Re: [PATCH v2 4/4] drm: require each CRTC to have a unique primary plane

[Jani Nikula <jani.nikula@linux.intel.com>]

On Fri, 11 Dec 2020, Simon Ser <contact@emersion.fr> wrote:
> User-space expects to be able to pick a primary plane for each CRTC
> exposed by the driver. Make sure this assumption holds in
> drm_mode_config_validate.
>
> Use the legacy drm_crtc.primary field to check this, because it's
> simpler and we require drivers to set it anyways. Accumulate a set of
> primary planes which are already used for a CRTC in a bitmask. Error out
> if a primary plane is re-used.
>
> Signed-off-by: Simon Ser <contact@emersion.fr>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> Cc: Pekka Paalanen <ppaalanen@gmail.com>
> ---
>  drivers/gpu/drm/drm_mode_config.c | 21 +++++++++++++++++++++
>  drivers/gpu/drm/drm_plane.c       |  6 ++++++
>  2 files changed, 27 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_mode_config.c b/drivers/gpu/drm/drm_mode_config.c
> index fbe680035129..f143f56f0820 100644
> --- a/drivers/gpu/drm/drm_mode_config.c
> +++ b/drivers/gpu/drm/drm_mode_config.c
> @@ -626,6 +626,9 @@ void drm_mode_config_validate(struct drm_device *dev)
>  {
>  	struct drm_encoder *encoder;
>  	struct drm_crtc *crtc;
> +	struct drm_plane *plane;
> +	__u64 primary_with_crtc = 0, cursor_with_crtc = 0;

Why __u64 and not u64?

> +	unsigned num_primary = 0;

I think the preference is to spell out the full type instead of using
just "unsigned".

BR,
Jani.

>  
>  	if (!drm_core_check_feature(dev, DRIVER_MODESET))
>  		return;
> @@ -647,12 +650,30 @@ void drm_mode_config_validate(struct drm_device *dev)
>  			     "Bogus primary plane possible_crtcs: [PLANE:%d:%s] must be compatible with [CRTC:%d:%s]\n",
>  			     crtc->primary->base.id, crtc->primary->name,
>  			     crtc->base.id, crtc->name);
> +			WARN(primary_with_crtc & BIT(crtc->primary->index),
> +			     "Primary plane [PLANE:%d:%s] used for two CRTCs",
> +			     crtc->primary->base.id, crtc->primary->name);
> +			primary_with_crtc |= BIT(crtc->primary->index);
>  		}
>  		if (crtc->cursor) {
>  			WARN(!(crtc->cursor->possible_crtcs & BIT(crtc->index)),
>  			     "Bogus cursor plane possible_crtcs: [PLANE:%d:%s] must be compatible with [CRTC:%d:%s]\n",
>  			     crtc->cursor->base.id, crtc->cursor->name,
>  			     crtc->base.id, crtc->name);
> +			WARN(cursor_with_crtc & BIT(crtc->cursor->index),
> +			     "Primary plane [PLANE:%d:%s] used for two CRTCs",
> +			     crtc->cursor->base.id, crtc->cursor->name);
> +			cursor_with_crtc |= BIT(crtc->cursor->index);
>  		}
>  	}
> +
> +	drm_for_each_plane(plane, dev) {
> +		if (plane->type == DRM_PLANE_TYPE_PRIMARY) {
> +			num_primary++;
> +		}
> +	}
> +
> +	WARN(num_primary != dev->mode_config.num_crtc,
> +	     "Must have as many primary planes as there are CRTCs, but have %u primary planes and %u CRTCs",
> +	     num_primary, dev->mode_config.num_crtc);
>  }
> diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
> index 49b0a8b9ac02..a1f4510efa83 100644
> --- a/drivers/gpu/drm/drm_plane.c
> +++ b/drivers/gpu/drm/drm_plane.c
> @@ -54,6 +54,12 @@
>   * enum drm_plane_type). A plane can be compatible with multiple CRTCs, see
>   * &drm_plane.possible_crtcs.
>   *
> + * Each CRTC must have a unique primary plane userspace can attach to enable
> + * the CRTC. In other words, userspace must be able to attach a different
> + * primary plane to each CRTC at the same time. Primary planes can still be
> + * compatible with multiple CRTCs. There must be exactly as many primary planes
> + * as there are CRTCs.
> + *
>   * 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

-- 
Jani Nikula, Intel Open Source Graphics Center
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