So you mean it would make more sense to be more explicit in handling
DRM_FORMAT_MOD_INVALID as an incoming modifier (which will, just like
DRM_FORMAT_MOD_LINEAR, will return true in
dm_plane_format_mod_supported)?
That's correct. Not passing any modifiers is the same as explicitly passing INVALID, both of which mean 'the driver will figure it out somehow'; that driver-specific determination is not the same as explicit LINEAR.
(I cannot regret enough that INVALID is not 0.)
Cheers,
Daniel