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

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Message-ID: <CADnq5_Nej3yoh2sSKYQEWaC-gQ4JrwsHFRScbZ5swbwfpgjw_g@mail.gmail.com>
Date:   Mon, 10 Apr 2017 12:28:59 -0400
From:   Alex Deucher <alexdeucher@...il.com>
To:     Gerd Hoffmann <kraxel@...hat.com>,
        Maling list - DRI developers 
        <dri-devel@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [RfC PATCH] drm: fourcc byteorder: brings header file comments in
 line with reality.

On Mon, Apr 10, 2017 at 8:02 AM, Daniel Vetter <daniel@...ll.ch> wrote:
> 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@...ux.intel.com>
>> Cc: Daniel Vetter <daniel.vetter@...el.com>
>> Cc: amd-gfx@...ts.freedesktop.org
>> Signed-off-by: Gerd Hoffmann <kraxel@...hat.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.

For radeon/amdgpu, we try to be good endian citizens when writing the
code, but as far as testing and support, it's all LE.  For a lot of hw
blocks, the surface format is the least of your worries.

Alex

> -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
>>
>> _______________________________________________
>> dri-devel mailing list
>> dri-devel@...ts.freedesktop.org
>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
> _______________________________________________
> amd-gfx mailing list
> amd-gfx@...ts.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/amd-gfx

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