[Intel-gfx] [PATCH] drm/i915: Improve the comment on ring_sync after seqno wrap

[Mika Kuoppala]

There are quite alot of nasty things happening inside the
seemingly bening intel_ring_begin() if seqno wrap happens.
Warn the reader about it.

Signed-off-by: Mika Kuoppala <mika.kuoppala at intel.com>
---
 drivers/gpu/drm/i915/intel_ringbuffer.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index d897a19..f99e2ae 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -727,7 +727,13 @@ gen6_ring_sync(struct intel_ring_buffer *waiter,
 	if (ret)
 		return ret;
 
-	/* If seqno wrap happened, omit the wait with no-ops */
+	/* Seqno allocation is inside intel_ring_begin() so the seqno
+	 * wrap (re init of driver seqno counter) might have happened.
+	 * This means that the seqno we are syncing to, just became
+	 * irrelevant in this brand new seqno universe. And as it does
+	 * not exists anymore, waiting for it to happen is unwise.
+	 * If this is the case, emit no-ops instead.
+	 */
 	if (likely(!i915_gem_has_seqno_wrapped(waiter->dev, seqno))) {
 		intel_ring_emit(waiter,
 				dw1 |
-- 
1.7.9.5