drm/i915/pmu: Fix synchronization of PMU callback with reset

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.

v2:
- The 2 reset counts captured in the PMU callback can end up being the
same if they were captured right after the count is incremented in the
reset flow. This can lead to a bad busyness state. Ensure that reset
is not in progress when the initial reset count is captured.

Signed-off-by: Umesh Nerlige Ramappa <[e-mail]>
Reviewed-by: Matthew Brost <[e-mail]>
Signed-off-by: John Harrison <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c

···
 	u64 total, gt_stamp_saved;
 	unsigned long flags;
 	u32 reset_count;
+	bool in_reset;
 
 	spin_lock_irqsave(&guc->timestamp.lock, flags);
 
 	/*
-	 * If a reset happened, we risk reading partially updated
-	 * engine busyness from GuC, so we just use the driver stored
-	 * copy of busyness. Synchronize with gt reset using reset_count.
+	 * If a reset happened, we risk reading partially updated engine
+	 * busyness from GuC, so we just use the driver stored copy of busyness.
+	 * Synchronize with gt reset using reset_count and the
+	 * I915_RESET_BACKOFF flag. Note that reset flow updates the reset_count
+	 * after I915_RESET_BACKOFF flag, so ensure that the reset_count is
+	 * usable by checking the flag afterwards.
 	 */
 	reset_count = i915_reset_count(gpu_error);
+	in_reset = test_bit(I915_RESET_BACKOFF, &gt->reset.flags);
 
 	*now = ktime_get();
 
···
 	 * 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;
 		guc_update_engine_gt_clks(engine);
 		guc_update_pm_timestamp(guc, engine, now);
 		intel_gt_pm_put_async(gt);