[RfC PATCH] drm: fourcc byteorder: brings header file comments in line with reality.

[Daniel Vetter]

On Mon, Apr 10, 2017 at 12:12:01PM +0200, Gerd Hoffmann wrote:
> Ok, this is really a kickstart for a discussion.  While working on
> graphics support for virtual machines on ppc64 (which exists in both
> little and big endian variants) I've figured the comments in the header
> file don't match reality.  They are not considered little endian (as
> suggested by the comments) but in practice are used as native endian.
> 
> So, go update the comments.
> 
> This patch switches all 32bpp / 8 bpc formats over to native endian.
> Those used/supported by ppc64 virtual machines (virtio-gpu/bochs-drm
> drivers).
> 
> Given that DRM_FORMAT_BIG_ENDIAN isn't used anywhere in the codebase
> I suspect this problem applies to more formats.  When looking at
> drm_mode_legacy_fb_format it seems *all* RGB formats are actually
> native endian not little endian.
> 
> Dunno where we stand in terms of YCbCr.
> 
> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Cc: Daniel Vetter <daniel.vetter at intel.com>
> Cc: amd-gfx at lists.freedesktop.org
> Signed-off-by: Gerd Hoffmann <kraxel at redhat.com>

I think we should go all-in on this and essentially remove the _BIG_ENDIAN
stuff. But in the end this is to be decided be the people who care about
big endian, afaik that's only you (for the pile of virt drivers we have)
and amdgpu/radeon.
-Daniel

> ---
>  include/uapi/drm/drm_fourcc.h | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/include/uapi/drm/drm_fourcc.h b/include/uapi/drm/drm_fourcc.h
> index 995c8f9..a7fc81d 100644
> --- a/include/uapi/drm/drm_fourcc.h
> +++ b/include/uapi/drm/drm_fourcc.h
> @@ -85,15 +85,15 @@ extern "C" {
>  #define DRM_FORMAT_BGR888	fourcc_code('B', 'G', '2', '4') /* [23:0] B:G:R little endian */
>  
>  /* 32 bpp RGB */
> -#define DRM_FORMAT_XRGB8888	fourcc_code('X', 'R', '2', '4') /* [31:0] x:R:G:B 8:8:8:8 little endian */
> -#define DRM_FORMAT_XBGR8888	fourcc_code('X', 'B', '2', '4') /* [31:0] x:B:G:R 8:8:8:8 little endian */
> -#define DRM_FORMAT_RGBX8888	fourcc_code('R', 'X', '2', '4') /* [31:0] R:G:B:x 8:8:8:8 little endian */
> -#define DRM_FORMAT_BGRX8888	fourcc_code('B', 'X', '2', '4') /* [31:0] B:G:R:x 8:8:8:8 little endian */
> +#define DRM_FORMAT_XRGB8888	fourcc_code('X', 'R', '2', '4') /* [31:0] x:R:G:B 8:8:8:8 native endian */
> +#define DRM_FORMAT_XBGR8888	fourcc_code('X', 'B', '2', '4') /* [31:0] x:B:G:R 8:8:8:8 native endian */
> +#define DRM_FORMAT_RGBX8888	fourcc_code('R', 'X', '2', '4') /* [31:0] R:G:B:x 8:8:8:8 native endian */
> +#define DRM_FORMAT_BGRX8888	fourcc_code('B', 'X', '2', '4') /* [31:0] B:G:R:x 8:8:8:8 native endian */
>  
> -#define DRM_FORMAT_ARGB8888	fourcc_code('A', 'R', '2', '4') /* [31:0] A:R:G:B 8:8:8:8 little endian */
> -#define DRM_FORMAT_ABGR8888	fourcc_code('A', 'B', '2', '4') /* [31:0] A:B:G:R 8:8:8:8 little endian */
> -#define DRM_FORMAT_RGBA8888	fourcc_code('R', 'A', '2', '4') /* [31:0] R:G:B:A 8:8:8:8 little endian */
> -#define DRM_FORMAT_BGRA8888	fourcc_code('B', 'A', '2', '4') /* [31:0] B:G:R:A 8:8:8:8 little endian */
> +#define DRM_FORMAT_ARGB8888	fourcc_code('A', 'R', '2', '4') /* [31:0] A:R:G:B 8:8:8:8 native endian */
> +#define DRM_FORMAT_ABGR8888	fourcc_code('A', 'B', '2', '4') /* [31:0] A:B:G:R 8:8:8:8 native endian */
> +#define DRM_FORMAT_RGBA8888	fourcc_code('R', 'A', '2', '4') /* [31:0] R:G:B:A 8:8:8:8 native endian */
> +#define DRM_FORMAT_BGRA8888	fourcc_code('B', 'A', '2', '4') /* [31:0] B:G:R:A 8:8:8:8 native endian */
>  
>  #define DRM_FORMAT_XRGB2101010	fourcc_code('X', 'R', '3', '0') /* [31:0] x:R:G:B 2:10:10:10 little endian */
>  #define DRM_FORMAT_XBGR2101010	fourcc_code('X', 'B', '3', '0') /* [31:0] x:B:G:R 2:10:10:10 little endian */
> -- 
> 2.9.3
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch