I couldn't find another issue for this so I decided to just make one so I can link to it.
Both RADV and ANV need support for
VK_EXT_image_drm_format_modifier in order to spec-legally import
VK_EXT_external_memory_dma_buf). You can sort of make it work currently by invoking UB and hard-coding driver specifics, but an official solution here would be nice. (Even if it only ends up internally hard-coding
VK_IMAGE_TILING_DRM_FORMAT_MODIFIER_EXT as the appropriate image tiling)
This is needed for e.g. vaapi hardware decoding support (cf. #2484 (closed)).