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

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Message-ID: <20170410180941.43922e25@eldfell>
Date:   Mon, 10 Apr 2017 18:09:41 +0300
From:   Pekka Paalanen <ppaalanen@...il.com>
To:     Gerd Hoffmann <kraxel@...hat.com>
Cc:     dri-devel@...ts.freedesktop.org,
        open list <linux-kernel@...r.kernel.org>,
        amd-gfx@...ts.freedesktop.org,
        Daniel Vetter <daniel.vetter@...el.com>,
        Ilia Mirkin <imirkin@...m.mit.edu>
Subject: Re: [RfC PATCH] drm: fourcc byteorder: brings header file comments
 in line with reality.

On Mon, 10 Apr 2017 16:17:27 +0200
Gerd Hoffmann <kraxel@...hat.com> wrote:

>   Hi,
> 
> > which software have you used as representative of "reality"?  
> 
> ppc64 (big endian) virtual machine, running with qemu stdvga & bochs-drm
> driver.  Xorg with modesetting driver uses DRM_FORMAT_XRGB8888 (one and
> only format supported by bochs-drm), and we have to interpret that in
> bigendian byte order on the host side to get a correct display.

I wonder if that is just an oversight from trying to match OpenGL
formats to DRM formats. It's full of gotcha's.

Did you try with GLAMOR? Do you see a difference with and without
GLAMOR? Hmm, but you have no GPU support, so GLAMOR would be through a
Mesa software renderer? I think I heard someone say something about
Mesa software on BE...

But even if this actually is a valid example of software we must keep
working as is, well, ouch. But if I cannot show that your fix breaks
anything, then I suppose you win. Your proposal would certainly solve
the dilemma we have with wl_shm formats in Wayland.

I also wonder if a real BE machine could have different results than
the virtual machine.

Thanks,
pq

> Didn't try wayland.  Can do, but will take a while.  Don't have a
> wayland-capable guest install at hand, and installing one takes a while
> because I don't have a physical pseries and emulation is slooooww.
> 
> > To solve that problem, we would like to know if anything existing would
> > break for each possible solution, but no developers using BE have really
> > turned up.  
> 
> That is part of the problem.
> And even ppc is moving to little endian these days ...
> 
> cheers,
>   Gerd
> 

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