[PATCH 17/49] drm/i915: Implement inter-engine read-read optimisations

[Chris Wilson <chris@chris-wilson.co.uk>]

Currently, we only track the last request globally across all engines.
This prevents us from issuing concurrent read requests on e.g. the RCS
and BCS engines (or more likely the render and media engines). Without
semaphores, we incur costly stalls as we synchronise between rings -
greatly impacting the current performance of Broadwell versus Haswell in
certain workloads (like video decode). With the introduction of
reference counted requests, it is much easier to track the last request
per ring, as well as the last global write request so that we can
optimise inter-engine read read requests (as well as better optimise
certain CPU waits).

v2: Fix inverted readonly condition for nonblocking waits.
v3: Handle non-continguous engine array after waits
v4: Rebase, tidy, rewrite ring list debugging
v5: Use obj->active as a bitfield, it looks cool
v6: Micro-optimise, mostly involving moving code around

Benchmark: igt/gem_read_read_speed
hsw:gt3e (with semaphores):
Before: Time to read-read 1024k:		275.794µs
After:  Time to read-read 1024k:		123.260µs

hsw:gt3e (w/o semaphores):
Before: Time to read-read 1024k:		230.433µs
After:  Time to read-read 1024k:		124.593µs

bdw-u (w/o semaphores):             Before          After
Time to read-read 1x1:            26.274µs       10.350µs
Time to read-read 128x128:        40.097µs       21.366µs
Time to read-read 256x256:        77.087µs       42.608µs
Time to read-read 512x512:       281.999µs      181.155µs
Time to read-read 1024x1024:    1196.141µs     1118.223µs
Time to read-read 2048x2048:    5639.072µs     5225.837µs
Time to read-read 4096x4096:   22401.662µs    21137.067µs
Time to read-read 8192x8192:   89617.735µs    85637.681µs

Testcase: igt/gem_concurrent_blit (read-read and friends)
Cc: Lionel Landwerlin <lionel.g.landwerlin@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_debugfs.c     |  16 +-
 drivers/gpu/drm/i915/i915_drv.h         |  19 +-
 drivers/gpu/drm/i915/i915_gem.c         | 516 +++++++++++++++++++-------------
 drivers/gpu/drm/i915/i915_gem_context.c |   2 -
 drivers/gpu/drm/i915/i915_gem_debug.c   |  92 ++----
 drivers/gpu/drm/i915/i915_gpu_error.c   |  19 +-
 drivers/gpu/drm/i915/intel_display.c    |   4 +-
 drivers/gpu/drm/i915/intel_lrc.c        |  15 +-
 drivers/gpu/drm/i915/intel_ringbuffer.c |  10 +-
 9 files changed, 386 insertions(+), 307 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
index 7ef6295438e9..5cea9a9c1cb9 100644
--- a/drivers/gpu/drm/i915/i915_debugfs.c
+++ b/drivers/gpu/drm/i915/i915_debugfs.c
@@ -120,10 +120,13 @@ static inline const char *get_global_flag(struct drm_i915_gem_object *obj)
 static void
 describe_obj(struct seq_file *m, struct drm_i915_gem_object *obj)
 {
+	struct drm_i915_private *dev_priv = to_i915(obj->base.dev);
+	struct intel_engine_cs *ring;
 	struct i915_vma *vma;
 	int pin_count = 0;
+	int i;
 
-	seq_printf(m, "%pK: %s%s%s%s %8zdKiB %02x %02x %x %x %x%s%s%s",
+	seq_printf(m, "%pK: %s%s%s%s %8zdKiB %02x %02x [ ",
 		   &obj->base,
 		   obj->active ? "*" : " ",
 		   get_pin_flag(obj),
@@ -131,8 +134,11 @@ describe_obj(struct seq_file *m, struct drm_i915_gem_object *obj)
 		   get_global_flag(obj),
 		   obj->base.size / 1024,
 		   obj->base.read_domains,
-		   obj->base.write_domain,
-		   i915_gem_request_get_seqno(obj->last_read_req),
+		   obj->base.write_domain);
+	for_each_ring(ring, dev_priv, i)
+		seq_printf(m, "%x ",
+				i915_gem_request_get_seqno(obj->last_read_req[i]));
+	seq_printf(m, "] %x %x%s%s%s",
 		   i915_gem_request_get_seqno(obj->last_write_req),
 		   i915_gem_request_get_seqno(obj->last_fenced_req),
 		   i915_cache_level_str(to_i915(obj->base.dev), obj->cache_level),
@@ -169,9 +175,9 @@ describe_obj(struct seq_file *m, struct drm_i915_gem_object *obj)
 		*t = '\0';
 		seq_printf(m, " (%s mappable)", s);
 	}
-	if (obj->last_read_req != NULL)
+	if (obj->last_write_req != NULL)
 		seq_printf(m, " (%s)",
-			   i915_gem_request_get_ring(obj->last_read_req)->name);
+			   i915_gem_request_get_ring(obj->last_write_req)->name);
 	if (obj->frontbuffer_bits)
 		seq_printf(m, " (frontbuffer: 0x%03x)", obj->frontbuffer_bits);
 }
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 896aae1c10ac..7cf5d1b0a749 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -500,7 +500,7 @@ struct drm_i915_error_state {
 	struct drm_i915_error_buffer {
 		u32 size;
 		u32 name;
-		u32 rseqno, wseqno;
+		u32 rseqno[I915_NUM_RINGS], wseqno;
 		u32 gtt_offset;
 		u32 read_domains;
 		u32 write_domain;
@@ -1905,7 +1905,7 @@ struct drm_i915_gem_object {
 	struct drm_mm_node *stolen;
 	struct list_head global_list;
 
-	struct list_head ring_list;
+	struct list_head ring_list[I915_NUM_RINGS];
 	/** Used in execbuf to temporarily hold a ref */
 	struct list_head obj_exec_link;
 
@@ -1916,7 +1916,7 @@ struct drm_i915_gem_object {
 	 * rendering and so a non-zero seqno), and is not set if it i s on
 	 * inactive (ready to be unbound) list.
 	 */
-	unsigned int active:1;
+	unsigned int active:I915_NUM_RINGS;
 
 	/**
 	 * This is set if the object has been written to since last bound
@@ -1987,8 +1987,17 @@ struct drm_i915_gem_object {
 	void *dma_buf_vmapping;
 	int vmapping_count;
 
-	/** Breadcrumb of last rendering to the buffer. */
-	struct drm_i915_gem_request *last_read_req;
+	/** Breadcrumb of last rendering to the buffer.
+	 * There can only be one writer, but we allow for multiple readers.
+	 * If there is a writer that necessarily implies that all other
+	 * read requests are complete - but we may only be lazily clearing
+	 * the read requests. A read request is naturally the most recent
+	 * request on a ring, so we may have two different write and read
+	 * requests on one ring where the write request is older than the
+	 * read request. This allows for the CPU to read from an active
+	 * buffer by only waiting for the write to complete.
+	 * */
+	struct drm_i915_gem_request *last_read_req[I915_NUM_RINGS];
 	struct drm_i915_gem_request *last_write_req;
 	/** Breadcrumb of last fenced GPU access to the buffer. */
 	struct drm_i915_gem_request *last_fenced_req;
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index dc3eafe7f7d4..7e6f2560bf35 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -38,14 +38,17 @@
 #include <linux/pci.h>
 #include <linux/dma-buf.h>
 
+#define RQ_BUG_ON(expr)
+
 static void i915_gem_object_flush_gtt_write_domain(struct drm_i915_gem_object *obj);
 static void i915_gem_object_flush_cpu_write_domain(struct drm_i915_gem_object *obj);
+static void
+i915_gem_object_retire__write(struct drm_i915_gem_object *obj);
+static void
+i915_gem_object_retire__read(struct drm_i915_gem_object *obj, int ring);
 static __must_check int
 i915_gem_object_wait_rendering(struct drm_i915_gem_object *obj,
 			       bool readonly);
-static void
-i915_gem_object_retire(struct drm_i915_gem_object *obj);
-
 static void i915_gem_write_fence(struct drm_device *dev, int reg,
 				 struct drm_i915_gem_object *obj);
 static void i915_gem_object_update_fence(struct drm_i915_gem_object *obj,
@@ -518,8 +521,6 @@ int i915_gem_obj_prepare_shmem_read(struct drm_i915_gem_object *obj,
 		ret = i915_gem_object_wait_rendering(obj, true);
 		if (ret)
 			return ret;
-
-		i915_gem_object_retire(obj);
 	}
 
 	ret = i915_gem_object_get_pages(obj);
@@ -939,8 +940,6 @@ i915_gem_shmem_pwrite(struct drm_device *dev,
 		ret = i915_gem_object_wait_rendering(obj, false);
 		if (ret)
 			return ret;
-
-		i915_gem_object_retire(obj);
 	}
 	/* Same trick applies to invalidate partially written cachelines read
 	 * before writing. */
@@ -1239,6 +1238,9 @@ int __i915_wait_request(struct drm_i915_gem_request *req,
 
 	WARN(!intel_irqs_enabled(dev_priv), "IRQs disabled");
 
+	if (list_empty(&req->list))
+		return 0;
+
 	if (i915_gem_request_completed(req, true))
 		return 0;
 
@@ -1338,6 +1340,56 @@ out:
 	return ret;
 }
 
+static inline void
+i915_gem_request_remove_from_client(struct drm_i915_gem_request *request)
+{
+	struct drm_i915_file_private *file_priv = request->file_priv;
+
+	if (!file_priv)
+		return;
+
+	spin_lock(&file_priv->mm.lock);
+	list_del(&request->client_list);
+	request->file_priv = NULL;
+	spin_unlock(&file_priv->mm.lock);
+}
+
+static void i915_gem_request_retire(struct drm_i915_gem_request *request)
+{
+	trace_i915_gem_request_retire(request);
+
+	list_del_init(&request->list);
+	i915_gem_request_remove_from_client(request);
+
+	put_pid(request->pid);
+
+	i915_gem_request_unreference(request);
+}
+
+static void
+__i915_gem_request_retire__upto(struct drm_i915_gem_request *rq)
+{
+	struct intel_engine_cs *engine = rq->ring;
+
+	lockdep_assert_held(&engine->dev->struct_mutex);
+
+	if (list_empty(&rq->list))
+		return;
+
+	rq->ringbuf->last_retired_head = rq->postfix;
+
+	do {
+		struct drm_i915_gem_request *prev =
+			list_entry(rq->list.prev, typeof(*rq), list);
+
+		i915_gem_request_retire(rq);
+
+		rq = prev;
+	} while (&rq->list != &engine->request_list);
+
+	WARN_ON(i915_verify_lists(engine->dev));
+}
+
 /**
  * Waits for a request to be signaled, and cleans up the
  * request and object lists appropriately for that event.
@@ -1348,7 +1400,6 @@ i915_wait_request(struct drm_i915_gem_request *req)
 	struct drm_device *dev;
 	struct drm_i915_private *dev_priv;
 	bool interruptible;
-	unsigned reset_counter;
 	int ret;
 
 	BUG_ON(req == NULL);
@@ -1367,29 +1418,13 @@ i915_wait_request(struct drm_i915_gem_request *req)
 	if (ret)
 		return ret;
 
-	reset_counter = atomic_read(&dev_priv->gpu_error.reset_counter);
-	i915_gem_request_reference(req);
-	ret = __i915_wait_request(req, reset_counter,
+	ret = __i915_wait_request(req,
+				  atomic_read(&dev_priv->gpu_error.reset_counter),
 				  interruptible, NULL, NULL);
-	i915_gem_request_unreference(req);
-	return ret;
-}
-
-static int
-i915_gem_object_wait_rendering__tail(struct drm_i915_gem_object *obj)
-{
-	if (!obj->active)
-		return 0;
-
-	/* Manually manage the write flush as we may have not yet
-	 * retired the buffer.
-	 *
-	 * Note that the last_write_req is always the earlier of
-	 * the two (read/write) requests, so if we haved successfully waited,
-	 * we know we have passed the last write.
-	 */
-	i915_gem_request_assign(&obj->last_write_req, NULL);
+	if (ret)
+		return ret;
 
+	__i915_gem_request_retire__upto(req);
 	return 0;
 }
 
@@ -1401,18 +1436,38 @@ static __must_check int
 i915_gem_object_wait_rendering(struct drm_i915_gem_object *obj,
 			       bool readonly)
 {
-	struct drm_i915_gem_request *req;
-	int ret;
+	int ret, i;
 
-	req = readonly ? obj->last_write_req : obj->last_read_req;
-	if (!req)
+	if (!obj->active)
 		return 0;
 
-	ret = i915_wait_request(req);
-	if (ret)
-		return ret;
+	if (readonly) {
+		if (obj->last_write_req != NULL) {
+			ret = i915_wait_request(obj->last_write_req);
+			if (ret)
+				return ret;
 
-	return i915_gem_object_wait_rendering__tail(obj);
+			i = obj->last_write_req->ring->id;
+			if (obj->last_read_req[i] == obj->last_write_req)
+				i915_gem_object_retire__read(obj, i);
+			else
+				i915_gem_object_retire__write(obj);
+		}
+	} else {
+		for (i = 0; i < I915_NUM_RINGS; i++) {
+			if (obj->last_read_req[i] == NULL)
+				continue;
+
+			ret = i915_wait_request(obj->last_read_req[i]);
+			if (ret)
+				return ret;
+
+			i915_gem_object_retire__read(obj, i);
+		}
+		RQ_BUG_ON(obj->active);
+	}
+
+	return 0;
 }
 
 /* A nonblocking variant of the above wait. This is a highly dangerous routine
@@ -1423,37 +1478,72 @@ i915_gem_object_wait_rendering__nonblocking(struct drm_i915_gem_object *obj,
 					    struct drm_i915_file_private *file_priv,
 					    bool readonly)
 {
-	struct drm_i915_gem_request *req;
 	struct drm_device *dev = obj->base.dev;
 	struct drm_i915_private *dev_priv = dev->dev_private;
+	struct drm_i915_gem_request *requests[I915_NUM_RINGS];
 	unsigned reset_counter;
-	int ret;
+	int ret, i, n = 0;
 
 	BUG_ON(!mutex_is_locked(&dev->struct_mutex));
 	BUG_ON(!dev_priv->mm.interruptible);
 
-	req = readonly ? obj->last_write_req : obj->last_read_req;
-	if (!req)
+	if (!obj->active)
 		return 0;
 
 	ret = i915_gem_check_wedge(&dev_priv->gpu_error, true);
 	if (ret)
 		return ret;
 
-	ret = i915_gem_check_olr(req);
-	if (ret)
-		return ret;
-
 	reset_counter = atomic_read(&dev_priv->gpu_error.reset_counter);
-	i915_gem_request_reference(req);
+
+	if (readonly) {
+		struct drm_i915_gem_request *rq;
+
+		rq = obj->last_write_req;
+		if (rq == NULL)
+			return 0;
+
+		ret = i915_gem_check_olr(rq);
+		if (ret)
+			goto err;
+
+		requests[n++] = i915_gem_request_reference(rq);
+	} else {
+		for (i = 0; i < I915_NUM_RINGS; i++) {
+			struct drm_i915_gem_request *rq;
+
+			rq = obj->last_read_req[i];
+			if (rq == NULL)
+				continue;
+
+			ret = i915_gem_check_olr(rq);
+			if (ret)
+				goto err;
+
+			requests[n++] = i915_gem_request_reference(rq);
+		}
+	}
+
 	mutex_unlock(&dev->struct_mutex);
-	ret = __i915_wait_request(req, reset_counter, true, NULL, file_priv);
+	for (i = 0; ret == 0 && i < n; i++)
+		ret = __i915_wait_request(requests[i], reset_counter, true,
+					  NULL, file_priv);
 	mutex_lock(&dev->struct_mutex);
-	i915_gem_request_unreference(req);
-	if (ret)
-		return ret;
 
-	return i915_gem_object_wait_rendering__tail(obj);
+err:
+	for (i = 0; i < n; i++) {
+		if (ret == 0) {
+			int ring = requests[i]->ring->id;
+			if (obj->last_read_req[ring] == requests[i])
+				i915_gem_object_retire__read(obj, ring);
+			if (obj->last_write_req == requests[i])
+				i915_gem_object_retire__write(obj);
+			__i915_gem_request_retire__upto(requests[i]);
+		}
+		i915_gem_request_unreference(requests[i]);
+	}
+
+	return ret;
 }
 
 /**
@@ -2204,78 +2294,58 @@ i915_gem_object_get_pages(struct drm_i915_gem_object *obj)
 	return 0;
 }
 
-static void
-i915_gem_object_move_to_active(struct drm_i915_gem_object *obj,
-			       struct intel_engine_cs *ring)
+void i915_vma_move_to_active(struct i915_vma *vma,
+			     struct intel_engine_cs *ring)
 {
-	struct drm_i915_gem_request *req;
-	struct intel_engine_cs *old_ring;
-
-	BUG_ON(ring == NULL);
-
-	req = intel_ring_get_request(ring);
-	old_ring = i915_gem_request_get_ring(obj->last_read_req);
-
-	if (old_ring != ring && obj->last_write_req) {
-		/* Keep the request relative to the current ring */
-		i915_gem_request_assign(&obj->last_write_req, req);
-	}
+	struct drm_i915_gem_object *obj = vma->obj;
 
 	/* Add a reference if we're newly entering the active list. */
-	if (!obj->active) {
+	if (obj->active == 0)
 		drm_gem_object_reference(&obj->base);
-		obj->active = 1;
-	}
+	obj->active |= intel_ring_flag(ring);
 
-	list_move_tail(&obj->ring_list, &ring->active_list);
+	list_move_tail(&obj->ring_list[ring->id], &ring->active_list);
+	i915_gem_request_assign(&obj->last_read_req[ring->id],
+				intel_ring_get_request(ring));
 
-	i915_gem_request_assign(&obj->last_read_req, req);
+	list_move_tail(&vma->mm_list, &vma->vm->active_list);
 }
 
-void i915_vma_move_to_active(struct i915_vma *vma,
-			     struct intel_engine_cs *ring)
+static void
+i915_gem_object_retire__write(struct drm_i915_gem_object *obj)
 {
-	list_move_tail(&vma->mm_list, &vma->vm->active_list);
-	return i915_gem_object_move_to_active(vma->obj, ring);
+	RQ_BUG_ON(obj->last_write_req == NULL);
+	RQ_BUG_ON(!(obj->active & intel_ring_flag(obj->last_write_req->ring)));
+
+	i915_gem_request_assign(&obj->last_write_req, NULL);
+	intel_fb_obj_flush(obj, true);
 }
 
 static void
-i915_gem_object_move_to_inactive(struct drm_i915_gem_object *obj)
+i915_gem_object_retire__read(struct drm_i915_gem_object *obj, int ring)
 {
 	struct i915_vma *vma;
 
-	BUG_ON(obj->base.write_domain & ~I915_GEM_GPU_DOMAINS);
-	BUG_ON(!obj->active);
+	RQ_BUG_ON(obj->last_read_req[ring] == NULL);
+	RQ_BUG_ON(!(obj->active & (1 << ring)));
+
+	list_del_init(&obj->ring_list[ring]);
+	i915_gem_request_assign(&obj->last_read_req[ring], NULL);
+
+	if (obj->last_write_req && obj->last_write_req->ring->id == ring)
+		i915_gem_object_retire__write(obj);
+
+	obj->active &= ~(1 << ring);
+	if (obj->active)
+		return;
 
 	list_for_each_entry(vma, &obj->vma_list, vma_link) {
 		if (!list_empty(&vma->mm_list))
 			list_move_tail(&vma->mm_list, &vma->vm->inactive_list);
 	}
 
-	intel_fb_obj_flush(obj, true);
-
-	list_del_init(&obj->ring_list);
-
-	i915_gem_request_assign(&obj->last_read_req, NULL);
-	i915_gem_request_assign(&obj->last_write_req, NULL);
-	obj->base.write_domain = 0;
-
 	i915_gem_request_assign(&obj->last_fenced_req, NULL);
-
-	obj->active = 0;
 	drm_gem_object_unreference(&obj->base);
-
-	WARN_ON(i915_verify_lists(dev));
-}
-
-static void
-i915_gem_object_retire(struct drm_i915_gem_object *obj)
-{
-	if (obj->last_read_req == NULL)
-		return;
-
-	if (i915_gem_request_completed(obj->last_read_req, true))
-		i915_gem_object_move_to_inactive(obj);
 }
 
 static int
@@ -2452,20 +2522,6 @@ int __i915_add_request(struct intel_engine_cs *ring,
 	return 0;
 }
 
-static inline void
-i915_gem_request_remove_from_client(struct drm_i915_gem_request *request)
-{
-	struct drm_i915_file_private *file_priv = request->file_priv;
-
-	if (!file_priv)
-		return;
-
-	spin_lock(&file_priv->mm.lock);
-	list_del(&request->client_list);
-	request->file_priv = NULL;
-	spin_unlock(&file_priv->mm.lock);
-}
-
 static bool i915_context_is_banned(struct drm_i915_private *dev_priv,
 				   const struct intel_context *ctx)
 {
@@ -2511,16 +2567,6 @@ static void i915_set_reset_status(struct drm_i915_private *dev_priv,
 	}
 }
 
-static void i915_gem_free_request(struct drm_i915_gem_request *request)
-{
-	list_del(&request->list);
-	i915_gem_request_remove_from_client(request);
-
-	put_pid(request->pid);
-
-	i915_gem_request_unreference(request);
-}
-
 void i915_gem_request_free(struct kref *req_ref)
 {
 	struct drm_i915_gem_request *req = container_of(req_ref,
@@ -2583,9 +2629,9 @@ static void i915_gem_reset_ring_cleanup(struct drm_i915_private *dev_priv,
 
 		obj = list_first_entry(&ring->active_list,
 				       struct drm_i915_gem_object,
-				       ring_list);
+				       ring_list[ring->id]);
 
-		i915_gem_object_move_to_inactive(obj);
+		i915_gem_object_retire__read(obj, ring->id);
 	}
 
 	/*
@@ -2622,7 +2668,8 @@ static void i915_gem_reset_ring_cleanup(struct drm_i915_private *dev_priv,
 					   struct drm_i915_gem_request,
 					   list);
 
-		i915_gem_free_request(request);
+		request->ringbuf->last_retired_head = request->tail;
+		i915_gem_request_retire(request);
 	}
 
 	/* This may not have been flushed before the reset, so clean it now */
@@ -2670,6 +2717,8 @@ void i915_gem_reset(struct drm_device *dev)
 	i915_gem_context_reset(dev);
 
 	i915_gem_restore_fences(dev);
+
+	WARN_ON(i915_verify_lists(dev));
 }
 
 /**
@@ -2678,11 +2727,11 @@ void i915_gem_reset(struct drm_device *dev)
 void
 i915_gem_retire_requests_ring(struct intel_engine_cs *ring)
 {
-	if (list_empty(&ring->request_list))
-		return;
-
 	WARN_ON(i915_verify_lists(ring->dev));
 
+	if (list_empty(&ring->active_list))
+		return;
+
 	/* Retire requests first as we use it above for the early return.
 	 * If we retire requests last, we may use a later seqno and so clear
 	 * the requests lists without clearing the active list, leading to
@@ -2698,16 +2747,13 @@ i915_gem_retire_requests_ring(struct intel_engine_cs *ring)
 		if (!i915_gem_request_completed(request, true))
 			break;
 
-		trace_i915_gem_request_retire(request);
-
 		/* We know the GPU must have read the request to have
 		 * sent us the seqno + interrupt, so use the position
 		 * of tail of the request to update the last known position
 		 * of the GPU head.
 		 */
 		request->ringbuf->last_retired_head = request->postfix;
-
-		i915_gem_free_request(request);
+		i915_gem_request_retire(request);
 	}
 
 	/* Move any buffers on the active list that are no longer referenced
@@ -2719,12 +2765,12 @@ i915_gem_retire_requests_ring(struct intel_engine_cs *ring)
 
 		obj = list_first_entry(&ring->active_list,
 				      struct drm_i915_gem_object,
-				      ring_list);
+				      ring_list[ring->id]);
 
-		if (!i915_gem_request_completed(obj->last_read_req, true))
+		if (!list_empty(&obj->last_read_req[ring->id]->list))
 			break;
 
-		i915_gem_object_move_to_inactive(obj);
+		i915_gem_object_retire__read(obj, ring->id);
 	}
 
 	if (unlikely(ring->trace_irq_req &&
@@ -2813,17 +2859,23 @@ i915_gem_idle_work_handler(struct work_struct *work)
 static int
 i915_gem_object_flush_active(struct drm_i915_gem_object *obj)
 {
-	struct intel_engine_cs *ring;
-	int ret;
+	int ret, i;
 
-	if (obj->active) {
-		ring = i915_gem_request_get_ring(obj->last_read_req);
+	if (!obj->active)
+		return 0;
+
+	for (i = 0; i < I915_NUM_RINGS; i++) {
+		struct drm_i915_gem_request *rq;
+
+		rq = obj->last_read_req[i];
+		if (rq == NULL)
+			continue;
 
-		ret = i915_gem_check_olr(obj->last_read_req);
+		ret = i915_gem_check_olr(rq);
 		if (ret)
 			return ret;
 
-		i915_gem_retire_requests_ring(ring);
+		i915_gem_retire_requests_ring(rq->ring);
 	}
 
 	return 0;
@@ -2857,9 +2909,10 @@ i915_gem_wait_ioctl(struct drm_device *dev, void *data, struct drm_file *file)
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	struct drm_i915_gem_wait *args = data;
 	struct drm_i915_gem_object *obj;
-	struct drm_i915_gem_request *req;
+	struct drm_i915_gem_request *req[I915_NUM_RINGS];
 	unsigned reset_counter;
-	int ret = 0;
+	int i, n = 0;
+	int ret;
 
 	if (args->flags != 0)
 		return -EINVAL;
@@ -2879,11 +2932,9 @@ i915_gem_wait_ioctl(struct drm_device *dev, void *data, struct drm_file *file)
 	if (ret)
 		goto out;
 
-	if (!obj->active || !obj->last_read_req)
+	if (!obj->active)
 		goto out;
 
-	req = obj->last_read_req;
-
 	/* Do this after OLR check to make sure we make forward progress polling
 	 * on this IOCTL with a timeout == 0 (like busy ioctl)
 	 */
@@ -2894,13 +2945,23 @@ i915_gem_wait_ioctl(struct drm_device *dev, void *data, struct drm_file *file)
 
 	drm_gem_object_unreference(&obj->base);
 	reset_counter = atomic_read(&dev_priv->gpu_error.reset_counter);
-	i915_gem_request_reference(req);
+
+	for (i = 0; i < I915_NUM_RINGS; i++) {
+		if (obj->last_read_req[i] == NULL)
+			continue;
+
+		req[n++] = i915_gem_request_reference(obj->last_read_req[i]);
+	}
+
 	mutex_unlock(&dev->struct_mutex);
 
-	ret = __i915_wait_request(req, reset_counter, true,
-				  args->timeout_ns > 0 ? &args->timeout_ns : NULL,
-				  file->driver_priv);
-	i915_gem_request_unreference__unlocked(req);
+	for (i = 0; i < n; i++) {
+		if (ret == 0)
+			ret = __i915_wait_request(req[i], reset_counter, true,
+						  args->timeout_ns > 0 ? &args->timeout_ns : NULL,
+						  file->driver_priv);
+		i915_gem_request_unreference__unlocked(req[i]);
+	}
 	return ret;
 
 out:
@@ -2909,6 +2970,62 @@ out:
 	return ret;
 }
 
+static int
+__i915_gem_object_sync(struct drm_i915_gem_object *obj,
+		       struct intel_engine_cs *to,
+		       struct drm_i915_gem_request *rq)
+{
+	struct intel_engine_cs *from;
+	int ret;
+
+	if (rq == NULL)
+		return 0;
+
+	from = i915_gem_request_get_ring(rq);
+	if (to == from)
+		return 0;
+
+	if (i915_gem_request_completed(rq, true))
+		return 0;
+
+	ret = i915_gem_check_olr(rq);
+	if (ret)
+		return ret;
+
+	if (!i915_semaphore_is_enabled(obj->base.dev)) {
+		ret = __i915_wait_request(rq,
+					  atomic_read(&to_i915(obj->base.dev)->gpu_error.reset_counter),
+					  to_i915(obj->base.dev)->mm.interruptible, NULL, NULL);
+		if (ret)
+			return ret;
+
+		if (obj->last_read_req[from->id] == rq)
+			i915_gem_object_retire__read(obj, from->id);
+		if (obj->last_write_req == rq)
+			i915_gem_object_retire__write(obj);
+	} else {
+		int idx = intel_ring_sync_index(from, to);
+		u32 seqno = i915_gem_request_get_seqno(rq);
+
+		if (seqno <= from->semaphore.sync_seqno[idx])
+			return 0;
+
+		trace_i915_gem_ring_sync_to(from, to, rq);
+		ret = to->semaphore.sync_to(to, from, seqno);
+		if (ret)
+			return ret;
+
+		/* We use last_read_req because sync_to()
+		 * might have just caused seqno wrap under
+		 * the radar.
+		 */
+		from->semaphore.sync_seqno[idx] =
+			i915_gem_request_get_seqno(obj->last_read_req[from->id]);
+	}
+
+	return 0;
+}
+
 /**
  * i915_gem_object_sync - sync an object to a ring.
  *
@@ -2917,7 +3034,17 @@ out:
  *
  * This code is meant to abstract object synchronization with the GPU.
  * Calling with NULL implies synchronizing the object with the CPU
- * rather than a particular GPU ring.
+ * rather than a particular GPU ring. Conceptually we serialise writes
+ * between engines inside the GPU. We only allow on engine to write
+ * into a buffer at any time, but multiple readers. To ensure each has
+ * a coherent view of memory, we must:
+ *
+ * - If there is an outstanding write request to the object, the new
+ *   request must wait for it to complete (either CPU or in hw, requests
+ *   on the same ring will be naturally ordered).
+ *
+ * - If we are a write request (pending_write_domain is set), the new
+ *   request must wait for outstanding read requests to complete.
  *
  * Returns 0 if successful, else propagates up the lower layer error.
  */
@@ -2925,39 +3052,25 @@ int
 i915_gem_object_sync(struct drm_i915_gem_object *obj,
 		     struct intel_engine_cs *to)
 {
-	struct intel_engine_cs *from;
-	u32 seqno;
-	int ret, idx;
-
-	from = i915_gem_request_get_ring(obj->last_read_req);
-
-	if (from == NULL || to == from)
-		return 0;
-
-	if (to == NULL || !i915_semaphore_is_enabled(obj->base.dev))
-		return i915_gem_object_wait_rendering(obj, false);
-
-	idx = intel_ring_sync_index(from, to);
+	const bool readonly = obj->base.pending_write_domain == 0;
+	int ret, i;
 
-	seqno = i915_gem_request_get_seqno(obj->last_read_req);
-	/* Optimization: Avoid semaphore sync when we are sure we already
-	 * waited for an object with higher seqno */
-	if (seqno <= from->semaphore.sync_seqno[idx])
+	if (!obj->active)
 		return 0;
 
-	ret = i915_gem_check_olr(obj->last_read_req);
-	if (ret)
-		return ret;
-
-	trace_i915_gem_ring_sync_to(from, to, obj->last_read_req);
-	ret = to->semaphore.sync_to(to, from, seqno);
-	if (!ret)
-		/* We use last_read_req because sync_to()
-		 * might have just caused seqno wrap under
-		 * the radar.
-		 */
-		from->semaphore.sync_seqno[idx] =
-				i915_gem_request_get_seqno(obj->last_read_req);
+	if (to == NULL) {
+		ret = i915_gem_object_wait_rendering(obj, readonly);
+	} else if (readonly) {
+		ret = __i915_gem_object_sync(obj, to,
+					     obj->last_write_req);
+	} else {
+		for (i = 0; i < I915_NUM_RINGS; i++) {
+			ret = __i915_gem_object_sync(obj, to,
+						     obj->last_read_req[i]);
+			if (ret)
+				break;
+		}
+	}
 
 	return ret;
 }
@@ -3044,10 +3157,6 @@ int i915_vma_unbind(struct i915_vma *vma)
 	/* Since the unbound list is global, only move to that list if
 	 * no more VMAs exist. */
 	if (list_empty(&obj->vma_list)) {
-		/* Throw away the active reference before
-		 * moving to the unbound list. */
-		i915_gem_object_retire(obj);
-
 		i915_gem_gtt_finish_object(obj);
 		list_move_tail(&obj->global_list, &dev_priv->mm.unbound_list);
 	}
@@ -3080,6 +3189,7 @@ int i915_gpu_idle(struct drm_device *dev)
 			return ret;
 	}
 
+	WARN_ON(i915_verify_lists(dev));
 	return 0;
 }
 
@@ -3713,8 +3823,6 @@ i915_gem_object_set_to_gtt_domain(struct drm_i915_gem_object *obj, bool write)
 	if (ret)
 		return ret;
 
-	i915_gem_object_retire(obj);
-
 	/* Flush and acquire obj->pages so that we are coherent through
 	 * direct access in memory with previous cached writes through
 	 * shmemfs and that our cache domain tracking remains valid.
@@ -3940,11 +4048,9 @@ i915_gem_object_pin_to_display_plane(struct drm_i915_gem_object *obj,
 	bool was_pin_display;
 	int ret;
 
-	if (pipelined != i915_gem_request_get_ring(obj->last_read_req)) {
-		ret = i915_gem_object_sync(obj, pipelined);
-		if (ret)
-			return ret;
-	}
+	ret = i915_gem_object_sync(obj, pipelined);
+	if (ret)
+		return ret;
 
 	/* Mark the pin_display early so that we account for the
 	 * display coherency whilst setting up the cache domains.
@@ -4049,7 +4155,6 @@ i915_gem_object_set_to_cpu_domain(struct drm_i915_gem_object *obj, bool write)
 	if (ret)
 		return ret;
 
-	i915_gem_object_retire(obj);
 	i915_gem_object_flush_gtt_write_domain(obj);
 
 	old_write_domain = obj->base.write_domain;
@@ -4359,15 +4464,15 @@ i915_gem_busy_ioctl(struct drm_device *dev, void *data,
 	 * necessary flushes here.
 	 */
 	ret = i915_gem_object_flush_active(obj);
+	if (ret)
+		goto unref;
 
-	args->busy = obj->active;
-	if (obj->last_read_req) {
-		struct intel_engine_cs *ring;
-		BUILD_BUG_ON(I915_NUM_RINGS > 16);
-		ring = i915_gem_request_get_ring(obj->last_read_req);
-		args->busy |= intel_ring_flag(ring) << 16;
-	}
+	BUILD_BUG_ON(I915_NUM_RINGS > 16);
+	args->busy = obj->active << 16;
+	if (obj->last_write_req)
+		args->busy |= intel_ring_flag(obj->last_write_req->ring);
 
+unref:
 	drm_gem_object_unreference(&obj->base);
 unlock:
 	mutex_unlock(&dev->struct_mutex);
@@ -4441,8 +4546,11 @@ unlock:
 void i915_gem_object_init(struct drm_i915_gem_object *obj,
 			  const struct drm_i915_gem_object_ops *ops)
 {
+	int i;
+
 	INIT_LIST_HEAD(&obj->global_list);
-	INIT_LIST_HEAD(&obj->ring_list);
+	for (i = 0; i < I915_NUM_RINGS; i++)
+		INIT_LIST_HEAD(&obj->ring_list[i]);
 	INIT_LIST_HEAD(&obj->obj_exec_link);
 	INIT_LIST_HEAD(&obj->vma_list);
 	INIT_LIST_HEAD(&obj->batch_pool_link);
diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
index f3e84c44d009..18900f745bc6 100644
--- a/drivers/gpu/drm/i915/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/i915_gem_context.c
@@ -768,8 +768,6 @@ static int do_switch(struct intel_engine_cs *ring,
 		 * swapped, but there is no way to do that yet.
 		 */
 		from->legacy_hw_ctx.rcs_state->dirty = 1;
-		BUG_ON(i915_gem_request_get_ring(
-			from->legacy_hw_ctx.rcs_state->last_read_req) != ring);
 
 		/* obj is kept alive until the next request by its active ref */
 		i915_gem_object_ggtt_unpin(from->legacy_hw_ctx.rcs_state);
diff --git a/drivers/gpu/drm/i915/i915_gem_debug.c b/drivers/gpu/drm/i915/i915_gem_debug.c
index f462d1b51d97..17299d04189f 100644
--- a/drivers/gpu/drm/i915/i915_gem_debug.c
+++ b/drivers/gpu/drm/i915/i915_gem_debug.c
@@ -34,82 +34,34 @@ int
 i915_verify_lists(struct drm_device *dev)
 {
 	static int warned;
-	struct drm_i915_private *dev_priv = dev->dev_private;
+	struct drm_i915_private *dev_priv = to_i915(dev);
 	struct drm_i915_gem_object *obj;
+	struct intel_engine_cs *ring;
 	int err = 0;
+	int i;
 
 	if (warned)
 		return 0;
 
-	list_for_each_entry(obj, &dev_priv->render_ring.active_list, list) {
-		if (obj->base.dev != dev ||
-		    !atomic_read(&obj->base.refcount.refcount)) {
-			DRM_ERROR("freed render active %p\n", obj);
-			err++;
-			break;
-		} else if (!obj->active ||
-			   (obj->base.read_domains & I915_GEM_GPU_DOMAINS) == 0) {
-			DRM_ERROR("invalid render active %p (a %d r %x)\n",
-				  obj,
-				  obj->active,
-				  obj->base.read_domains);
-			err++;
-		} else if (obj->base.write_domain && list_empty(&obj->gpu_write_list)) {
-			DRM_ERROR("invalid render active %p (w %x, gwl %d)\n",
-				  obj,
-				  obj->base.write_domain,
-				  !list_empty(&obj->gpu_write_list));
-			err++;
-		}
-	}
-
-	list_for_each_entry(obj, &dev_priv->mm.flushing_list, list) {
-		if (obj->base.dev != dev ||
-		    !atomic_read(&obj->base.refcount.refcount)) {
-			DRM_ERROR("freed flushing %p\n", obj);
-			err++;
-			break;
-		} else if (!obj->active ||
-			   (obj->base.write_domain & I915_GEM_GPU_DOMAINS) == 0 ||
-			   list_empty(&obj->gpu_write_list)) {
-			DRM_ERROR("invalid flushing %p (a %d w %x gwl %d)\n",
-				  obj,
-				  obj->active,
-				  obj->base.write_domain,
-				  !list_empty(&obj->gpu_write_list));
-			err++;
-		}
-	}
-
-	list_for_each_entry(obj, &dev_priv->mm.gpu_write_list, gpu_write_list) {
-		if (obj->base.dev != dev ||
-		    !atomic_read(&obj->base.refcount.refcount)) {
-			DRM_ERROR("freed gpu write %p\n", obj);
-			err++;
-			break;
-		} else if (!obj->active ||
-			   (obj->base.write_domain & I915_GEM_GPU_DOMAINS) == 0) {
-			DRM_ERROR("invalid gpu write %p (a %d w %x)\n",
-				  obj,
-				  obj->active,
-				  obj->base.write_domain);
-			err++;
-		}
-	}
-
-	list_for_each_entry(obj, &i915_gtt_vm->inactive_list, list) {
-		if (obj->base.dev != dev ||
-		    !atomic_read(&obj->base.refcount.refcount)) {
-			DRM_ERROR("freed inactive %p\n", obj);
-			err++;
-			break;
-		} else if (obj->pin_count || obj->active ||
-			   (obj->base.write_domain & I915_GEM_GPU_DOMAINS)) {
-			DRM_ERROR("invalid inactive %p (p %d a %d w %x)\n",
-				  obj,
-				  obj->pin_count, obj->active,
-				  obj->base.write_domain);
-			err++;
+	for_each_ring(ring, dev_priv, i) {
+		list_for_each_entry(obj, &ring->active_list, ring_list[ring->id]) {
+			if (obj->base.dev != dev ||
+			    !atomic_read(&obj->base.refcount.refcount)) {
+				DRM_ERROR("%s: freed active obj %p\n",
+					  ring->name, obj);
+				err++;
+				break;
+			} else if (!obj->active ||
+				   obj->last_read_req[ring->id] == NULL) {
+				DRM_ERROR("%s: invalid active obj %p\n",
+					  ring->name, obj);
+				err++;
+			} else if (obj->base.write_domain) {
+				DRM_ERROR("%s: invalid write obj %p (w %x)\n",
+					  ring->name,
+					  obj, obj->base.write_domain);
+				err++;
+			}
 		}
 	}
 
diff --git a/drivers/gpu/drm/i915/i915_gpu_error.c b/drivers/gpu/drm/i915/i915_gpu_error.c
index 1d4e60df8883..5f798961266f 100644
--- a/drivers/gpu/drm/i915/i915_gpu_error.c
+++ b/drivers/gpu/drm/i915/i915_gpu_error.c
@@ -192,15 +192,20 @@ static void print_error_buffers(struct drm_i915_error_state_buf *m,
 				struct drm_i915_error_buffer *err,
 				int count)
 {
+	int i;
+
 	err_printf(m, "  %s [%d]:\n", name, count);
 
 	while (count--) {
-		err_printf(m, "    %08x %8u %02x %02x %x %x",
+		err_printf(m, "    %08x %8u %02x %02x [ ",
 			   err->gtt_offset,
 			   err->size,
 			   err->read_domains,
-			   err->write_domain,
-			   err->rseqno, err->wseqno);
+			   err->write_domain);
+		for (i = 0; i < I915_NUM_RINGS; i++)
+			err_printf(m, "%02x ", err->rseqno[i]);
+
+		err_printf(m, "] %02x", err->wseqno);
 		err_puts(m, pin_flag(err->pinned));
 		err_puts(m, tiling_flag(err->tiling));
 		err_puts(m, dirty_flag(err->dirty));
@@ -679,10 +684,12 @@ static void capture_bo(struct drm_i915_error_buffer *err,
 		       struct i915_vma *vma)
 {
 	struct drm_i915_gem_object *obj = vma->obj;
+	int i;
 
 	err->size = obj->base.size;
 	err->name = obj->base.name;
-	err->rseqno = i915_gem_request_get_seqno(obj->last_read_req);
+	for (i = 0; i < I915_NUM_RINGS; i++)
+		err->rseqno[i] = i915_gem_request_get_seqno(obj->last_read_req[i]);
 	err->wseqno = i915_gem_request_get_seqno(obj->last_write_req);
 	err->gtt_offset = vma->node.start;
 	err->read_domains = obj->base.read_domains;
@@ -695,8 +702,8 @@ static void capture_bo(struct drm_i915_error_buffer *err,
 	err->dirty = obj->dirty;
 	err->purgeable = obj->madv != I915_MADV_WILLNEED;
 	err->userptr = obj->userptr.mm != NULL;
-	err->ring = obj->last_read_req ?
-			i915_gem_request_get_ring(obj->last_read_req)->id : -1;
+	err->ring = obj->last_write_req ?
+			i915_gem_request_get_ring(obj->last_write_req)->id : -1;
 	err->cache_level = obj->cache_level;
 }
 
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 5eb159bcd599..64b67df94d33 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -9895,7 +9895,7 @@ static bool use_mmio_flip(struct intel_engine_cs *ring,
 	else if (i915.enable_execlists)
 		return true;
 	else
-		return ring != i915_gem_request_get_ring(obj->last_read_req);
+		return ring != i915_gem_request_get_ring(obj->last_write_req);
 }
 
 static void skl_do_mmio_flip(struct intel_crtc *intel_crtc)
@@ -10199,7 +10199,7 @@ static int intel_crtc_page_flip(struct drm_crtc *crtc,
 	} else if (IS_IVYBRIDGE(dev) || IS_HASWELL(dev)) {
 		ring = &dev_priv->ring[BCS];
 	} else if (INTEL_INFO(dev)->gen >= 7) {
-		ring = i915_gem_request_get_ring(obj->last_read_req);
+		ring = i915_gem_request_get_ring(obj->last_write_req);
 		if (ring == NULL || ring->id != RCS)
 			ring = &dev_priv->ring[BCS];
 	} else {
diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 1d0fb8450adc..fb4f3792fd78 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -901,6 +901,7 @@ static int logical_ring_wait_request(struct intel_ringbuffer *ringbuf,
 {
 	struct intel_engine_cs *ring = ringbuf->ring;
 	struct drm_i915_gem_request *request;
+	unsigned space;
 	int ret;
 
 	if (intel_ring_space(ringbuf) >= bytes)
@@ -912,15 +913,14 @@ static int logical_ring_wait_request(struct intel_ringbuffer *ringbuf,
 		 * from multiple ringbuffers. Here, we must ignore any that
 		 * aren't from the ringbuffer we're considering.
 		 */
-		struct intel_context *ctx = request->ctx;
-		if (ctx->engine[ring->id].ringbuf != ringbuf)
+		if (request->ringbuf != ringbuf)
 			continue;
 
 		/* Would completion of this request free enough space? */
-		if (__intel_ring_space(request->tail, ringbuf->tail,
-				       ringbuf->size) >= bytes) {
+		space = __intel_ring_space(request->tail, ringbuf->tail,
+					   ringbuf->size);
+		if (space >= bytes)
 			break;
-		}
 	}
 
 	if (&request->list == &ring->request_list)
@@ -930,9 +930,8 @@ static int logical_ring_wait_request(struct intel_ringbuffer *ringbuf,
 	if (ret)
 		return ret;
 
-	i915_gem_retire_requests_ring(ring);
-
-	return intel_ring_space(ringbuf) >= bytes ? 0 : -ENOSPC;
+	ringbuf->space = bytes;
+	return 0;
 }
 
 static int logical_ring_wait_for_space(struct intel_ringbuffer *ringbuf,
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index a351178913f7..a1184e700d1d 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -2061,16 +2061,17 @@ static int intel_ring_wait_request(struct intel_engine_cs *ring, int n)
 {
 	struct intel_ringbuffer *ringbuf = ring->buffer;
 	struct drm_i915_gem_request *request;
+	unsigned space;
 	int ret;
 
 	if (intel_ring_space(ringbuf) >= n)
 		return 0;
 
 	list_for_each_entry(request, &ring->request_list, list) {
-		if (__intel_ring_space(request->postfix, ringbuf->tail,
-				       ringbuf->size) >= n) {
+		space = __intel_ring_space(request->postfix, ringbuf->tail,
+					   ringbuf->size);
+		if (space >= n)
 			break;
-		}
 	}
 
 	if (&request->list == &ring->request_list)
@@ -2080,8 +2081,7 @@ static int intel_ring_wait_request(struct intel_engine_cs *ring, int n)
 	if (ret)
 		return ret;
 
-	i915_gem_retire_requests_ring(ring);
-
+	ringbuf->space = space;
 	return 0;
 }
 
-- 
2.1.4

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