[PATCH RFC] drm/vblank: Replace WARN with drm_err for vblank wait timeouts

["syzbot" <syzbot@kernel.org>]

On PREEMPT_RT kernels, a user-space task with elevated Real-Time (RT)
priority can starve essential kernel threads. For example, the VKMS driver
simulates vblank interrupts using hrtimers. On PREEMPT_RT, these timers run
in the per-CPU timer threads at a low RT priority. If a user-space task
elevates its priority above the timer thread and monopolizes the CPU, the
timer thread is starved and the vblank hrtimer never fires.

This leads to a timeout when a worker thread waits for the vblank event.
For instance, a console update triggers a framebuffer update, scheduling
drm_fb_helper_damage_work() on the system workqueue. The worker thread
eventually calls drm_crtc_wait_one_vblank() to synchronize the screen
update with the vblank interval. Due to the starved timer, the wait times
out and triggers a drm_WARN. A similar issue exists in
drm_atomic_helper_wait_for_vblanks() where a WARN is triggered upon
timeout.

WARN macros must not be used for conditions that can legitimately happen,
and pr_err or drm_err should be used instead if necessary. Since this
timeout is an expected consequence of RT scheduling starvation rather than
a kernel bug, replace the warnings with drm_err(). This ensures the timeout
is still logged for diagnostic purposes without triggering an unnecessary
backtrace.

Fixes: 74afeb812850 ("drm/vblank: Add vblank timer")
Assisted-by: Gemini:gemini-3.5-flash Gemini:gemini-3.1-pro-preview syzbot
Reported-by: syzbot+f59157955aba9d0cb43b@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=f59157955aba9d0cb43b
Link: https://syzkaller.appspot.com/ai_job?id=396433fb-a59e-4936-97d4-4d2e7ced2ee9
To: "David Airlie" <airlied@gmail.com>
To: <dri-devel@lists.freedesktop.org>
To: "Maarten Lankhorst" <maarten.lankhorst@linux.intel.com>
To: "Maxime Ripard" <mripard@kernel.org>
To: "Simona Vetter" <simona@ffwll.ch>
To: "Thomas Zimmermann" <tzimmermann@suse.de>
Cc: <linux-kernel@vger.kernel.org>

---
diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
index 51f39edc3..0266beff9 100644
--- a/drivers/gpu/drm/drm_atomic_helper.c
+++ b/drivers/gpu/drm/drm_atomic_helper.c
@@ -1918,8 +1918,9 @@ drm_atomic_helper_wait_for_vblanks(struct drm_device *dev,
 						drm_crtc_vblank_count(crtc),
 					 msecs_to_jiffies(1000));
 
-		WARN(!ret, "[CRTC:%d:%s] vblank wait timed out\n",
-		     crtc->base.id, crtc->name);
+		if (!ret)
+			drm_err(dev, "[CRTC:%d:%s] vblank wait timed out\n",
+				crtc->base.id, crtc->name);
 
 		drm_crtc_vblank_put(crtc);
 	}
diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
index f90fb2d13..7f7f4dd35 100644
--- a/drivers/gpu/drm/drm_vblank.c
+++ b/drivers/gpu/drm/drm_vblank.c
@@ -1326,7 +1326,8 @@ int drm_crtc_wait_one_vblank(struct drm_crtc *crtc)
 				 last != drm_vblank_count(dev, pipe),
 				 msecs_to_jiffies(1000));
 
-	drm_WARN(dev, ret == 0, "vblank wait timed out on crtc %i\n", pipe);
+	if (ret == 0)
+		drm_err(dev, "vblank wait timed out on crtc %i\n", pipe);
 
 	drm_vblank_put(dev, pipe);
 


base-commit: f5098b6bae761e346ebcd9da7f95622c04733cff
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