[PATCH 1/2] RFT drm/i915/execlists: Flush memory before signaling ELSQ

[Chris Wilson]

We observe that the ordering of writes for a CS event is not as strong
from the GPU as we would like, and that on occasions we see the
ringbuffer tail updated before the event is written into the ringbuffer,
leading us to reuse the stale data.

Through around a big hammer to try and batter ELSQ into submission with
the presumption that perhaps the UC mmio write is not flushing our
writes into the context images.

References: https://bugs.freedesktop.org/show_bug.cgi?id=108315
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/intel_lrc.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 22b57b8926fc..ba61849fbb9b 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -454,8 +454,10 @@ static void execlists_submit_ports(struct intel_engine_cs *engine)
 	}
 
 	/* we need to manually load the submit queue */
-	if (execlists->ctrl_reg)
+	if (execlists->ctrl_reg) {
+		wmb(); /* XXX Big hammer or paper? XXX */
 		writel(EL_CTRL_LOAD, execlists->ctrl_reg);
+	}
 
 	execlists_clear_active(execlists, EXECLISTS_ACTIVE_HWACK);
 }
-- 
2.19.1