[Intel-gfx] [PATCH 2/3] drm/i915: Prefer checking the wakeref itself rather than the counter

[Chris Wilson]

The counter goes to zero at the start of the parking cycle, but the
wakeref itself is held until the end. Likewise, the counter becomes one
at the end of the unparking, but the wakeref is taken first. If we check
the wakeref instead of the counter, we include the unpark/unparking time
as intel_wakeref_is_active(), and do not spuriously declare inactive if
we fail to park (i.e. the parking and wakeref drop is postponed).

The premature inactive deactivation may result us in randomly stopping
the retire worker too early with a potential for a livelock if that was
the only means by which we were retiring at the time (e.g. in handling
i915_drop_caches).

Testcase: igt/gem_concurrent_blit
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/intel_wakeref.c | 20 +++++++++++++++++---
 drivers/gpu/drm/i915/intel_wakeref.h |  2 +-
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_wakeref.c b/drivers/gpu/drm/i915/intel_wakeref.c
index 1f94bc4ff9e4..91196d9612bb 100644
--- a/drivers/gpu/drm/i915/intel_wakeref.c
+++ b/drivers/gpu/drm/i915/intel_wakeref.c
@@ -7,6 +7,19 @@
 #include "intel_drv.h"
 #include "intel_wakeref.h"
 
+static void rpm_get(struct drm_i915_private *i915, struct intel_wakeref *wf)
+{
+	wf->wakeref = intel_runtime_pm_get(i915);
+}
+
+static void rpm_put(struct drm_i915_private *i915, struct intel_wakeref *wf)
+{
+	intel_wakeref_t wakeref = fetch_and_zero(&wf->wakeref);
+
+	intel_runtime_pm_put(i915, wakeref);
+	GEM_BUG_ON(!wakeref);
+}
+
 int __intel_wakeref_get_first(struct drm_i915_private *i915,
 			      struct intel_wakeref *wf,
 			      int (*fn)(struct intel_wakeref *wf))
@@ -21,11 +34,11 @@ int __intel_wakeref_get_first(struct drm_i915_private *i915,
 	if (!atomic_read(&wf->count)) {
 		int err;
 
-		wf->wakeref = intel_runtime_pm_get(i915);
+		rpm_get(i915, wf);
 
 		err = fn(wf);
 		if (unlikely(err)) {
-			intel_runtime_pm_put(i915, wf->wakeref);
+			rpm_put(i915, wf);
 			mutex_unlock(&wf->mutex);
 			return err;
 		}
@@ -46,7 +59,7 @@ int __intel_wakeref_put_last(struct drm_i915_private *i915,
 
 	err = fn(wf);
 	if (likely(!err))
-		intel_runtime_pm_put(i915, wf->wakeref);
+		rpm_put(i915, wf);
 	else
 		atomic_inc(&wf->count);
 	mutex_unlock(&wf->mutex);
@@ -58,4 +71,5 @@ void __intel_wakeref_init(struct intel_wakeref *wf, struct lock_class_key *key)
 {
 	__mutex_init(&wf->mutex, "wakeref", key);
 	atomic_set(&wf->count, 0);
+	wf->wakeref = 0;
 }
diff --git a/drivers/gpu/drm/i915/intel_wakeref.h b/drivers/gpu/drm/i915/intel_wakeref.h
index a979d638344b..db742291211c 100644
--- a/drivers/gpu/drm/i915/intel_wakeref.h
+++ b/drivers/gpu/drm/i915/intel_wakeref.h
@@ -127,7 +127,7 @@ intel_wakeref_unlock(struct intel_wakeref *wf)
 static inline bool
 intel_wakeref_active(struct intel_wakeref *wf)
 {
-	return atomic_read(&wf->count);
+	return READ_ONCE(wf->wakeref);
 }
 
 #endif /* INTEL_WAKEREF_H */
-- 
2.20.1