[Intel-gfx] [PATCH 1/4] drm/i915/gem: Use chained reloc batches

[Chris Wilson]

The ring is a precious resource: we anticipate to only use a few hundred
bytes for a request, and only try to reserve that before we start. If we
go beyond our guess in building the request, then instead of waiting at
the start of execbuf before we hold any locks or other resources, we
may trigger a wait inside a critical region. One example is in using gpu
relocations, where currently we emit a new MI_BB_START from the ring
every time we overflow a page of relocation entries. However, instead of
insert the command into the precious ring, we can chain the next page of
relocation entries as MI_BB_START from the end of the previous.

Testcase: igt/gem_exec_reloc/basic-many-active
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 .../gpu/drm/i915/gem/i915_gem_execbuffer.c    | 104 +++++++++++++++---
 1 file changed, 91 insertions(+), 13 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
index 414859fa2673..47b1192a159e 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
@@ -975,20 +975,95 @@ static inline struct i915_ggtt *cache_to_ggtt(struct reloc_cache *cache)
 	return &i915->ggtt;
 }
 
+static int reloc_gpu_chain(struct reloc_cache *cache)
+{
+	struct intel_gt_buffer_pool_node *pool;
+	struct i915_request *rq = cache->rq;
+	struct i915_vma *batch;
+	u32 *cmd;
+	int err;
+
+	pool = intel_gt_get_buffer_pool(rq->engine->gt, PAGE_SIZE);
+	if (IS_ERR(pool))
+		return PTR_ERR(pool);
+
+	batch = i915_vma_instance(pool->obj, rq->context->vm, NULL);
+	if (IS_ERR(batch)) {
+		err = PTR_ERR(batch);
+		goto out_pool;
+	}
+
+	err = i915_vma_pin(batch, 0, 0, PIN_USER | PIN_NONBLOCK);
+	if (err)
+		goto out_pool;
+
+	cmd = cache->rq_cmd + cache->rq_size;
+	*cmd++ = MI_ARB_CHECK;
+	if (cache->gen >= 8) {
+		*cmd++ = MI_BATCH_BUFFER_START_GEN8;
+		*cmd++ = lower_32_bits(batch->node.start);
+		*cmd++ = upper_32_bits(batch->node.start);
+	} else {
+		*cmd++ = MI_BATCH_BUFFER_START;
+		*cmd++ = lower_32_bits(batch->node.start);
+	}
+	i915_gem_object_flush_map(rq->batch->obj);
+	i915_gem_object_unpin_map(rq->batch->obj);
+	rq->batch = NULL;
+
+	err = intel_gt_buffer_pool_mark_active(pool, rq);
+	if (err == 0) {
+		i915_vma_lock(batch);
+		err = i915_request_await_object(rq, batch->obj, false);
+		if (err == 0)
+			err = i915_vma_move_to_active(batch, rq, 0);
+		i915_vma_unlock(batch);
+	}
+	i915_vma_unpin(batch);
+	if (err)
+		goto out_pool;
+
+	cmd = i915_gem_object_pin_map(pool->obj,
+				      cache->has_llc ?
+				      I915_MAP_FORCE_WB :
+				      I915_MAP_FORCE_WC);
+	if (IS_ERR(cmd)) {
+		err = PTR_ERR(cmd);
+		goto out_pool;
+	}
+
+	cache->rq_cmd = cmd;
+	cache->rq_size = 0;
+
+	/* Return with batch mapping (cmd) still pinned */
+	rq->batch = batch;
+
+out_pool:
+	intel_gt_buffer_pool_put(pool);
+	return err;
+}
+
 static void reloc_gpu_flush(struct reloc_cache *cache)
 {
-	struct drm_i915_gem_object *obj = cache->rq->batch->obj;
+	struct i915_request *rq;
 
-	GEM_BUG_ON(cache->rq_size >= obj->base.size / sizeof(u32));
-	cache->rq_cmd[cache->rq_size] = MI_BATCH_BUFFER_END;
+	rq = fetch_and_zero(&cache->rq);
+	if (!rq)
+		return;
 
-	__i915_gem_object_flush_map(obj, 0, sizeof(u32) * (cache->rq_size + 1));
-	i915_gem_object_unpin_map(obj);
+	if (rq->batch) {
+		struct drm_i915_gem_object *obj = rq->batch->obj;
 
-	intel_gt_chipset_flush(cache->rq->engine->gt);
+		GEM_BUG_ON(cache->rq_size >= obj->base.size / sizeof(u32));
+		cache->rq_cmd[cache->rq_size++] = MI_BATCH_BUFFER_END;
 
-	i915_request_add(cache->rq);
-	cache->rq = NULL;
+		__i915_gem_object_flush_map(obj,
+					    0, sizeof(u32) * cache->rq_size);
+		i915_gem_object_unpin_map(obj);
+	}
+
+	intel_gt_chipset_flush(rq->engine->gt);
+	i915_request_add(rq);
 }
 
 static void reloc_cache_reset(struct reloc_cache *cache)
@@ -1280,13 +1355,9 @@ static u32 *reloc_gpu(struct i915_execbuffer *eb,
 {
 	struct reloc_cache *cache = &eb->reloc_cache;
 	u32 *cmd;
-
-	if (cache->rq_size > PAGE_SIZE/sizeof(u32) - (len + 1))
-		reloc_gpu_flush(cache);
+	int err;
 
 	if (unlikely(!cache->rq)) {
-		int err;
-
 		if (!intel_engine_can_store_dword(eb->engine))
 			return ERR_PTR(-ENODEV);
 
@@ -1295,6 +1366,13 @@ static u32 *reloc_gpu(struct i915_execbuffer *eb,
 			return ERR_PTR(err);
 	}
 
+	if (unlikely(cache->rq_size > PAGE_SIZE / sizeof(u32) - len - 4)) {
+		err = reloc_gpu_chain(cache);
+		if (unlikely(err))
+			return ERR_PTR(err);
+	}
+
+	GEM_BUG_ON(cache->rq_size + len >= PAGE_SIZE  / sizeof(u32));
 	cmd = cache->rq_cmd + cache->rq_size;
 	cache->rq_size += len;
 
-- 
2.20.1