DRM Format Modifiers in v4l2

Hi all,

I couldn't find this topic talked about elsewhere, but apologies if it's a duplicate - I'll be glad to be steered in the direction of a thread.

We'd like to support DRM format modifiers in v4l2 in order to share the description of different (mostly proprietary) buffer formats between e.g. a v4l2 device and a DRM device.

DRM format modifiers are defined in include/uapi/drm/drm_fourcc.h and are a vendor-namespaced 64-bit value used to describe various vendor-specific buffer layouts. They are combined with a (DRM) FourCC code to give a complete description of the data contained in a buffer.

The same modifier definition is used in the Khronos EGL extension EGL_EXT_image_dma_buf_import_modifiers, and is supported in the Wayland linux-dmabuf protocol.

This buffer information could of course be described in the vendor-specific part of V4L2_PIX_FMT_*, but this would duplicate the information already defined in drm_fourcc.h. Additionally, there would be quite a format explosion where a device supports a dozen or more formats, all of which can use one or more different layouts/compression schemes.

So, I'm wondering if anyone has views on how/whether this could be incorporated?

I spoke briefly about this to Laurent at LPC last year, and he suggested v4l2_control as one approach.

I also wondered if could be added in v4l2_pix_format_mplane - looks like there's 8 bytes left before it exceeds the 200 bytes, or could go in the reserved portion of v4l2_plane_pix_format.

Thanks for any thoughts, -Brian