Re: [PATCH] drm: document struct drm_mode_fb_cmd2

On Thu, 20 Jan 2022 at 12:48, Simon Ser contact@emersion.fr wrote:

Follow-up for the DRM_IOCTL_MODE_GETFB2 docs.

Yeah, thanks a lot for doing this, it's really awesome. <3

+/**

• • struct drm_mode_fb_cmd2 - Frame-buffer metadata.
• • This struct holds frame-buffer metadata. There are two ways to use it:
• • • User-space can fill this struct and perform an &DRM_IOCTL_MODE_ADDFB2
• • ioctl to register a new frame-buffer. The new frame-buffer object ID will
• • be set by the kernel in @fb_id.
• • • User-space can set @fb_id and perform a &DRM_IOCTL_MODE_GETFB2 ioctl to
• • fetch metadata about an existing frame-buffer.
• • In case of planar formats, this struct allows up to 4 buffer objects with
• • offsets and pitches per plane. The pitch and offset order is dictated by the
• • fourcc, e.g. NV12 (https://fourcc.org/yuv.php#NV12) is described as:

• • YUV 4:2:0 image with a plane of 8 bit Y samples followed by an
    

• • interleaved U/V plane containing 8 bit 2x2 subsampled colour difference
    

• • samples.
    

• • So it would consist of Y as ``offsets[0]`` and UV as ``offsets[1]``. Note
• • that ``offsets[0]`` will generally be 0 (but this is not required).

I think the note about offsets[0] usually being zero should be dropped. Equally, rather than referring to fourcc.org, why don't we just refer to drm_fourcc.h and maybe note in there that fourcc.org can be helpful for some formats, but others are missing or mismatching.

•   /**
  

•    * @handles: GEM buffer handle, one per plane. Set to 0 if the plane is
  

•    * unused.
     */
    __u32 handles[4];
  

•   __u32 pitches[4]; /* pitch for each plane */
  

•   __u32 offsets[4]; /* offset of each plane */
  

•   __u64 modifier[4]; /* ie, tiling, compress */
  

•   /** @pitches: Pitch (aka. stride), one per plane. */
  

•   __u32 pitches[4];
  

•   /** @offsets: Offset into the buffer, one per plane. */
  

•   __u32 offsets[4];
  

•   /**
  

•    * @modifier: Format modifier, one per plane. See ``DRM_FORMAT_MOD_*``
  

•    * constants in ``drm_fourcc.h``. All planes must use the same
  

•    * modifier. Ignored unless &DRM_MODE_FB_MODIFIERS is set in @flags.
  

•    */
  

•   __u64 modifier[4];
  

All these fields MBZ (not INVALID!) if the plane is unused.

Cheers, Daniel