[Intel-gfx] [PATCH v12 3/3] drm/i915: peel dma-fence-chains wait fences

[Lionel Landwerlin]

To allow faster engine to engine synchronization, peel the layer of
dma-fence-chain to expose potential i915 fences so that the
i915-request code can emit HW semaphore wait/signal operations in the
ring which is faster than waking up the host to submit unblocked
workloads after interrupt notification.

v2: Also deal with chains where the last node is not a dma-fence-chain

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
---
 .../gpu/drm/i915/gem/i915_gem_execbuffer.c    | 39 ++++++++++++++++++-
 1 file changed, 38 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
index d8814e637e71..3ffd95d1dc2c 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
@@ -2403,6 +2403,7 @@ await_fence_array(struct i915_execbuffer *eb)
 
 	for (n = 0; n < eb->n_fences; n++) {
 		struct drm_syncobj *syncobj;
+		struct dma_fence_chain *chain;
 		struct dma_fence *fence;
 		unsigned int flags;
 
@@ -2423,7 +2424,43 @@ await_fence_array(struct i915_execbuffer *eb)
 				continue;
 		}
 
-		err = i915_request_await_dma_fence(eb->request, fence);
+		chain = to_dma_fence_chain(fence);
+		if (chain) {
+			struct dma_fence *iter;
+
+			/*
+			 * If we're dealing with a dma-fence-chain, peel the
+			 * chain by adding all of the unsignaled fences
+			 * (dma_fence_chain_for_each does that for us) the
+			 * chain points to.
+			 *
+			 * This enables us to identify waits on i915 fences
+			 * and allows for faster engine-to-engine
+			 * synchronization using HW semaphores.
+			 */
+			dma_fence_chain_for_each(iter, fence) {
+				struct dma_fence_chain *iter_chain =
+					to_dma_fence_chain(iter);
+
+				/*
+				 * It is possible that the last item in the
+				 * chain is not a dma_fence_chain.
+				 */
+				if (iter_chain) {
+					err = i915_request_await_dma_fence(eb->request,
+									   iter_chain->fence);
+				} else {
+					err = i915_request_await_dma_fence(eb->request, iter);
+				}
+				if (err < 0) {
+					dma_fence_put(iter);
+					break;
+				}
+			}
+		} else {
+			err = i915_request_await_dma_fence(eb->request, fence);
+		}
+
 		dma_fence_put(fence);
 		if (err < 0)
 			return err;
-- 
2.27.0