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

Ville Syrjälä
2025-10-02T11:40:11Z

On Wed, Oct 01, 2025 at 07:57:23PM -0700, Chintan Patel wrote:
> 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.

"invalid userspace calls" should never reach this far.
That would be 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);

It should at the very least be a drm_err(). Though the backtrace can
be useful in figuring out where the problem is coming from, so not
too happy about this change.

> + }
>
> drm_vblank_put(dev, pipe);
> }
> --
> 2.43.0

--
Ville Syrjälä
Intel