There are a few details specific to the GETFB2 IOCTL. It's not immediately clear how user-space should check for the number of planes. Suggest using the handles field or the pitches field. The modifier array is filled with zeroes, ie. DRM_FORMAT_MOD_LINEAR. So explicitly tell user-space to not look at it unless the flag is set. Changes in v2 (Daniel): - Mention that handles should be used to compute the number of planes, and only refer to pitches as a fallback. - Reword bit about undefined modifier. Signed-off-by: Simon Ser <contact@emersion.fr> Acked-by: Daniel Vetter <daniel@ffwll.ch> Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com> Acked-by: Daniel Stone <daniels@collabora.com> Link: https://patchwork.freedesktop.org/patch/msgid/20211123112400.22245-1-contact@emersion.fr