[Intel-gfx] [PATCH] drm/i915/selftest: Synchronise with the GPU timestamp

[Chris Wilson]

Wait for the GPU to wake up from the semaphore before measuring the
time, so that we coordinate the sampling on both the CPU and GPU for
more accurate comparisons.

Reported-by: Bruce Chang <yu.bruce.chang at intel.com>
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: CQ Tang <cq.tang at intel.com>
---
 drivers/gpu/drm/i915/gt/selftest_engine_pm.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/gt/selftest_engine_pm.c b/drivers/gpu/drm/i915/gt/selftest_engine_pm.c
index 3ce8cb3329f3..007a7c790778 100644
--- a/drivers/gpu/drm/i915/gt/selftest_engine_pm.c
+++ b/drivers/gpu/drm/i915/gt/selftest_engine_pm.c
@@ -111,8 +111,10 @@ static int __measure_timestamps(struct intel_context *ce,
 
 	/* Run the request for a 100us, sampling timestamps before/after */
 	preempt_disable();
-	*dt = local_clock();
 	write_semaphore(&sema[2], 0);
+	while (READ_ONCE(sema[1]) == 0) /* wait for the gpu to catch up */
+		cpu_relax();
+	*dt = local_clock();
 	udelay(100);
 	*dt = local_clock() - *dt;
 	write_semaphore(&sema[2], 1);
-- 
2.20.1