[PATCH] drm/i915/gt: Add a delay to let engine resumes correctly

[Nitin Gote]

Sometimes engine reset fails because the engine resumes from an
incorrect RING_HEAD. Engine head failed to set to zero even after
writing into it. This is a timing issue and we experimented
different values and found out that 20ms delay works best based
on testing.

So, add a 20ms delay to let engine resumes from correct RING_HEAD.

Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/13650
Signed-off-by: Nitin Gote <nitin.r.gote at intel.com>
---
 drivers/gpu/drm/i915/gt/intel_ring_submission.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/gpu/drm/i915/gt/intel_ring_submission.c b/drivers/gpu/drm/i915/gt/intel_ring_submission.c
index 6e9977b2d180..5576f000e965 100644
--- a/drivers/gpu/drm/i915/gt/intel_ring_submission.c
+++ b/drivers/gpu/drm/i915/gt/intel_ring_submission.c
@@ -365,6 +365,13 @@ static void reset_prepare(struct intel_engine_cs *engine)
 			     ENGINE_READ_FW(engine, RING_HEAD),
 			     ENGINE_READ_FW(engine, RING_TAIL),
 			     ENGINE_READ_FW(engine, RING_START));
+		/*
+		 * Sometimes engine head failed to set to zero even after writing into it.
+		 * Use 20ms delay to let engine resumes from correct RING_HEAD.
+		 * Experimented different values and determined that 20ms works best
+		 * based on testing.
+		 */
+		mdelay(20);
 		if (!stop_ring(engine)) {
 			drm_err(&engine->i915->drm,
 				"failed to set %s head to zero "
-- 
2.25.1