[PATCH 2/3] i915/guc: Ensure busyness counter increases motonically

[Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>]

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: 77cdd054dd2c ("drm/i915/pmu: Connect engine busyness stats from GuC to pmu")
Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
---
 drivers/gpu/drm/i915/gt/intel_engine_types.h      | 5 +++++
 drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c | 5 ++++-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_engine_types.h b/drivers/gpu/drm/i915/gt/intel_engine_types.h
index ba55c059063d..fe1f85e5dda3 100644
--- a/drivers/gpu/drm/i915/gt/intel_engine_types.h
+++ b/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 {
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 c71aedcbce43..56be9f385270 100644
--- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
+++ b/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)
-- 
2.34.1