[PATCH 1/3] drm/plane: Mention format_mod_supported in IN_FORMATS docs

["José Expósito" <jose.exposito89@gmail.com>]

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.

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.

[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 <jose.exposito89@gmail.com>
---
 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 &DRM_CAP_ADDFB2_MODIFIERS driver
  *     capability for general modifier support. If this flag is set then every
-- 
2.25.1


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