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

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Message-ID: <668bc373-1614-a095-6085-15c040f322b8@daenzer.net>
Date:   Thu, 27 Apr 2017 16:02:58 +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 27/04/17 03:45 PM, Gerd Hoffmann wrote:
>   Hi,
> 
>>> That is done using the RADEON_TILING_SWAP_{16,32}BIT flag mentioned in
>>> another thread?
>>
>> Right.
>>
>>
>>> What about dumb bos?  You've mentioned the swap flag isn't used for
>>> those.  Which implies they are in little endian byte order (both gpu and
>>> cpu view).
>>
>> Right, AFAICT from looking at the code.
> 
> Ok.
> 
> And I also don't see an easy way to make them big endian (cpu view)
> using swapping with the existing drm interfaces, given we apply a format
> when we put the bo into use as framebuffer, not when creating it.  So
> userspace can: (1) create dumb bo, (2) map bo, (3) write something bo,
> (4) create fb + attach to crtc.  And at (3) we don't know the format
> yet, so we can't configure swapping accordingly.
> 
> So just not using the swapping indeed looks like the only sensible
> option.  Which in turn implies there is no BGRA8888 support for dumb
> bos.  Hmm, I can see the problem.  Userspace expectation appears to be
> that ADDFB configures a native endian framebuffer, which the driver
> simply can't do on bigendian.

... with pre-R600 GPUs.


> So, what can/should the driver do here?  Throw errors for ADDFB and
> force userspace to use ADDFB2?  From a design point of view the best
> option, but in the other hand I suspect that could break the xorg radeon
> driver ...

Yes, it would.

One thing we could do is provide a way for userspace to query the
effective format(s) as seen by the GPU and/or CPU.

It might also make sense for the radeon driver to set the
RADEON_TILING_SWAP_{16,32}BIT flags for dumb BOs. I wonder about the
status of apps using dumb BOs directly wrt this discussion.


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