linux-kernel - Re: [PATCH 3/6] drm: fourcc byteorder: add bigendian support to drm_mode_legacy_fb

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Date:   Wed, 26 Apr 2017 18:21:10 +0900
From:   Michel Dänzer <michel@...nzer.net>
To:     Gerd Hoffmann <kraxel@...hat.com>
Cc:     Daniel Vetter <daniel.vetter@...el.com>,
        amd-gfx@...ts.freedesktop.org,
        open list <linux-kernel@...r.kernel.org>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 3/6] drm: fourcc byteorder: add bigendian support to
 drm_mode_legacy_fb_format

On 26/04/17 02:53 PM, Gerd Hoffmann wrote:
> On Di, 2017-04-25 at 12:18 +0900, Michel Dänzer wrote:
>> On 24/04/17 03:25 PM, Gerd Hoffmann wrote:
>>> Return correct fourcc codes on bigendian.  Drivers must be adapted to
>>> this change.
>>>
>>> Signed-off-by: Gerd Hoffmann <kraxel@...hat.com>
>>
>> Just to reiterate, this won't work for the radeon driver, which programs
>> the GPU to use (effectively, per the current definition that these are
>> little endian GPU formats) DRM_FORMAT_XRGB8888 with pre-R600 and
>> DRM_FORMAT_BGRX8888 with >= R600.
> 
> Hmm, ok, how does bigendian fbdev emulation work on pre-R600 then?

Using a GPU byte swapping mechanism which only affects CPU access to
video RAM.


>>> +#ifdef __BIG_ENDIAN
>>> +	switch (bpp) {
>>> +	case 8:
>>> +		fmt = DRM_FORMAT_C8;
>>> +		break;
>>> +	case 24:
>>> +		fmt = DRM_FORMAT_BGR888;
>>> +		break;
>>
>> BTW, endianness as a concept cannot apply to 8 or 24 bpp formats.
> 
> I could move the 8 bpp case out of the #ifdef somehow, but code
> readability will suffer then I think ...

How so?

At least it would make clearer which formats are affected by endianness
and which aren't.


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer