[Intel-gfx] [PATCH 1/2] drm/i915/selftest: Fix engine timestamp and ktime disparity

[Anshuman Gupta]

While reading the engine timestamps there can be uncontrollable
concurrent mmio access via other i915 child drivers and by GuC,
which is not truly atomic context as expected by this selftest,
which may cause mmio latency to read the engine timestamps,
Account such latency to calculate time to read engine timestamp
such that selftest can validate the timestamp and ktime pair.

Cc: Chris Wilson <chris.p.wilson at intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta at intel.com>
---
 drivers/gpu/drm/i915/gt/selftest_gt_pm.c | 2 +-
 drivers/gpu/drm/i915/gt/selftest_rps.c   | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/selftest_gt_pm.c b/drivers/gpu/drm/i915/gt/selftest_gt_pm.c
index b46425aeb2f0..0971241707ce 100644
--- a/drivers/gpu/drm/i915/gt/selftest_gt_pm.c
+++ b/drivers/gpu/drm/i915/gt/selftest_gt_pm.c
@@ -63,8 +63,8 @@ static void measure_clocks(struct intel_engine_cs *engine,
 
 		udelay(1000);
 
-		dt[i] = ktime_sub(ktime_get(), dt[i]);
 		cycles[i] += read_timestamp(engine);
+		dt[i] = ktime_sub(ktime_get(), dt[i]);
 		local_irq_enable();
 	}
 
diff --git a/drivers/gpu/drm/i915/gt/selftest_rps.c b/drivers/gpu/drm/i915/gt/selftest_rps.c
index 6755bbc4ebda..c0cc0dd78c7c 100644
--- a/drivers/gpu/drm/i915/gt/selftest_rps.c
+++ b/drivers/gpu/drm/i915/gt/selftest_rps.c
@@ -299,13 +299,13 @@ int live_rps_clock_interval(void *arg)
 			for (i = 0; i < 5; i++) {
 				preempt_disable();
 
-				dt_[i] = ktime_get();
 				cycles_[i] = -intel_uncore_read_fw(gt->uncore, GEN6_RP_CUR_UP_EI);
+				dt_[i] = ktime_get();
 
 				udelay(1000);
 
-				dt_[i] = ktime_sub(ktime_get(), dt_[i]);
 				cycles_[i] += intel_uncore_read_fw(gt->uncore, GEN6_RP_CUR_UP_EI);
+				dt_[i] = ktime_sub(ktime_get(), dt_[i]);
 
 				preempt_enable();
 			}
-- 
2.25.1