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

Pekka Paalanen ppaalanen at gmail.com
Tue Apr 18 11:18:27 UTC 2017

On Tue, 18 Apr 2017 12:00:17 +0200
Gerd Hoffmann <kraxel at redhat.com> wrote:

>   Hi,
> > > 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...  
> So, did some more testing to see where we stand.
> Historical note:  RHEL-6.9 (gnome 2) works fine.  Not of much interest
> here, it drives the qemu stdvga with offb, not bochs-drm.

I suppose this proves the virtual machine itself is correct about
framebuffer endianess? Except you are running it on a little-endian
host machine I presume...

> More interesting:  RHEL-7.3 (gnome 3.14) works fine too.  kernel 3.10,
> but drm drivers updated to roughly 4.6 level.  Runs bochs-drm.  mesa
> 11.2.2.  glamour not used.
> Most recent:  Fedora 25 (gnome 3.22) looks mostly ok, but there are
> rendering glitches, for example in the gnome activities screen (the one
> you get when you press the windows key).  kernel 4.10, mesa 13.0.4.
> glamor not used, but I think gnome-shell uses opengl (via llvmpipe) for
> compositing.

I believe glitches are irrelevant for this topic, what we are
interested in is if the colors are right or byte-swapped (also mind
alpha/blue etc. swaps).

> btw: is there some way to start a wayland session from a shell (i.e.
> what startx does for xorg)?

Depends on which display server you want to start I believe. I don't
know about anything else than Weston, which is 'weston' for a
logind-enabled system.

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