[PATCH 15/15] drm/nouveau: Skip fence context seqno check if notifies are enabled

[Chris Wilson]

If the fence context has notifies enabled, each of the fences'
FENCE_FLAG_SIGNALED_BIT will be updated from the interrupt. We can rely
on this status for reporting the current fence_is_signaled() and so
avoid an expensive uncached read.

v2: Keep the uncached read after first enabling the notify.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Ben Skeggs <bskeggs at redhat.com>
Cc: dri-devel at lists.freedesktop.org
Cc: nouveau at lists.freedesktop.org
---
 drivers/gpu/drm/nouveau/nouveau_fence.c | 29 ++++++++++++++++++++---------
 1 file changed, 20 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c b/drivers/gpu/drm/nouveau/nouveau_fence.c
index 4bb9ab892ae1..e4d6e478cc44 100644
--- a/drivers/gpu/drm/nouveau/nouveau_fence.c
+++ b/drivers/gpu/drm/nouveau/nouveau_fence.c
@@ -510,6 +510,16 @@ static bool nouveau_fence_is_signaled(struct fence *f)
 	struct nouveau_channel *chan;
 	bool ret = false;
 
+	/* If the notify is already active, we can rely on
+	 * FENCE_FLAG_SIGNALED_BIT being accurate and skip checking the
+	 * current seqno as fctx->read() requires an expensive uncached read.
+	 */
+	if (test_bit(FENCE_FLAG_USER_BITS, &fence->base.flags))
+		return false;
+
+	if (READ_ONCE(fctx->notify_ref))
+		return false;
+
 	rcu_read_lock();
 	chan = rcu_dereference(fence->channel);
 	if (chan)
@@ -566,18 +576,19 @@ static bool nouveau_fence_enable_signaling(struct fence *f)
 {
 	struct nouveau_fence *fence = from_fence(f);
 	struct nouveau_fence_chan *fctx = nouveau_fctx(fence);
-	bool ret;
 
-	if (!fctx->notify_ref++)
+	if (!fctx->notify_ref) {
 		nvif_notify_get(&fctx->notify);
+		/* Check for a missed signal when installing the interrupt */
+		if (!nouveau_fence_no_signaling(f)) {
+			nvif_notify_put(&fctx->notify);
+			return false;
+		}
+	}
 
-	ret = nouveau_fence_no_signaling(f);
-	if (ret)
-		set_bit(FENCE_FLAG_USER_BITS, &fence->base.flags);
-	else if (!--fctx->notify_ref)
-		nvif_notify_put(&fctx->notify);
-
-	return ret;
+	fctx->notify_ref++;
+	set_bit(FENCE_FLAG_USER_BITS, &fence->base.flags);
+	return true;
 }
 
 static const struct fence_ops nouveau_fence_ops_uevent = {
-- 
2.9.3