[Intel-gfx] [CI 1/2] 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    | 51 +++++++++---------------
 drivers/gpu/drm/i915/gt/intel_gt_types.h |  7 ----
 2 files changed, 18 insertions(+), 40 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_gt_pm.c 
b/drivers/gpu/drm/i915/gt/intel_gt_pm.c
index aef3084e8b16..0bd303d2823e 100644
--- a/drivers/gpu/drm/i915/gt/intel_gt_pm.c
+++ b/drivers/gpu/drm/i915/gt/intel_gt_pm.c
@@ -40,24 +40,20 @@ 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.start, ktime_get());
 }
 
 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();
+       ktime_t total;
+
+       total = ktime_sub(ktime_get(), gt->stats.start);
+       total = ktime_add(gt->stats.total, total);
+
+       WRITE_ONCE(gt->stats.start, 0);
+       smp_wmb(); /* pairs with intel_gt_get_busy_time() */
+       gt->stats.total = total;
 }
 
 static int __gt_unpark(struct intel_wakeref *wf)
@@ -129,7 +125,6 @@ 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)
@@ -363,28 +358,18 @@ 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;
 
-       do {
-               seq = read_seqcount_begin(&gt->stats.lock);
-               total = __intel_gt_get_awake_time(gt);
-       } while (read_seqcount_retry(&gt->stats.lock, seq));
+       start = READ_ONCE(gt->stats.start);
+       if (start) {
+               smp_rmb(); /* pairs with runtime_begin/end */
+               start = ktime_sub(ktime_get(), start);
+       }
 
-       return total;
+       return ktime_add(total, start);
 }
 
 #if IS_ENABLED(CONFIG_DRM_I915_SELFTEST)
diff --git a/drivers/gpu/drm/i915/gt/intel_gt_types.h 
b/drivers/gpu/drm/i915/gt/intel_gt_types.h
index a83d3e18254d..91d20daca536 100644
--- a/drivers/gpu/drm/i915/gt/intel_gt_types.h
+++ b/drivers/gpu/drm/i915/gt/intel_gt_types.h
@@ -89,13 +89,6 @@ struct intel_gt {
        u32 pm_guc_events;
 
        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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