bug#36786: Warn of AMD GPUs unusable with Guix System

["pelzflorian (Florian Pelz)" <pelzflorian@pelzflorian.de>]

Back in
<https://lists.gnu.org/archive/html/guix-devel/2019-04/msg00503.html>,
I wrote:

On Fri, Apr 26, 2019 at 10:35:43AM +0200, pelzflorian (Florian Pelz) wrote:
> On Sat, Apr 20, 2019 at 11:47:56AM +0200, Ludovic Courtès wrote:
> > "pelzflorian (Florian Pelz)" <pelzflorian@pelzflorian.de> skribis:
> > > I believe issues with (some?) AMD GPUs should be mentioned in the
> > > manual.  Also, on the downloads page perhaps it should be mentioned
> > > that there may be issues with some hardware, referring users to the
> > > Hardware Considerations in the Guix manual.
> > 
> > We could do that, but these kind of kernel-side issues tend to appear
> > and vanish quickly, no?  But I don’t know much about these AMD GPU
> > issues, so I’m happy to do what people consider appropriate.
> > 
> 
> My AMD GPU machine started working with VESA Xorg drivers at full resolution.
> I reconfigured a while ago but did not test back then.

Aside from the fact that 3d acceleration is out of reach for modern
AMD GPUs on a linux-libre kernel, Guix had at least two further
reports of trouble with old and very new AMD GPUs like:

https://lists.gnu.org/archive/html/help-guix/2019-06/msg00227.html

and

https://lists.gnu.org/archive/html/help-guix/2019-07/msg00179.html


Part of the reason a warning about AMD GPUs should be considered is
that AMD Radeon is often claimed to be usable with drivers that are
“entirely free and open-source software”
(<https://en.wikipedia.org/wiki/Radeon#Free_and_open-source_graphics_device_driver_%22Radeon%22>),
making people believe they do not require nonfree firmware.

I suppose but do not know that recent Nvidia hardware works without 3d
acceleration, even when the Nouveau Xorg driver does not support it
yet.  I suppose but am unsure if recent Intel GPUs work out of the box
without blobs.  If correct, this would make AMD in particular notable.

On the other hand, Pierre Neidhard commented at
<https://lists.gnu.org/archive/html/guix-devel/2019-04/msg00355.html>
that many AMD GPUs work well.

Possibly I am overly negative because my computer is affected as well.
Still, I think the hardware considerations section in the manual
should list at least that Xorg graphics on some AMD GPUs do not work
and that using the console might require tweaks like
modprobe.blacklist=radeon.  The website too should tell users on the
downloads page that, even though most machines work well, wi-fi
vendors like Broadcom and graphics card vendor AMD refuse to make *some*
of their devices work with the linux-libre kernel used by Guix System.

Regards,
Florian