drm/msm/kms: Use nested locking for crtc lock instead of custom classes

We don't need to make up custom lock classes here, we can simply use
mutex_lock_nested() and pass in the index of the crtc to the locking
APIs instead. This helps lockdep understand that these are really
different locks while avoiding having to allocate custom lockdep
classes.

Cc: Krishna Manikandan <mkrishn@codeaurora.org>
Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Fixes: b3d91800 ("drm/msm: Fix race condition in msm driver with async layer updates")
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>

drivers/gpu/drm/msm/msm_atomic.c

@@ -57,10 +57,13 @@ static void vblank_put(struct msm_kms *kms, unsigned crtc_mask)
 
 static void lock_crtcs(struct msm_kms *kms, unsigned int crtc_mask)
 {
+	int crtc_index;
 	struct drm_crtc *crtc;
 
-	for_each_crtc_mask(kms->dev, crtc, crtc_mask)
-		mutex_lock(&kms->commit_lock[drm_crtc_index(crtc)]);
+	for_each_crtc_mask(kms->dev, crtc, crtc_mask) {
+		crtc_index = drm_crtc_index(crtc);
+		mutex_lock_nested(&kms->commit_lock[crtc_index], crtc_index);
+	}
 }
 
 static void unlock_crtcs(struct msm_kms *kms, unsigned int crtc_mask)

drivers/gpu/drm/msm/msm_kms.h

@@ -157,7 +157,6 @@ struct msm_kms {
 	 * from the crtc's pending_timer close to end of the frame:
 	 */
 	struct mutex commit_lock[MAX_CRTCS];
-	struct lock_class_key commit_lock_keys[MAX_CRTCS];
 	unsigned pending_crtc_mask;
 	struct msm_pending_timer pending_timers[MAX_CRTCS];
 };
@@ -167,11 +166,8 @@ static inline int msm_kms_init(struct msm_kms *kms,
 {
 	unsigned i, ret;
 
-	for (i = 0; i < ARRAY_SIZE(kms->commit_lock); i++) {
-		lockdep_register_key(&kms->commit_lock_keys[i]);
-		__mutex_init(&kms->commit_lock[i], "&kms->commit_lock[i]",
-			     &kms->commit_lock_keys[i]);
-	}
+	for (i = 0; i < ARRAY_SIZE(kms->commit_lock); i++)
+		mutex_init(&kms->commit_lock[i]);
 
 	kms->funcs = funcs;