[Intel-gfx] [PATCH v2 3/3] drm/i915/gt: Eliminate the trylock for reading a timeline's hwsp

[Chris Wilson]

As we stash a pointer to the HWSP cacheline on the request, when reading
it we only need confirm that the cacheline is still valid by checking
that the request and timeline are still intact.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
---
 drivers/gpu/drm/i915/gt/intel_timeline.c | 38 ++++++++----------------
 1 file changed, 13 insertions(+), 25 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_timeline.c b/drivers/gpu/drm/i915/gt/intel_timeline.c
index d71aafb66d6e..e852bd142ddf 100644
--- a/drivers/gpu/drm/i915/gt/intel_timeline.c
+++ b/drivers/gpu/drm/i915/gt/intel_timeline.c
@@ -514,6 +514,7 @@ int intel_timeline_read_hwsp(struct i915_request *from,
 			     struct i915_request *to,
 			     u32 *hwsp)
 {
+	struct intel_timeline_cacheline *cl = from->hwsp_cacheline;
 	struct intel_timeline *tl;
 	int err;
 
@@ -526,33 +527,20 @@ int intel_timeline_read_hwsp(struct i915_request *from,
 		return 1;
 
 	GEM_BUG_ON(rcu_access_pointer(to->timeline) == tl);
-
-	err = -EAGAIN;
-	if (mutex_trylock(&tl->mutex)) {
-		struct intel_timeline_cacheline *cl = from->hwsp_cacheline;
-
-		if (i915_request_completed(from)) {
-			err = 1;
-			goto unlock;
-		}
-
-		err = cacheline_ref(cl, to);
-		if (err)
-			goto unlock;
-
-		if (likely(cl == tl->hwsp_cacheline)) {
-			*hwsp = tl->hwsp_offset;
-		} else { /* across a seqno wrap, recover the original offset */
-			*hwsp = i915_ggtt_offset(cl->hwsp->vma) +
-				ptr_unmask_bits(cl->vaddr, CACHELINE_BITS) *
-				CACHELINE_BYTES;
-		}
-
-unlock:
-		mutex_unlock(&tl->mutex);
+	err = cacheline_ref(cl, to);
+	if (err)
+		goto out;
+
+	*hwsp = tl->hwsp_offset;
+	if (unlikely(cl != READ_ONCE(tl->hwsp_cacheline))) {
+		/* across a seqno wrap, recover the original offset */
+		*hwsp = i915_ggtt_offset(cl->hwsp->vma) +
+			ptr_unmask_bits(cl->vaddr, CACHELINE_BITS) *
+			CACHELINE_BYTES;
 	}
-	intel_timeline_put(tl);
 
+out:
+	intel_timeline_put(tl);
 	return err;
 }
 
-- 
2.24.0