[PATCH 3/3] drm: require a non_NULL drm_crtc.primary

[Daniel Vetter]

On Thu, Dec 10, 2020 at 03:50:35PM +0000, Simon Ser wrote:
> If a CRTC is missing a legacy primary plane pointer, a lot of things
> will be broken for user-space: fbdev stops working and the entire legacy
> uAPI stops working.
> 
> Require all drivers to populate drm_crtc.primary to prevent these
> issues. Warn if it's NULL.
> 
> Signed-off-by: Simon Ser <contact at emersion.fr>
> Cc: Daniel Vetter <daniel at ffwll.ch>
> Cc: Pekka Paalanen <ppaalanen at gmail.com>

For context we even kinda guarantee that xrgb8888 is available, with
in-kernel conversion, because just too much of the simplistic userspace
falls over otherwise. If we don't have a primary plane then kms is
practically useless I think :-)

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

> ---
>  drivers/gpu/drm/drm_mode_config.c | 3 +++
>  drivers/gpu/drm/drm_plane.c       | 2 +-
>  2 files changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_mode_config.c b/drivers/gpu/drm/drm_mode_config.c
> index 2c73a60e8765..fbe680035129 100644
> --- a/drivers/gpu/drm/drm_mode_config.c
> +++ b/drivers/gpu/drm/drm_mode_config.c
> @@ -639,6 +639,9 @@ void drm_mode_config_validate(struct drm_device *dev)
>  	}
>  
>  	drm_for_each_crtc(crtc, dev) {
> +		WARN(!crtc->primary, "Missing primary plane on [CRTC:%d:%s]\n",
> +		     crtc->base.id, crtc->name);
> +
>  		if (crtc->primary) {
>  			WARN(!(crtc->primary->possible_crtcs & BIT(crtc->index)),
>  			     "Bogus primary plane possible_crtcs: [PLANE:%d:%s] must be compatible with [CRTC:%d:%s]\n",
> diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
> index 5d33ca9f0032..49b0a8b9ac02 100644
> --- a/drivers/gpu/drm/drm_plane.c
> +++ b/drivers/gpu/drm/drm_plane.c
> @@ -57,7 +57,7 @@
>   * Legacy uAPI doesn't expose the primary and cursor planes directly. DRM core
>   * relies on the driver to set the primary and optionally the cursor plane used
>   * for legacy IOCTLs. This is done by calling drm_crtc_init_with_planes(). All
> - * drivers should provide one primary plane per CRTC to avoid surprising legacy
> + * drivers must provide one primary plane per CRTC to avoid surprising legacy
>   * userspace too much.
>   */
>  
> -- 
> 2.29.2
> 
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch