On Tue, 21 Dec 2021 at 13:13, José Expósito wrote:
>
> Adding format modifiers without implementing the function
> "drm_plane_funcs.format_mod_supported" exposes an invalid IN_FORMATS
> blob with modifiers but no formats to user-space.
I think the fix should be applied to the generic code. The docs at
drm_plane.h clearly state that this callback is optional:
* This optional hook is used for the DRM to determine if the given
* format/modifier combination is valid for the plane. This allows the
* DRM to generate the correct format bitmask (which formats apply to
* which modifier), and to valdiate modifiers at atomic_check time.
*
* If not present, then any modifier in the plane's modifier
* list is allowed with any of the plane's formats.
So, one should fix the core code in create_in_format_blob() to include
all combinations.
>
> This breaks the latest Weston [1]. For testing purposes, I extracted the
> affected code to a standalone program [2].
>
> Make clear in the IN_FORMATS documentation that implementing
> "format_mod_supported" is mandatory.
format_mod_supported() being optional or mandatory should be
documented in the format_mod_supported() docs, not in the IN_FORMAT
docs.
>
> [1]
> https://gitlab.freedesktop.org/wayland/weston/-/blob/9.0/libweston/backend-drm/kms.c#L431
> [2] https://github.com/JoseExposito/drm-sandbox/blob/main/in_formats.c
>
> Signed-off-by: José Expósito
> ---
> drivers/gpu/drm/drm_plane.c | 7 +--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_plane.c b/drivers/gpu/drm/drm_plane.c
> index 82afb854141b..347571f8909a 100644
> --- a/drivers/gpu/drm/drm_plane.c
> +++ b/drivers/gpu/drm/drm_plane.c
> @@ -126,8 +126,11 @@
> * IN_FORMATS:
> * Blob property which contains the set of buffer format and modifier
> * 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.
> + * drm_format_modifier_blob and the drm_plane_funcs.format_mod_supported
> + * function must be implemented by the driver to determine if the given
> + * format/modifier combination is valid for the plane. Without this
> property
> + * the plane doesn't support buffers with modifiers. Userspace cannot
> change
> + * this property.
> *
> * Note that userspace can check the _CAP_ADDFB2_MODIFIERS driver
> * capability for general modifier support. If this flag is set then
> every
> --
> 2.25.1
>
--
With best wishes
Dmitry
