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

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Message-ID: <20170419100956.78e4a7ab@eldfell>
Date:   Wed, 19 Apr 2017 10:09:56 +0300
From:   Pekka Paalanen <ppaalanen@...il.com>
To:     Michel Dänzer <michel@...nzer.net>
Cc:     Gerd Hoffmann <kraxel@...hat.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        amd-gfx@...ts.freedesktop.org,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [RfC PATCH] drm: fourcc byteorder: brings header file comments
 in line with reality.

On Wed, 19 Apr 2017 10:01:47 +0900
Michel Dänzer <michel@...nzer.net> wrote:

> On 18/04/17 07:14 PM, Gerd Hoffmann wrote:
> >   Hi,
> >   
> >>> Quite true that this proves nothing. However one should note that
> >>> fbcon -> fbdev works,  
> >>
> >> BTW, this supports Gerd's patch, since the KMS fbdev emulation code uses
> >> e.g. DRM_FORMAT_XRGB8888 for depth/bpp 24/32, and the fbdev API uses
> >> native endian packed colour values.  
> > 
> > Same is true for DRM_IOCTL_MODE_ADDFB, with depth/bpp 24/32 you'll get
> > DRM_FORMAT_XRGB8888 (only DRM_IOCTL_MODE_ADDFB2 allows userspace specify
> > fourcc formats directly).  
> 
> Right, and since all major Xorg drivers use DRM_IOCTL_MODE_ADDFB,
> they're effectively using DRM_FORMAT_XRGB8888 as native endianness as well.

I sincerely hope this doesn't actually force us into a place where we
have XRGB8888 (and ARGB8888?) as native-endian, but the other format
codes - since being used explicitly - must be kept as little-endian
because they were used like that honouring the documentation we have
atm. It's starting to resemble the wl_shm format codes problem we have
on Wayland for BE.

Has this now turned into a question of what the kernel drivers do
with the DRM pixel format codes?

Hmm, I suppose that has been the question all along... userspace uses
whatever formats that look right, new kernel drivers come along and fix
themselves to work with whatever userspace uses? And the people who get
it wrong are the ones who only ever test on LE and trust the docs...

Thanks,
pq

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