Bug#867555: gnome-shell: Tries to drive monitor at unsupported refresh rate

[Simon McVittie]

On Fri, 07 Jul 2017 at 13:20:28 +0100, Mark Brown wrote:
> On Fri, Jul 07, 2017 at 12:44:46PM +0100, Simon McVittie wrote:
> 
> > My first question is, what graphics hardware and driver is this?
> 
> It's this:
> 
> 05:00.0 VGA compatible controller: Advanced Micro Devices, Inc.  [AMD/ATI] Cedar [Radeon HD 5000/6000/7350/8350 Series]
> 
> using whatever Debian drives it with by default.

That would be the radeon or possibly amdgpu kernel driver, together with
corresponding DRM, Mesa and X drivers in user-space, assuming you haven't
installed the proprietary fglrx driver and forgotten you did so.
I'm afraid I don't know the finer points of which driver gets used
for which hardware under which conditions. lsmod would probably tell you.

> > * Move your ~/.config/monitors.xml out of the way (don't delete it; if
> >   this works, it would be useful to see what's in it). This is where
> >   GNOME stores display settings. You could also compare it with
> >   ~/.config/monitors.xml~ which is the second-most-recent version.
> 
> Removing my monitors.xml appears to fix things, the difference appears
> to be that if I try to make any change to the default layout of the
> monitors monitor 1 goes blank.

Thanks. It seems that the mode in which GNOME Shell brings up your
displays when unconfigured is fine, but when you start reconfiguring
(for which I assume you're using gnome-control-center, aka "Settings"?),
some code that only runs during reconfiguration chooses an impossible
mode. Hopefully this will be enough for someone more knowledgeable than
me to narrow down which module has the problem.

> > * Look for mentions of EDID, DDC or mode in the Xorg log
> >   (could be /var/log/Xorg.*.log, ~/.cache/gdm/* or the systemd Journal
> >   depending how your X server was started)
> 
> I'm using a default Debian system with systemd, I don't know how
> specifically the X server is started. The X logs haven't been updated
> for a long time.  There's no obvious X logs in .cache/gdm and I've no
> idea how to get X logs from systemd.

Please check /var/log/syslog (as root or a member of group adm), assuming
you haven't removed rsyslogd. X logs go there via the systemd Journal;
the Journal is in-memory-only by default, because writing it to disk
is mostly redundant with having a syslogd.

> XWAYLAND0 connected 2560x1440+0+0 (0x26) normal (normal left inverted right x axis y axis) 550mm x 310mm

That's XWayland, which uses a Wayland compositor as its "hardware",
and can't do mode switching (it reports that the current Wayland mode
is the only mode possible). When using Wayland, GNOME Shell does the
mode-switching without any X involvement.

If you didn't mean to be using Wayland, please try xrandr under an
X11 GNOME session or a non-GNOME X11 session.

If you are intentionally living in the future, someone who knows more
about the DRI/DRM stack than I do will have to tell you what the
Wayland equivalent of xrandr is. Rummaging in /sys/class/drm/card*/modes,
or using parse-edid < /sys/class/drm/cardwhatever/edid (parse-edid is
in the read-edid package), might be informative?

(GNOME in Debian is probably going to default to Wayland when we upgrade to
GNOME 3.24, but for the 3.22 versions currently in stretch and buster/sid,
using Wayland has too many papercuts to be the default.)

    S