[PATCH v3] drm/vblank: downgrade vblank wait timeout from WARN to error

[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 heavy scheduling pressure or in rare cases of delayed vblank
handling, and are not always a kernel bug.

Replace the WARN with drm_err() messages that report the timeout
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

v3:
 - Use drm_err() instead of drm_dbg_kms() (suggested by Ville Syrjälä)
 - Remove unnecessary curr = drm_vblank_count() (suggested by Thomas Zimmermann)
 - Fix commit message wording (“invalid userspace calls” → “delayed vblank handling”)
---
 drivers/gpu/drm/drm_vblank.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
index 46f59883183d..0664aea1b924 100644
--- a/drivers/gpu/drm/drm_vblank.c
+++ b/drivers/gpu/drm/drm_vblank.c
@@ -1305,7 +1305,8 @@ 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);
+	if (!ret)
+		drm_err(dev, "vblank wait timed out on crtc %i\n", pipe);
 
 	drm_vblank_put(dev, pipe);
 }
-- 
2.43.0