drm/i915/pmu: Fix KMD and GuC race on accessing busyness

GuC updates shared memory and KMD reads it. Since this is not
synchronized, we run into a race where the value read is inconsistent.
Sometimes the inconsistency is in reading the upper MSB bytes of the
last_switch_in value. 2 types of cases are seen - upper 8 bits are zero
and upper 24 bits are zero. Since these are non-zero values, it is
not trivial to determine validity of these values. Instead we read the
values multiple times until they are consistent. In test runs, 3
attempts results in consistent values. The upper bound is set to 6
attempts and may need to be tuned as per any new occurences.

Since the duration that gt is parked can vary, the patch also updates
the gt timestamp on unpark before starting the worker.

v2:
- Initialize i
- Use READ_ONCE to access engine record

Fixes: 77cdd054 ("drm/i915/pmu: Connect engine busyness stats from GuC to pmu")
Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Alan Previn <alan.previn.teres.alexis@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220125020124.788679-2-umesh.nerlige.ramappa@intel.com

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

@@ -1113,6 +1113,19 @@ __extend_last_switch(struct intel_guc *guc, u64 *prev_start, u32 new_start)
 	if (new_start == lower_32_bits(*prev_start))
 		return;
 
+	/*
+	 * When gt is unparked, we update the gt timestamp and start the ping
+	 * worker that updates the gt_stamp every POLL_TIME_CLKS. As long as gt
+	 * is unparked, all switched in contexts will have a start time that is
+	 * within +/- POLL_TIME_CLKS of the most recent gt_stamp.
+	 *
+	 * If neither gt_stamp nor new_start has rolled over, then the
+	 * gt_stamp_hi does not need to be adjusted, however if one of them has
+	 * rolled over, we need to adjust gt_stamp_hi accordingly.
+	 *
+	 * The below conditions address the cases of new_start rollover and
+	 * gt_stamp_last rollover respectively.
+	 */
 	if (new_start < gt_stamp_last &&
 	    (new_start - gt_stamp_last) <= POLL_TIME_CLKS)
 		gt_stamp_hi++;
@@ -1124,17 +1137,45 @@ __extend_last_switch(struct intel_guc *guc, u64 *prev_start, u32 new_start)
 	*prev_start = ((u64)gt_stamp_hi << 32) | new_start;
 }
 
-static void guc_update_engine_gt_clks(struct intel_engine_cs *engine)
+/*
+ * GuC updates shared memory and KMD reads it. Since this is not synchronized,
+ * we run into a race where the value read is inconsistent. Sometimes the
+ * inconsistency is in reading the upper MSB bytes of the last_in value when
+ * this race occurs. 2 types of cases are seen - upper 8 bits are zero and upper
+ * 24 bits are zero. Since these are non-zero values, it is non-trivial to
+ * determine validity of these values. Instead we read the values multiple times
+ * until they are consistent. In test runs, 3 attempts results in consistent
+ * values. The upper bound is set to 6 attempts and may need to be tuned as per
+ * any new occurences.
+ */
+static void __get_engine_usage_record(struct intel_engine_cs *engine,
+				      u32 *last_in, u32 *id, u32 *total)
 {
 	struct guc_engine_usage_record *rec = intel_guc_engine_usage(engine);
+	int i = 0;
+
+	do {
+		*last_in = READ_ONCE(rec->last_switch_in_stamp);
+		*id = READ_ONCE(rec->current_context_index);
+		*total = READ_ONCE(rec->total_runtime);
+
+		if (READ_ONCE(rec->last_switch_in_stamp) == *last_in &&
+		    READ_ONCE(rec->current_context_index) == *id &&
+		    READ_ONCE(rec->total_runtime) == *total)
+			break;
+	} while (++i < 6);
+}
+
+static void guc_update_engine_gt_clks(struct intel_engine_cs *engine)
+{
 	struct intel_engine_guc_stats *stats = &engine->stats.guc;
 	struct intel_guc *guc = &engine->gt->uc.guc;
-	u32 last_switch = rec->last_switch_in_stamp;
-	u32 ctx_id = rec->current_context_index;
-	u32 total = rec->total_runtime;
+	u32 last_switch, ctx_id, total;
 
 	lockdep_assert_held(&guc->timestamp.lock);
 
+	__get_engine_usage_record(engine, &last_switch, &ctx_id, &total);
+
 	stats->running = ctx_id != ~0U && last_switch;
 	if (stats->running)
 		__extend_last_switch(guc, &stats->start_gt_clk, last_switch);
@@ -1236,6 +1277,10 @@ static ktime_t guc_engine_busyness(struct intel_engine_cs *engine, ktime_t *now)
 	if (!in_reset && intel_gt_pm_get_if_awake(gt)) {
 		stats_saved = *stats;
 		gt_stamp_saved = guc->timestamp.gt_stamp;
+		/*
+		 * Update gt_clks, then gt timestamp to simplify the 'gt_stamp -
+		 * start_gt_clk' calculation below for active engines.
+		 */
 		guc_update_engine_gt_clks(engine);
 		guc_update_pm_timestamp(guc, now);
 		intel_gt_pm_put_async(gt);
@@ -1364,10 +1409,15 @@ void intel_guc_busyness_park(struct intel_gt *gt)
 void intel_guc_busyness_unpark(struct intel_gt *gt)
 {
 	struct intel_guc *guc = &gt->uc.guc;
+	unsigned long flags;
+	ktime_t unused;
 
 	if (!guc_submission_initialized(guc))
 		return;
 
+	spin_lock_irqsave(&guc->timestamp.lock, flags);
+	guc_update_pm_timestamp(guc, &unused);
+	spin_unlock_irqrestore(&guc->timestamp.lock, flags);
 	mod_delayed_work(system_highpri_wq, &guc->timestamp.work,
 			 guc->timestamp.ping_delay);
 }