Re: [PATCH v4] drm/vblank: Don't arm vblank timer with invalid frame duration

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Mon, Sep 14, 2026 at 12:57:26PM +0200, Thomas Zimmermann wrote:
> From: Roman Ilin <me@romanilin.is>
> 
> When a CRTC's display mode carries a too small pixel clock,
> drm_calc_timestamping_constants() computes a frame duration that
> exceeds INT_MAX. drm_vblank_crtc.framedur_ns becomes negative.
> drm_crtc_vblank_start_timer() then arms the vblank hrtimer with this
> interval, after which vblank events are no longer delivered. Pending
> page flips never complete and the display appears frozen.
> 
> This could be triggered on virtio-gpu guests that have dynamic resolution
> enabled: when the SPICE agent or the X server resizes the output, it
> submits a mode whose pixel clock is off by a factor of 1000, e.g.:
> 
>     clock = 406 kHz, htotal = 3152, vtotal = 2148

How about actually "fixing" the clock in the driver?

> 
>     framedur_ns = 3152 * 2148 * 1000000 / 406 = 16675852216 ns (~16.7 s)
> 
> 16675852216 does not fit into an int and wraps to roughly -504000000.
> ns_to_ktime() then yields a negative interval and the timer stops working.
> 
> Found by bisection, which pointed at commit a036f5fceedb ("drm/virtgpu:
> Use vblank timer"). That commit merely made virtio-gpu use the vblank
> timer and thereby exposed the pre-existing problem in the timer setup
> added by commit 74afeb812850 ("drm/vblank: Add vblank timer").
> 
> To fix this, modify drm_calc_timestamping_constants() to use u64 for
> calculations, check for INT_MAX overflows, and return an error code.
> drm_crtc_vblank_start_timer() will then propagate the error, enabling
> the driver to fall back to immediate vblank events. Valid modes are
> unaffected, and the timer self-heals on the next mode with a sane clock.
> 
> v4:
> - avoid log spamming with drm_warn_once()
> 
> Fixes: 74afeb812850 ("drm/vblank: Add vblank timer")
> Suggested-by: Thomas Zimmermann <tzimmermann@suse.de>
> Signed-off-by: Roman Ilin <me@romanilin.is>
> ---
> I took Roman's patch from [1] and replaced the error messages with
> warn_once, as suggestde in my review.
> 
> [1] https://lore.kernel.org/dri-devel/20260702181027.98526-1-me@romanilin.is/
> ---
>  drivers/gpu/drm/drm_vblank.c | 73 +++++++++++++++++++++++-------------
>  include/drm/drm_vblank.h     |  4 +-
>  2 files changed, 49 insertions(+), 28 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
> index f90fb2d13e42..72b543bca9e3 100644
> --- a/drivers/gpu/drm/drm_vblank.c
> +++ b/drivers/gpu/drm/drm_vblank.c
> @@ -631,45 +631,54 @@ EXPORT_SYMBOL(drm_crtc_vblank_waitqueue);
>   * drm_crtc_vblank_helper_get_vblank_timestamp(). They are derived from
>   * CRTC's true scanout timing, so they take things like panel scaling or
>   * other adjustments into account.
> + *
> + * Returns:
> + * 0 on success, or a negative errno code otherwise.
>   */
> -void drm_calc_timestamping_constants(struct drm_crtc *crtc,
> -				     const struct drm_display_mode *mode)
> +int drm_calc_timestamping_constants(struct drm_crtc *crtc,
> +				    const struct drm_display_mode *mode)
>  {
>  	struct drm_device *dev = crtc->dev;
>  	unsigned int pipe = drm_crtc_index(crtc);
>  	struct drm_vblank_crtc *vblank = drm_crtc_vblank_crtc(crtc);
> -	int linedur_ns = 0, framedur_ns = 0;
> +	u64 linedur_ns, framedur_ns;
>  	int dotclock = mode->crtc_clock;
> +	unsigned int frame_size;
>  
>  	if (!drm_dev_has_vblank(dev))
> -		return;
> +		return 0;
>  
>  	if (drm_WARN_ON(dev, pipe >= dev->num_crtcs))
> -		return;
> +		return -EINVAL;
>  
> -	/* Valid dotclock? */
> -	if (dotclock > 0) {
> -		int frame_size = mode->crtc_htotal * mode->crtc_vtotal;
> +	if (dotclock <= 0) {
> +		drm_err_once(dev, "crtc %u: Can't calculate constants, dotclock = %d!\n",
> +			     crtc->base.id, dotclock);
> +		goto error;
> +	}
>  
> -		/*
> -		 * Convert scanline length in pixels and video
> -		 * dot clock to line duration and frame duration
> -		 * in nanoseconds:
> -		 */
> -		linedur_ns  = div_u64((u64) mode->crtc_htotal * 1000000, dotclock);
> -		framedur_ns = div_u64((u64) frame_size * 1000000, dotclock);
> +	frame_size = (unsigned int)mode->crtc_htotal * (unsigned int)mode->crtc_vtotal;
>  
> -		/*
> -		 * Fields of interlaced scanout modes are only half a frame duration.
> -		 */
> -		if (mode->flags & DRM_MODE_FLAG_INTERLACE)
> -			framedur_ns /= 2;
> -	} else {
> -		drm_err(dev, "crtc %u: Can't calculate constants, dotclock = 0!\n",
> -			crtc->base.id);
> +	/*
> +	 * Convert scanline length in pixels and video dot clock to line duration
> +	 * and frame duration in nanoseconds.
> +	 */
> +	linedur_ns  = div_u64((u64)mode->crtc_htotal * 1000000, dotclock);
> +	framedur_ns = div_u64((u64)frame_size * 1000000, dotclock);
> +
> +	/*
> +	 * Fields of interlaced scanout modes are only half a frame duration.
> +	 */
> +	if (mode->flags & DRM_MODE_FLAG_INTERLACE)
> +		framedur_ns /= 2;
> +
> +	if (linedur_ns > INT_MAX || framedur_ns > INT_MAX) {
> +		drm_err_once(dev, "crtc %u: Can't calculate constants, mode clock too small!\n",
> +			     crtc->base.id);
> +		goto error;
>  	}
>  
> -	vblank->linedur_ns  = linedur_ns;
> +	vblank->linedur_ns = linedur_ns;
>  	vblank->framedur_ns = framedur_ns;
>  	drm_mode_copy(&vblank->hwmode, mode);
>  
> @@ -678,7 +687,16 @@ void drm_calc_timestamping_constants(struct drm_crtc *crtc,
>  		     crtc->base.id, mode->crtc_htotal,
>  		     mode->crtc_vtotal, mode->crtc_vdisplay);
>  	drm_dbg_core(dev, "crtc %u: clock %d kHz framedur %d linedur %d\n",
> -		     crtc->base.id, dotclock, framedur_ns, linedur_ns);
> +		     crtc->base.id, dotclock,
> +		     vblank->framedur_ns, vblank->linedur_ns);
> +
> +	return 0;
> +
> +error:
> +	vblank->linedur_ns = 0;
> +	vblank->framedur_ns = 0;
> +	drm_mode_copy(&vblank->hwmode, mode);
> +	return -EINVAL;
>  }
>  EXPORT_SYMBOL(drm_calc_timestamping_constants);
>  
> @@ -2221,6 +2239,7 @@ int drm_crtc_vblank_start_timer(struct drm_crtc *crtc)
>  	struct drm_vblank_crtc *vblank = drm_crtc_vblank_crtc(crtc);
>  	struct drm_vblank_crtc_timer *vtimer = &vblank->vblank_timer;
>  	unsigned long flags;
> +	int ret;
>  
>  	if (!vtimer->crtc) {
>  		/*
> @@ -2239,7 +2258,9 @@ int drm_crtc_vblank_start_timer(struct drm_crtc *crtc)
>  			hrtimer_try_to_cancel(&vtimer->timer);
>  	}
>  
> -	drm_calc_timestamping_constants(crtc, &crtc->mode);
> +	ret = drm_calc_timestamping_constants(crtc, &crtc->mode);
> +	if (ret)
> +		return ret;
>  
>  	spin_lock_irqsave(&vtimer->interval_lock, flags);
>  	vtimer->interval = ns_to_ktime(vblank->framedur_ns);
> diff --git a/include/drm/drm_vblank.h b/include/drm/drm_vblank.h
> index 2fcef9c0f5b1..d99772dfa034 100644
> --- a/include/drm/drm_vblank.h
> +++ b/include/drm/drm_vblank.h
> @@ -311,8 +311,8 @@ void drm_crtc_vblank_on(struct drm_crtc *crtc);
>  u64 drm_crtc_accurate_vblank_count(struct drm_crtc *crtc);
>  void drm_crtc_vblank_restore(struct drm_crtc *crtc);
>  
> -void drm_calc_timestamping_constants(struct drm_crtc *crtc,
> -				     const struct drm_display_mode *mode);
> +int drm_calc_timestamping_constants(struct drm_crtc *crtc,
> +				    const struct drm_display_mode *mode);
>  wait_queue_head_t *drm_crtc_vblank_waitqueue(struct drm_crtc *crtc);
>  void drm_crtc_set_max_vblank_count(struct drm_crtc *crtc,
>  				   u32 max_vblank_count);
> 
> base-commit: 06576c2b43750d990297b8ff1989b94ac72e855f
> prerequisite-patch-id: 6729bb10d5e338678e78ccda2dd24f232a13cd22
> prerequisite-patch-id: 9d94d20bd45967493b4dc339b58082e1c8da0f4f
> prerequisite-patch-id: 6ac8d22df34db9e7d467b1ab95ad30f94841d5db
> prerequisite-patch-id: f548dbde3f944ca3415c808be628719acebe898a
> prerequisite-patch-id: 2ae91ee8e2bbcd33b8325be046e7214185aba6aa
> prerequisite-patch-id: f97bbde08a9996d7c62ca3e3ebfcd387e50bc5c3
> prerequisite-patch-id: ecc3c825277bc9978c6979455517249a074c0cb1
> prerequisite-patch-id: 0449925cca36664b566f614bfb7d573a11f0575f
> prerequisite-patch-id: 9fb2c2787244e9621847af09bea03e575e6276c7
> prerequisite-patch-id: eb4384d581805e1178d22e7df45f134223c38305
> prerequisite-patch-id: 1a7a529e4b2864abe8071aa44598d28ce72483f4
> prerequisite-patch-id: 844a85f4c2fe2f72209b82d46614004c3cfe4123
> prerequisite-patch-id: 13f7cb22114c648d35f8c9fa74e9165656ce31eb
> prerequisite-patch-id: 20ee46bd6fdd91e39f71d22f52ef6682a4e41512
> prerequisite-patch-id: 1d26c0ff91796ff1e62d3831d00bee18723788ef
> prerequisite-patch-id: 007f6ea0d09bef54b689bae0eb5abeb497bdfaad
> prerequisite-patch-id: 2d89c1507c909520adf9bfb1a137d458a819ae8f
> prerequisite-patch-id: d90dc4134468870ab8ccbd71547c9552dff74fbd
> prerequisite-patch-id: c67e5d886a47b7d0266d81100837557fda34cb24
> prerequisite-patch-id: a5a973e527c88a5b47053d7a72aefe0b550197cb
> prerequisite-patch-id: b9adc9622920a3e70168e672c2c92795b3e3a106
> prerequisite-patch-id: 5030de433a01c2e99056cadb676a8e2ba35f055a
> -- 
> 2.55.0

-- 
Ville Syrjälä
Intel