[Intel-gfx] [PATCH] drm/i915/pmu: Fix synchronization of PMU callback with reset

[Umesh Nerlige Ramappa]

Since the PMU callback runs in irq context, it synchronizes with gt
reset using the reset count. We could run into a case where the PMU
callback could read the reset count before it is updated. This has a
potential of corrupting the busyness stats.

In addition to the reset count, check if the reset bit is set before
capturing busyness.

In addition save the previous stats only if you intend to update them.

Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa at intel.com>
---
 drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
index 5cc49c0b3889..d83ade77ca07 100644
--- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
+++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
@@ -1183,6 +1183,7 @@ static ktime_t guc_engine_busyness(struct intel_engine_cs *engine, ktime_t *now)
 	u64 total, gt_stamp_saved;
 	unsigned long flags;
 	u32 reset_count;
+	bool in_reset;
 
 	spin_lock_irqsave(&guc->timestamp.lock, flags);
 
@@ -1191,7 +1192,9 @@ static ktime_t guc_engine_busyness(struct intel_engine_cs *engine, ktime_t *now)
 	 * engine busyness from GuC, so we just use the driver stored
 	 * copy of busyness. Synchronize with gt reset using reset_count.
 	 */
-	reset_count = i915_reset_count(gpu_error);
+	rcu_read_lock();
+	in_reset = test_bit(I915_RESET_BACKOFF, &gt->reset.flags);
+	rcu_read_unlock();
 
 	*now = ktime_get();
 
@@ -1201,9 +1204,10 @@ static ktime_t guc_engine_busyness(struct intel_engine_cs *engine, ktime_t *now)
 	 * start_gt_clk is derived from GuC state. To get a consistent
 	 * view of activity, we query the GuC state only if gt is awake.
 	 */
-	stats_saved = *stats;
-	gt_stamp_saved = guc->timestamp.gt_stamp;
-	if (intel_gt_pm_get_if_awake(gt)) {
+	if (intel_gt_pm_get_if_awake(gt) && !in_reset) {
+		stats_saved = *stats;
+		gt_stamp_saved = guc->timestamp.gt_stamp;
+		reset_count = i915_reset_count(gpu_error);
 		guc_update_engine_gt_clks(engine);
 		guc_update_pm_timestamp(guc, engine, now);
 		intel_gt_pm_put_async(gt);
-- 
2.20.1