On 7/3/26 09:26, Thomas Zimmermann wrote: > Am 02.07.26 um 20:23 schrieb Roman Ilin: >> Apologies, I accidentally fired off send-email before saving my final >> changelog notes. Please ignore the changelog and notes in the original >> v3 email. >> >> The actual changes in v3 are: >> >> - Changed the WARN_ON_ONCE to drm_dbg_kms (to avoid user-triggerable >> panics). > > We want to know when this happens. And user space will only be able to > trigger this once, so there's no risk of spamming the kernel log.
Hmmm, just wondering: what's the status here? Roman Ilin, did you loose interest? Or was this continued somewhere or maybe even resolved and I just missed that? Thomas: if Roman left this behind, could you maybe handle this, as it fixes a regressions that iirc is caused by a change of yours? Or is it fine for some reason to ignore this, even if the userland problem that triggers this is still unfixed afaics? Side note: I by chance saw that the latter a few days ago got a new comment from someone that ran into it. https://gitlab.freedesktop.org/spice/linux/vd_agent/-/work_items/52 Ciao, Thorsten > So this is not really a problem. drm_WARN_ON_ONCE was ok for that. You > can also use a regular DRM print macro. But instead of drm_dbg_kms() > should use drm_err_once(). But please also output linedur_ns and > framedur_ns in the error. We want to know which of them is incorrect. > You can also add more information to the error message. See [1] for the > mode-formating macros. > > [1] https://elixir.bootlin.com/linux/v7.1.2/source/include/drm/ > drm_modes.h#L422 > > >> - Updated drm_calc_timestamping_constants with the goto error fallback >> to clear the stale state. >> >> Also, an automated review bot pointed out an AB-BA deadlock in >> drm_crtc_vblank_start_timer(). But I am leaving this out of the patch to >> keep the fixes orthogonal. >> >> Sorry for the noise. >> >>> On Jul 2, 2026, at 21:10, Roman Ilin <[email protected]> wrote: >>> >>> 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 >>> >>> 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. >>> >>> Fixes: 74afeb812850 ("drm/vblank: Add vblank timer") >>> Suggested-by: Thomas Zimmermann <[email protected]> >>> Signed-off-by: Roman Ilin <[email protected]> >>> --- >>> Changes in v3: >>> >>> - Changed the WARN_ON_ONCE to drm_err_once >>> >>> Notes: >>> >>> >>> >>> drivers/gpu/drm/drm_vblank.c | 71 +++++++++++++++++++++++------------- >>> include/drm/drm_vblank.h | 4 +- >>> 2 files changed, 48 insertions(+), 27 deletions(-) >>> >>> diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c >>> index f90fb2d13..629b9fcc7 100644 >>> --- a/drivers/gpu/drm/drm_vblank.c >>> +++ b/drivers/gpu/drm/drm_vblank.c >>> @@ -631,42 +631,51 @@ 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(dev, "crtc %u: Can't calculate constants, dotclock = %d!\n", > > Please turn this into drm_err_once() because this call can actually be > triggered repeatedly from userspace. > > Best regards > Thomas > > >>> + 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_dbg_kms(dev, "crtc %u: Can't calculate constants, mode clock >>> too small!\n", >>> + crtc->base.id); >>> + goto error; >>> } >>> >>> vblank->linedur_ns = linedur_ns; >>> @@ -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 2fcef9c0f..d99772dfa 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); >>> -- >>> 2.54.0 >>> >
