[PATCH v2] drm/vblank: downgrade vblank wait timeout from WARN to debug

[Chintan Patel <chintanlike@gmail.com>]

When wait_event_timeout() in drm_wait_one_vblank() times out, the
current WARN can cause unnecessary kernel panics in environments
with panic_on_warn set (e.g. CI, fuzzing). These timeouts can happen
under scheduler pressure or from invalid userspace calls, so they are
not always a kernel bug.

Replace the WARN with drm_dbg_kms() messages that provide useful
context (last and current vblank counters) without crashing the
system. Developers can still enable drm.debug to diagnose genuine
problems.

Reported-by: syzbot+147ba789658184f0ce04@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=147ba789658184f0ce04
Tested-by: syzbot+147ba789658184f0ce04@syzkaller.appspotmail.com

Signed-off-by: Chintan Patel <chintanlike@gmail.com>

v2:
 - Drop unnecessary in-code comment (suggested by Thomas Zimmermann)
 - Remove else branch, only log timeout case
---
 drivers/gpu/drm/drm_vblank.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
index 46f59883183d..a94570668cba 100644
--- a/drivers/gpu/drm/drm_vblank.c
+++ b/drivers/gpu/drm/drm_vblank.c
@@ -1289,7 +1289,7 @@ void drm_wait_one_vblank(struct drm_device *dev, unsigned int pipe)
 {
 	struct drm_vblank_crtc *vblank = drm_vblank_crtc(dev, pipe);
 	int ret;
-	u64 last;
+	u64 last, curr;
 
 	if (drm_WARN_ON(dev, pipe >= dev->num_crtcs))
 		return;
@@ -1305,7 +1305,12 @@ void drm_wait_one_vblank(struct drm_device *dev, unsigned int pipe)
 				 last != drm_vblank_count(dev, pipe),
 				 msecs_to_jiffies(100));
 
-	drm_WARN(dev, ret == 0, "vblank wait timed out on crtc %i\n", pipe);
+	curr = drm_vblank_count(dev, pipe);
+
+	if (ret == 0) {
+		drm_dbg_kms(dev, "WAIT_VBLANK: timeout crtc=%d, last=%llu, curr=%llu\n",
+			pipe, last, curr);
+	}
 
 	drm_vblank_put(dev, pipe);
 }
-- 
2.43.0