[PATCH] drm/modifiers: Enforce consistency between the cap an IN_FORMATS

[Daniel Vetter]

On Thu, May 06, 2021 at 03:23:43PM +0200, Daniel Vetter wrote:
> It's very confusing for userspace to have to deal with inconsistencies
> here, and some drivers screwed this up a bit. Most just ommitted the
> format list when they meant to say that only linear modifier is
> allowed, but some also meant that only implied modifiers are
> acceptable (because actually none of the planes registered supported
> modifiers).
> 
> Now that this is all done consistently across all drivers, document
> the rules and enforce it in the drm core.
> 
> v2:
> - Make the capability a link (Simon)
> - Note that all is lost before 5.1.
> 
> v3:
> - Use drm_WARN_ON (Lyude)
> 
> Acked-by: Pekka Paalanen <pekka.paalanen at collabora.com>
> Reviewed-by: Emil Velikov <emil.velikov at collabora.com>
> Reviewed-by: Lyude Paul <lyude at redhat.com>
> Acked-by: Maxime Ripard <maxime at cerno.tech>
> Cc: Simon Ser <contact at emersion.fr>
> Reviewed-by: Lucas Stach <l.stach at pengutronix.de>
> Cc: Pekka Paalanen <pekka.paalanen at collabora.com>
> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> Cc: Maxime Ripard <mripard at kernel.org>
> Cc: Thomas Zimmermann <tzimmermann at suse.de>
> Cc: David Airlie <airlied at linux.ie>
> Cc: Daniel Vetter <daniel at ffwll.ch>

Pushed to drm-misc-next after another round of intel-gfx CI testing.
-Daniel

> ---
>  drivers/gpu/drm/drm_plane.c   | 18 +++++++++++++++++-
>  include/drm/drm_mode_config.h |  2 ++
>  2 files changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
> index 0dd43882fe7c..b373958ecb30 100644
> --- a/drivers/gpu/drm/drm_plane.c
> +++ b/drivers/gpu/drm/drm_plane.c
> @@ -128,6 +128,13 @@
>   *     pairs supported by this plane. The blob is a struct
>   *     drm_format_modifier_blob. Without this property the plane doesn't
>   *     support buffers with modifiers. Userspace cannot change this property.
> + *
> + *     Note that userspace can check the &DRM_CAP_ADDFB2_MODIFIERS driver
> + *     capability for general modifier support. If this flag is set then every
> + *     plane will have the IN_FORMATS property, even when it only supports
> + *     DRM_FORMAT_MOD_LINEAR. Before linux kernel release v5.1 there have been
> + *     various bugs in this area with inconsistencies between the capability
> + *     flag and per-plane properties.
>   */
>  
>  static unsigned int drm_num_planes(struct drm_device *dev)
> @@ -277,8 +284,14 @@ static int __drm_universal_plane_init(struct drm_device *dev,
>  			format_modifier_count++;
>  	}
>  
> -	if (format_modifier_count)
> +	/* autoset the cap and check for consistency across all planes */
> +	if (format_modifier_count) {
> +		drm_WARN_ON(dev, !config->allow_fb_modifiers &&
> +			    !list_empty(&config->plane_list));
>  		config->allow_fb_modifiers = true;
> +	} else {
> +		drm_WARN_ON(dev, config->allow_fb_modifiers);
> +	}
>  
>  	plane->modifier_count = format_modifier_count;
>  	plane->modifiers = kmalloc_array(format_modifier_count,
> @@ -360,6 +373,9 @@ static int __drm_universal_plane_init(struct drm_device *dev,
>   * drm_universal_plane_init() to let the DRM managed resource infrastructure
>   * take care of cleanup and deallocation.
>   *
> + * Drivers supporting modifiers must set @format_modifiers on all their planes,
> + * even those that only support DRM_FORMAT_MOD_LINEAR.
> + *
>   * Returns:
>   * Zero on success, error code on failure.
>   */
> diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
> index ab424ddd7665..1ddf7783fdf7 100644
> --- a/include/drm/drm_mode_config.h
> +++ b/include/drm/drm_mode_config.h
> @@ -909,6 +909,8 @@ struct drm_mode_config {
>  	 * @allow_fb_modifiers:
>  	 *
>  	 * Whether the driver supports fb modifiers in the ADDFB2.1 ioctl call.
> +	 * Note that drivers should not set this directly, it is automatically
> +	 * set in drm_universal_plane_init().
>  	 *
>  	 * IMPORTANT:
>  	 *
> -- 
> 2.31.0
> 

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