Re: [PATCH 5/5] drm/vblank: Require all CRTCs implement vblank support in drm_vblank_init()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] drm_vblank_init() causes a NULL pointer dereference or driver probe failure due to accessing uninitialized or empty crtc_list
--

--- Patch [5]: [PATCH 5/5] drm/vblank: Require all CRTCs implement vblank support in drm_vblank_init() ---
commit 178bbb448a16663eaad60425c62b446801e0f387
Author: Lyude Paul <lyude@redhat.com>

drm/vblank: Require all CRTCs implement vblank support in drm_vblank_init()

This commit modifies drm_vblank_init() to verify that all CRTCs implement
vblank support, ensuring an all-or-nothing configuration. It returns -EINVAL
if support is mixed, and -ENODEV if no CRTCs support vblank.

> diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
> index d57405050554f..17affae23e7a6 100644
> --- a/drivers/gpu/drm/drm_vblank.c
> +++ b/drivers/gpu/drm/drm_vblank.c
> @@ -542,12 +542,31 @@ static void drm_vblank_init_release(struct drm_device *dev, void *ptr)
>   * drmm_add_action_or_reset().
>   *
>   * Returns:
> - * Zero on success or a negative error code on failure.
> + * Zero on success or a negative error code on failure. If no CRTCs with vblank
> + * support implemented were found, -ENODEV will be returned.
>   */
>  int drm_vblank_init(struct drm_device *dev)
>  {
>  	int ret;
>  	struct drm_crtc *crtc;
> +	bool found_vbl_crtc = false, found_non_vbl_crtc = false;
> +
> +	/* Vblank hooks are per-CRTC, not per-device - which is contrary to how
> +	 * the vblank core was written. Since hardware that only has vblank
> +	 * support on some CRTCs but not all is non-existent and would likely
> +	 * lead to UB, ensure that vblank support is all or nothing.
> +	 */
> +	drm_for_each_crtc(crtc, dev) {

[Severity: High]
Can iterating over dev->mode_config.crtc_list here cause a NULL pointer
dereference or a false -ENODEV return during driver probe?

Several DRM drivers call drm_vblank_init() before initializing mode_config
or creating CRTCs. For example:

In i915, intel_display_driver_probe_noirq() calls drm_vblank_init() before
intel_mode_config_init() sets up the crtc_list, meaning the list head is
uninitialized.

In vkms, vkms_create() calls drm_vblank_init() before vkms_modeset_init()
initializes the crtc_list.

In zynqmp_kms, zynqmp_dpsub_drm_init() initializes the list but calls
drm_vblank_init() before any CRTCs are created, meaning the list is empty
and this function will unconditionally return -ENODEV.

Does drm_vblank_init() need to support being called before CRTCs are
registered?

> +		if (crtc->funcs->enable_vblank)
> +			found_vbl_crtc = true;
> +		else
> +			found_non_vbl_crtc = true;
> +
> +		if (drm_WARN_ON(dev, found_vbl_crtc && found_non_vbl_crtc))
> +			return -EINVAL;
> +	}
> +	if (!found_vbl_crtc)
> +		return -ENODEV;

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260814203542.1405135-1-lyude@redhat.com?part=5