drm/i915/gem: Async GPU relocations only

Reduce the 3 relocation paths down to the single path that accommodates
all. The primary motivation for this is to guard the relocations with a
natural fence (derived from the i915_request used to write the
relocation from the GPU).

The tradeoff in using async gpu relocations is that it increases latency
over using direct CPU relocations, for the cases where the target is
idle and accessible by the CPU. The benefit is greatly reduced lock
contention and improved concurrency by pipelining.

Note that forcing the async gpu relocations does reveal a few issues
they have. Firstly, is that they are visible as writes to gem_busy,
causing to mark some buffers are being to written to by the GPU even
though userspace only reads. Secondly is that, in combination with the
cmdparser, they can cause priority inversions. This should be the case
where the work is being put into a common workqueue losing our priority
information and so being executed in FIFO from the worker, denying us
the opportunity to reorder the requests afterwards.

Signed-off-by: Chris Wilson <[e-mail]>
Reviewed-by: Matthew Auld <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c

···
 	struct eb_vma vma[];
 };
 
-enum {
-	FORCE_CPU_RELOC = 1,
-	FORCE_GTT_RELOC,
-	FORCE_GPU_RELOC,
-#define DBG_FORCE_RELOC 0 /* choose one of the above! */
-};
-
 #define __EXEC_OBJECT_HAS_PIN		BIT(31)
 #define __EXEC_OBJECT_HAS_FENCE		BIT(30)
 #define __EXEC_OBJECT_NEEDS_MAP		BIT(29)
···
 	 */
 	struct reloc_cache {
 		struct drm_mm_node node; /** temporary GTT binding */
-		unsigned long vaddr; /** Current kmap address */
-		unsigned long page; /** Currently mapped page index */
 		unsigned int gen; /** Cached value of INTEL_GEN */
 		bool use_64bit_reloc : 1;
 		bool has_llc : 1;
···
 	}
 }
 
-static inline int use_cpu_reloc(const struct reloc_cache *cache,
-				const struct drm_i915_gem_object *obj)
-{
-	if (!i915_gem_object_has_struct_page(obj))
-		return false;
-
-	if (DBG_FORCE_RELOC == FORCE_CPU_RELOC)
-		return true;
-
-	if (DBG_FORCE_RELOC == FORCE_GTT_RELOC)
-		return false;
-
-	return (cache->has_llc ||
-		obj->cache_dirty ||
-		obj->cache_level != I915_CACHE_NONE);
-}
-
 static int eb_reserve_vma(const struct i915_execbuffer *eb,
 			  struct eb_vma *ev,
 			  u64 pin_flags)
···
 static void reloc_cache_init(struct reloc_cache *cache,
 			     struct drm_i915_private *i915)
 {
-	cache->page = -1;
-	cache->vaddr = 0;
 	/* Must be a variable in the struct to allow GCC to unroll. */
 	cache->gen = INTEL_GEN(i915);
 	cache->has_llc = HAS_LLC(i915);
···
 	i915_request_add(rq);
 
 	return err;
-}
-
-static void reloc_cache_reset(struct reloc_cache *cache)
-{
-	void *vaddr;
-
-	if (!cache->vaddr)
-		return;
-
-	vaddr = unmask_page(cache->vaddr);
-	if (cache->vaddr & KMAP) {
-		if (cache->vaddr & CLFLUSH_AFTER)
-			mb();
-
-		kunmap_atomic(vaddr);
-		i915_gem_object_finish_access((struct drm_i915_gem_object *)cache->node.mm);
-	} else {
-		struct i915_ggtt *ggtt = cache_to_ggtt(cache);
-
-		intel_gt_flush_ggtt_writes(ggtt->vm.gt);
-		io_mapping_unmap_atomic((void __iomem *)vaddr);
-
-		if (drm_mm_node_allocated(&cache->node)) {
-			ggtt->vm.clear_range(&ggtt->vm,
-					     cache->node.start,
-					     cache->node.size);
-			mutex_lock(&ggtt->vm.mutex);
-			drm_mm_remove_node(&cache->node);
-			mutex_unlock(&ggtt->vm.mutex);
-		} else {
-			i915_vma_unpin((struct i915_vma *)cache->node.mm);
-		}
-	}
-
-	cache->vaddr = 0;
-	cache->page = -1;
-}
-
-static void *reloc_kmap(struct drm_i915_gem_object *obj,
-			struct reloc_cache *cache,
-			unsigned long page)
-{
-	void *vaddr;
-
-	if (cache->vaddr) {
-		kunmap_atomic(unmask_page(cache->vaddr));
-	} else {
-		unsigned int flushes;
-		int err;
-
-		err = i915_gem_object_prepare_write(obj, &flushes);
-		if (err)
-			return ERR_PTR(err);
-
-		BUILD_BUG_ON(KMAP & CLFLUSH_FLAGS);
-		BUILD_BUG_ON((KMAP | CLFLUSH_FLAGS) & PAGE_MASK);
-
-		cache->vaddr = flushes | KMAP;
-		cache->node.mm = (void *)obj;
-		if (flushes)
-			mb();
-	}
-
-	vaddr = kmap_atomic(i915_gem_object_get_dirty_page(obj, page));
-	cache->vaddr = unmask_flags(cache->vaddr) | (unsigned long)vaddr;
-	cache->page = page;
-
-	return vaddr;
-}
-
-static void *reloc_iomap(struct drm_i915_gem_object *obj,
-			 struct reloc_cache *cache,
-			 unsigned long page)
-{
-	struct i915_ggtt *ggtt = cache_to_ggtt(cache);
-	unsigned long offset;
-	void *vaddr;
-
-	if (cache->vaddr) {
-		intel_gt_flush_ggtt_writes(ggtt->vm.gt);
-		io_mapping_unmap_atomic((void __force __iomem *) unmask_page(cache->vaddr));
-	} else {
-		struct i915_vma *vma;
-		int err;
-
-		if (i915_gem_object_is_tiled(obj))
-			return ERR_PTR(-EINVAL);
-
-		if (use_cpu_reloc(cache, obj))
-			return NULL;
-
-		i915_gem_object_lock(obj);
-		err = i915_gem_object_set_to_gtt_domain(obj, true);
-		i915_gem_object_unlock(obj);
-		if (err)
-			return ERR_PTR(err);
-
-		vma = i915_gem_object_ggtt_pin(obj, NULL, 0, 0,
-					       PIN_MAPPABLE |
-					       PIN_NONBLOCK /* NOWARN */ |
-					       PIN_NOEVICT);
-		if (IS_ERR(vma)) {
-			memset(&cache->node, 0, sizeof(cache->node));
-			mutex_lock(&ggtt->vm.mutex);
-			err = drm_mm_insert_node_in_range
-				(&ggtt->vm.mm, &cache->node,
-				 PAGE_SIZE, 0, I915_COLOR_UNEVICTABLE,
-				 0, ggtt->mappable_end,
-				 DRM_MM_INSERT_LOW);
-			mutex_unlock(&ggtt->vm.mutex);
-			if (err) /* no inactive aperture space, use cpu reloc */
-				return NULL;
-		} else {
-			cache->node.start = vma->node.start;
-			cache->node.mm = (void *)vma;
-		}
-	}
-
-	offset = cache->node.start;
-	if (drm_mm_node_allocated(&cache->node)) {
-		ggtt->vm.insert_page(&ggtt->vm,
-				     i915_gem_object_get_dma_address(obj, page),
-				     offset, I915_CACHE_NONE, 0);
-	} else {
-		offset += page << PAGE_SHIFT;
-	}
-
-	vaddr = (void __force *)io_mapping_map_atomic_wc(&ggtt->iomap,
-							 offset);
-	cache->page = page;
-	cache->vaddr = (unsigned long)vaddr;
-
-	return vaddr;
-}
-
-static void *reloc_vaddr(struct drm_i915_gem_object *obj,
-			 struct reloc_cache *cache,
-			 unsigned long page)
-{
-	void *vaddr;
-
-	if (cache->page == page) {
-		vaddr = unmask_page(cache->vaddr);
-	} else {
-		vaddr = NULL;
-		if ((cache->vaddr & KMAP) == 0)
-			vaddr = reloc_iomap(obj, cache, page);
-		if (!vaddr)
-			vaddr = reloc_kmap(obj, cache, page);
-	}
-
-	return vaddr;
-}
-
-static void clflush_write32(u32 *addr, u32 value, unsigned int flushes)
-{
-	if (unlikely(flushes & (CLFLUSH_BEFORE | CLFLUSH_AFTER))) {
-		if (flushes & CLFLUSH_BEFORE) {
-			clflushopt(addr);
-			mb();
-		}
-
-		*addr = value;
-
-		/*
-		 * Writes to the same cacheline are serialised by the CPU
-		 * (including clflush). On the write path, we only require
-		 * that it hits memory in an orderly fashion and place
-		 * mb barriers at the start and end of the relocation phase
-		 * to ensure ordering of clflush wrt to the system.
-		 */
-		if (flushes & CLFLUSH_AFTER)
-			clflushopt(addr);
-	} else
-		*addr = value;
 }
 
 static int reloc_move_to_gpu(struct i915_request *rq, struct i915_vma *vma)
···
 	return cmd;
 }
 
-static inline bool use_reloc_gpu(struct i915_vma *vma)
-{
-	if (DBG_FORCE_RELOC == FORCE_GPU_RELOC)
-		return true;
-
-	if (DBG_FORCE_RELOC)
-		return false;
-
-	return !dma_resv_test_signaled_rcu(vma->resv, true);
-}
-
 static unsigned long vma_phys_addr(struct i915_vma *vma, u32 offset)
 {
 	struct page *page;
···
 	return addr + offset_in_page(offset);
 }
 
-static bool __reloc_entry_gpu(struct i915_execbuffer *eb,
-			      struct i915_vma *vma,
-			      u64 offset,
-			      u64 target_addr)
+static int __reloc_entry_gpu(struct i915_execbuffer *eb,
+			     struct i915_vma *vma,
+			     u64 offset,
+			     u64 target_addr)
 {
 	const unsigned int gen = eb->reloc_cache.gen;
 	unsigned int len;
···
 
 	batch = reloc_gpu(eb, vma, len);
 	if (IS_ERR(batch))
-		return false;
+		return PTR_ERR(batch);
 
 	addr = gen8_canonical_addr(vma->node.start + offset);
 	if (gen >= 8) {
···
 		*batch++ = target_addr;
 	}
 
-	return true;
-}
-
-static bool reloc_entry_gpu(struct i915_execbuffer *eb,
-			    struct i915_vma *vma,
-			    u64 offset,
-			    u64 target_addr)
-{
-	if (eb->reloc_cache.vaddr)
-		return false;
-
-	if (!use_reloc_gpu(vma))
-		return false;
-
-	return __reloc_entry_gpu(eb, vma, offset, target_addr);
+	return 0;
 }
 
 static u64
-relocate_entry(struct i915_vma *vma,
+relocate_entry(struct i915_execbuffer *eb,
+	       struct i915_vma *vma,
 	       const struct drm_i915_gem_relocation_entry *reloc,
-	       struct i915_execbuffer *eb,
 	       const struct i915_vma *target)
 {
 	u64 target_addr = relocation_target(reloc, target);
-	u64 offset = reloc->offset;
+	int err;
 
-	if (!reloc_entry_gpu(eb, vma, offset, target_addr)) {
-		bool wide = eb->reloc_cache.use_64bit_reloc;
-		void *vaddr;
-
-repeat:
-		vaddr = reloc_vaddr(vma->obj,
-				    &eb->reloc_cache,
-				    offset >> PAGE_SHIFT);
-		if (IS_ERR(vaddr))
-			return PTR_ERR(vaddr);
-
-		GEM_BUG_ON(!IS_ALIGNED(offset, sizeof(u32)));
-		clflush_write32(vaddr + offset_in_page(offset),
-				lower_32_bits(target_addr),
-				eb->reloc_cache.vaddr);
-
-		if (wide) {
-			offset += sizeof(u32);
-			target_addr >>= 32;
-			wide = false;
-			goto repeat;
-		}
-	}
+	err = __reloc_entry_gpu(eb, vma, reloc->offset, target_addr);
+	if (err)
+		return err;
 
 	return target->node.start | UPDATE;
 }
···
 	 * If the relocation already has the right value in it, no
 	 * more work needs to be done.
 	 */
-	if (!DBG_FORCE_RELOC &&
-	    gen8_canonical_addr(target->vma->node.start) == reloc->presumed_offset)
+	if (gen8_canonical_addr(target->vma->node.start) == reloc->presumed_offset)
 		return 0;
 
 	/* Check that the relocation address is valid... */
···
 	ev->flags &= ~EXEC_OBJECT_ASYNC;
 
 	/* and update the user's relocation entry */
-	return relocate_entry(ev->vma, reloc, eb, target->vma);
+	return relocate_entry(eb, ev->vma, reloc, target->vma);
 }
 
 static int eb_relocate_vma(struct i915_execbuffer *eb, struct eb_vma *ev)
···
 		 * this is bad and so lockdep complains vehemently.
 		 */
 		copied = __copy_from_user(r, urelocs, count * sizeof(r[0]));
-		if (unlikely(copied)) {
-			remain = -EFAULT;
-			goto out;
-		}
+		if (unlikely(copied))
+			return -EFAULT;
 
 		remain -= count;
 		do {
···
 
 			if (likely(offset == 0)) {
 			} else if ((s64)offset < 0) {
-				remain = (int)offset;
-				goto out;
+				return (int)offset;
 			} else {
 				/*
 				 * Note that reporting an error now
···
 		} while (r++, --count);
 		urelocs += ARRAY_SIZE(stack);
 	} while (remain);
-out:
-	reloc_cache_reset(&eb->reloc_cache);
-	return remain;
+
+	return 0;
 }
 
 static int eb_relocate(struct i915_execbuffer *eb)
···
 	eb.i915 = i915;
 	eb.file = file;
 	eb.args = args;
-	if (DBG_FORCE_RELOC || !(args->flags & I915_EXEC_NO_RELOC))
+	if (!(args->flags & I915_EXEC_NO_RELOC))
 		args->flags |= __EXEC_HAS_RELOC;
 
 	eb.exec = exec;

drivers/gpu/drm/i915/gem/selftests/i915_gem_execbuffer.c

···
 		return err;
 
 	/* 8-Byte aligned */
-	if (!__reloc_entry_gpu(eb, vma,
-			       offsets[0] * sizeof(u32),
-			       0)) {
-		err = -EIO;
+	err = __reloc_entry_gpu(eb, vma, offsets[0] * sizeof(u32), 0);
+	if (err)
 		goto unpin_vma;
-	}
 
 	/* !8-Byte aligned */
-	if (!__reloc_entry_gpu(eb, vma,
-			       offsets[1] * sizeof(u32),
-			       1)) {
-		err = -EIO;
+	err = __reloc_entry_gpu(eb, vma, offsets[1] * sizeof(u32), 1);
+	if (err)
 		goto unpin_vma;
-	}
 
 	/* Skip to the end of the cmd page */
 	i = PAGE_SIZE / sizeof(u32) - RELOC_TAIL - 1;
···
 	eb->reloc_cache.rq_size += i;
 
 	/* Force batch chaining */
-	if (!__reloc_entry_gpu(eb, vma,
-			       offsets[2] * sizeof(u32),
-			       2)) {
-		err = -EIO;
+	err = __reloc_entry_gpu(eb, vma, offsets[2] * sizeof(u32), 2);
+	if (err)
 		goto unpin_vma;
-	}
 
 	GEM_BUG_ON(!eb->reloc_cache.rq);
 	rq = i915_request_get(eb->reloc_cache.rq);