We had a complaint recently from a customer that received a server we shipped
out that their monitor just showed a black screen. It turns out that they’d
hooked it up to an ancient POS with 800x600 as its best resolution, and gdm in
CentOS 7 apparently assumes at least 1024x768. It was apparently a major
hassle for them to hunt down a reasonable monitor for us.
Since we almost never use the GUI on these things, we thought we’d solve that
problem going forward by shipping these boxes out to use a fixed 800x600, so
they’ll work on any monitor the user plugs in.
The first problem you run into is that the Settings > Devices > Display GUI
only affects the login you do it under. GDM doesn’t pay attention to this, not
even to the configuration set as root!
Then you go web searching and find solutions involving xrandr and advice to
just put it in /etc/gdm/Init/Default, but that doesn’t seem to work.
I then got soooo close when I found the advice to set the monitor configuration
in the GUI, then copy ~/.config/monitors.xml to /var/lib/gdm/.config. That
works…until you plug in a different monitor, because that file has a hard tie
to the monitor brand, model and *serial number*!
The monitors.xml file looks like this:
<monitors version="2">
<configuration>
<migrated/>
<logicalmonitor>
<x>0</x>
<y>0</y>
<primary>yes</primary>
<monitor>
<monitorspec>
<connector>VGA-1</connector>
<vendor>SAM</vendor>
<product>SyncMaster</product>
<serial>H1AK500000</serial>
</monitorspec>
<mode>
<width>800</width>
<height>600</height>
<rate>60</rate>
</mode>
</monitor>
</logicalmonitor>
</configuration>
</monitors>
I have tried:
1. Removing the <monitorspec> section entirely. gdm then gives me a black
screen on “systemctl restart gdm” from a text console.
2. Filling out all fields except <connector> with values like “default”,
“generic”, “any”, “*”, etc. GDM decides you couldn’t possibly mean for this
configuration to apply to that monitor, since it differs, so it ignores you and
does whatever it likes instead.
3. Lots and lots of web searching for documentation on the file format to see
if I can find out if there is a different way of specifying “any monitor, I
don’t care”.
I was about to go reading the source code, but I thought I’d ask here first.
Keep in mind that we ship these systems out without knowing what monitor
they’ll plug in, and once the systems are out in the field, it’s often
difficult to get remote access to them. That means we can’t just log in with
SSH, probe it with xrandr, and fix it remotely; we’re probably having the
customer log in via the GUI to fix the problem that prevents us from SSHing in
in the first place!
It’s fine in our case to pin the configuration to “VGA-1”, because we know what
connectors are on the server itself. We just don’t know what the customer will
plug into it.
I assume there’s a minimum screen resolution for CentOS 7, probably 1024x768,
but we’re installing these systems with a reasonable screen. We only need this
low resolution post-install, and then only so the customer can pull up a
terminal and maybe run the Network settings tool. For that, 800x600 is fine.
You do have to drag the Settings app around a bit to see everything, but it
works.
Our current fallback is Ctrl-Alt-F2 and nano, which is tolerable for some
things — “Okay, type ‘ifconfig’…now find the line that has ‘inet’ in it, and
read that back to me…’ — but remotely talking someone through changing
ifcfg-noisenoise via nano is a minor nightmare, especially now that Confusing
Network Device Naming is the default.
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