[Intel-gfx] [CI 5/5] drm/i915/gt: Reduce GT runtime stats from seqlock to a latch

[Chris Wilson]

Since we can compute the elapsed time to add to the total, during the
PMU sample we only need to have a consistent view of the (start, total,
active) tuple to be able to locally determine the runtime. That can be
arrange by a pair of memory bariiers and carefully sequencing of the
writes and reads.

Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
Reviewed-by: Andi Shyti <andi.sh...@intel.com>
---
 drivers/gpu/drm/i915/gt/intel_gt_pm.c    | 38 +++++++-----------------
 drivers/gpu/drm/i915/gt/intel_gt_types.h |  5 ----
 2 files changed, 10 insertions(+), 33 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_gt_pm.c 
b/drivers/gpu/drm/i915/gt/intel_gt_pm.c
index c94e8ac884eb..fb8be4e1f63e 100644
--- a/drivers/gpu/drm/i915/gt/intel_gt_pm.c
+++ b/drivers/gpu/drm/i915/gt/intel_gt_pm.c
@@ -41,24 +41,18 @@ static void user_forcewake(struct intel_gt *gt, bool 
suspend)
 
 static void runtime_begin(struct intel_gt *gt)
 {
-       local_irq_disable();
-       write_seqcount_begin(&gt->stats.lock);
        gt->stats.start = ktime_get();
-       gt->stats.active = true;
-       write_seqcount_end(&gt->stats.lock);
-       local_irq_enable();
+       smp_wmb(); /* pairs with intel_gt_get_busy_time() */
+       WRITE_ONCE(gt->stats.active, true);
 }
 
 static void runtime_end(struct intel_gt *gt)
 {
-       local_irq_disable();
-       write_seqcount_begin(&gt->stats.lock);
-       gt->stats.active = false;
        gt->stats.total =
                ktime_add(gt->stats.total,
                          ktime_sub(ktime_get(), gt->stats.start));
-       write_seqcount_end(&gt->stats.lock);
-       local_irq_enable();
+       smp_wmb(); /* pairs with intel_gt_get_busy_time() */
+       WRITE_ONCE(gt->stats.active, false);
 }
 
 static int __gt_unpark(struct intel_wakeref *wf)
@@ -130,7 +124,6 @@ static const struct intel_wakeref_ops wf_ops = {
 void intel_gt_pm_init_early(struct intel_gt *gt)
 {
        intel_wakeref_init(&gt->wakeref, gt->uncore->rpm, &wf_ops);
-       seqcount_mutex_init(&gt->stats.lock, &gt->wakeref.mutex);
 }
 
 void intel_gt_pm_init(struct intel_gt *gt)
@@ -364,26 +357,15 @@ int intel_gt_runtime_resume(struct intel_gt *gt)
        return intel_uc_runtime_resume(&gt->uc);
 }
 
-static ktime_t __intel_gt_get_awake_time(const struct intel_gt *gt)
-{
-       ktime_t total = gt->stats.total;
-
-       if (gt->stats.active)
-               total = ktime_add(total,
-                                 ktime_sub(ktime_get(), gt->stats.start));
-
-       return total;
-}
-
 ktime_t intel_gt_get_awake_time(const struct intel_gt *gt)
 {
-       unsigned int seq;
-       ktime_t total;
+       ktime_t total = gt->stats.total;
+       ktime_t start = gt->stats.start;
 
-       do {
-               seq = read_seqcount_begin(&gt->stats.lock);
-               total = __intel_gt_get_awake_time(gt);
-       } while (read_seqcount_retry(&gt->stats.lock, seq));
+       if (READ_ONCE(gt->stats.active)) {
+               smp_rmb(); /* pairs with runtime_begin/end */
+               total = ktime_add(total, ktime_sub(ktime_get(), start));
+       }
 
        return total;
 }
diff --git a/drivers/gpu/drm/i915/gt/intel_gt_types.h 
b/drivers/gpu/drm/i915/gt/intel_gt_types.h
index a83d3e18254d..4205c3f022ad 100644
--- a/drivers/gpu/drm/i915/gt/intel_gt_types.h
+++ b/drivers/gpu/drm/i915/gt/intel_gt_types.h
@@ -91,11 +91,6 @@ struct intel_gt {
        struct {
                bool active;
 
-               /**
-                * @lock: Lock protecting the below fields.
-                */
-               seqcount_mutex_t lock;
-
                /**
                 * @total: Total time this engine was busy.
                 *
-- 
2.20.1

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