i915/guc: Ensure busyness counter increases motonically

Active busyness of an engine is calculated using gt timestamp and the
context switch in time. While capturing the gt timestamp, it's possible
that the context switches out. This race could result in an active
busyness value that is greater than the actual context runtime value by a
small amount. This leads to a negative delta and throws off busyness
calculations for the user.

If a subsequent count is smaller than the previous one, just return the
previous one, since we expect the busyness to catch up.

Fixes: 77cdd05 ("drm/i915/pmu: Connect engine busyness stats from GuC to pmu")
Signed-off-by: Umesh Nerlige Ramappa <[email protected]>
Reviewed-by: John Harrison <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

Author: unerlige

drivers/gpu/drm/i915/gt/intel_engine_types.h

@@ -343,6 +343,11 @@ struct intel_engine_guc_stats {
 	 * @start_gt_clk: GT clock time of last idle to active transition.
 	 */
 	u64 start_gt_clk;
+
+	/**
+	 * @total: The last value of total returned
+	 */
+	u64 total;
 };
 
 union intel_engine_tlb_inv_reg {

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

@@ -1378,9 +1378,12 @@ static ktime_t guc_engine_busyness(struct intel_engine_cs *engine, ktime_t *now)
 		total += intel_gt_clock_interval_to_ns(gt, clk);
 	}
 
+	if (total > stats->total)
+		stats->total = total;
+
 	spin_unlock_irqrestore(&guc->timestamp.lock, flags);
 
-	return ns_to_ktime(total);
+	return ns_to_ktime(stats->total);
 }
 
 static void guc_enable_busyness_worker(struct intel_guc *guc)