On Mon, Sep 14, 2026 at 12:57:26PM +0200, Thomas Zimmermann wrote:
> From: Roman Ilin <[email protected]>
>
> 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 <[email protected]>
> Signed-off-by: Roman Ilin <[email protected]>
> ---
> 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/[email protected]/
> ---
> 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
