struct file_operations amdgpu_ttm_gtt_fops = {
 
 #endif
 
+/**
+ * amdgpu_iomem_read - Virtual read access to GPU mapped memory
+ *
+ * This function is used to read memory that has been mapped to the
+ * GPU and the known addresses are not physical addresses but instead
+ * bus addresses (e.g., what you'd put in an IB or ring buffer).
+ */
 static ssize_t amdgpu_iomem_read(struct file *f, char __user *buf,
                                  size_t size, loff_t *pos)
 {
@@ -2050,6 +2372,7 @@ static ssize_t amdgpu_iomem_read(struct file *f, char __user *buf,
         ssize_t result = 0;
         int r;
 
+        /* retrieve the IOMMU domain if any for this device */
         dom = iommu_get_domain_for_dev(adev->dev);
 
         while (size) {
@@ -2062,6 +2385,10 @@ static ssize_t amdgpu_iomem_read(struct file *f, char __user *buf,
 
                 bytes = bytes < size ? bytes : size;
 
+                /* Translate the bus address to a physical address.  If
+                 * the domain is NULL it means there is no IOMMU active
+                 * and the address translation is the identity
+                 */
                 addr = dom ? iommu_iova_to_phys(dom, addr) : addr;
 
                 pfn = addr >> PAGE_SHIFT;
@@ -2086,6 +2413,13 @@ static ssize_t amdgpu_iomem_read(struct file *f, char __user *buf,
         return result;
 }
 
+/**
+ * amdgpu_iomem_write - Virtual write access to GPU mapped memory
+ *
+ * This function is used to write memory that has been mapped to the
+ * GPU and the known addresses are not physical addresses but instead
+ * bus addresses (e.g., what you'd put in an IB or ring buffer).
+ */
 static ssize_t amdgpu_iomem_write(struct file *f, const char __user *buf,
                                  size_t size, loff_t *pos)
 {