Linux: Permanent graphics mode (resolution) on Cinnamon

This post was written by eli on August 4, 2019
Posted Under: cinnamon,Linux

The goal

Quite simple: Set a fixed graphics mode on the computer screen.

More precisely, make Cinnamon (version 3.2.6) on Linux Mint 18.1 (Serena) show the desktop with a predefined resolution, no matter what happens. Spoiler: I failed. But I got close enough for practical purposes, and collected a lot of knowledge while trying. So here it is.

The reason I need this: On the machine mentioned here, I have an two screens connected through an HDMI splitter, so the monitor identification is somewhat shaky, and it’s not clear which piece of info the computer gets each time. To make it even trickier, the graphics mode I need is only listed in the EDID information submitted by one of the monitors. In other words: More often than not, the computer doesn’t know it’s allowed to use the mode I want it to use.

This situation meets the somewhat arrogant “I know what’s best, I never fail” attitude often seen by graphics software. There is more than one automatic mechanism for changing the resolution to “what is correct”, so just changing the resolution with xrandr doesn’t cut. The underlying mechanisms seem to change frequently from one version to another, and having them documented is probably too much to ask for. It seems like there are some race conditions taking place between different utilities that have a say on this matter. Possibly the reason for the problem I tried to solve on this post.

For clarity: EDID is a chunk of data that is typically stored on a small flash memory on the monitor. This data is fetched through I2C wires that are part of an DVI / HDMI / VGA connector when the monitor is plugged in. This is how the computer knows not only the commercial name of the monitor, but also what graphics modes it supports and prefers.

How cinnamon selects the resolution to use

So — the first question is: How does (my very specific) Cinnamon determine which screen resolution is the “correct” one?

This is a journey into the realm of mystery and uncertainty, but it seems like the rationale is to remember previously connected monitors, along with a separate user-selected graphics mode for each.

So the steps are probably something like:

  • Grab the list of allowed resolution modes, as presented by xrandr, for the relevant monitor (through libxrandr?). This is typically the set of modes listed in the monitor’s EDID information, but it’s possible to add modes as well (see below).
  • If there’s a user logged in, look up .config/monitors.xml in that user’s home directory. If there’s a match between the monitor’s product identification, apply the selected resolution. This file is changed by Cinnamon’s Display setting utility (among others, I guess), and presents the user’s preferences.
  • There’s possibly also a globally default monitor.xml at /etc/gnome-settings-daemon/xrandr/. I don’t have such file, and it’s not clear if it’s in effect had it existed. I haven’t tried this one.
  • If there’s no matching (or adequate?) mode setting in monitor.xml (or no user logged in), choose the preferred mode, as pointed at by xrandr.

This way or another, monitors.xml only lists width, height and rate for each graphics mode, without the timing details that are required to run it properly. So if the resolution requested in monitors.xml isn’t listed by xrandr, there is no way to request it, as there is crucial information missing. This isn’t supposed to happen ever, since the utility that sets the user’s preferences isn’t supposed to select a mode that the monitor doesn’t support. But if it does, the logical thing would be to ignore the resolution in monitors.xml, and go on with the monitor’s preferred mode. In reality, it appears like this causes the blank screen that I’ve mentioned on this post.

The automatic setting of resolution seems to take place when some kind of X session starts (the login screen and after the user logs in) as well as when a new monitor is hotplugged. Setting a monitor’s mode with xrandr seems to trigger an automatic setting as well sometimes. Having tried to set the resolution with xrandr a few times, it reverts sometimes to the automatic setting, and sometimes it stays with the one I set. Go figure.

How I got it done

Since there are all kinds of ghosts in the system that insist on “fixing” the display resolution, I might as well play along. So the trick is as follows:

  • Edit ~/.config/monitors.xml (manually), setting the resolution for all monitors listed to the one I want.
  • Make sure that the desired graphics mode, along with its timing parameters, is listed by xrandr, even if the monitor didn’t mention it in its EDID info.The first step is relatively easy. The entries in the XML file look like this:
      <output name="HDMI3">
          <vendor>SNY</vendor>
          <product>0x0801</product>
          <serial>0x01010101</serial>
          <width>1360</width>
          <height>768</height>
          <rate>60</rate>
          <x>0</x>
          <y>0</y>
          <rotation>normal</rotation>
          <reflect_x>no</reflect_x>
          <reflect_y>no</reflect_y>
          <primary>yes</primary>
      </output>

This is after editing the file. I needed 1360 x 768 @60 Hz, as shown above. So just set the width, height and rate tags in the XML file for all entries. No matter what monitor the system thinks it sees, the “user preference” is the same.

Now making sure that the mode exists: Add something like the following as /etc/X11/Xsession.d/10add-xrandr-mode (owned by root, not executable, no shebang):

xrandr -d :0 --newmode "hdmi_splitter" 85.5 1360 1424 1536 1792 768 771 777 795 +hsync +vsync
xrandr -d :0 --addmode HDMI3 hdmi_splitter
xrandr -d :0 --output HDMI3 --mode hdmi_splitter

Needless to say (?), this relates to the specific graphics mode.

So this file is executed every time X is started (and hence the xrandr modes list is cleared). All it does is making sure that the relevant output port (HDMI3) knows how to display 1360 x 768. Note that the name of the mode has no particular significance, and that the frame rate isn’t given explicitly, but is calculated by the tools. I got these figures from an xrandr readout with the desired monitor connected directly. See the full listing at the end of this post. It’s the first entry there.

The third command actually switches the display to the desired mode. It can be removed actually, because it’s overridden very soon anyhow. Nevertheless, it shows the command that can be used manually on console, given the two earlier commands (should not be needed, given that the mode is invoked automatically, fingers crossed).

That’s it. Except for occasional glitches (getting full control of this was too much to expect), the two actions mentioned above are enough to get the mode I wanted. Not the “no matter what” I wanted, but close enough.

As for the -d :0 flags, it’s required in remote sessions and scripts. Alternatively, start with an

$ export DISPLAY=:0

Using cvt to obtain the timing parameters (not!)

It’s suggested on some websites to obtain the timing parameters with something like

$ cvt 1360 768 60
# 1360x768 59.80 Hz (CVT) hsync: 47.72 kHz; pclk: 84.75 MHz
Modeline "1360x768_60.00"   84.75  1360 1432 1568 1776  768 771 781 798 -hsync +vsync

I tried this, and the monitor didn’t sync on the signal. It’s indeed a pretty lousy monitor to miss on a DVI signals, and still.

Note the small differences between the timing parameters — that’s probably the reason for this failure. So when the real parameters can be obtained, use them. There is no secret catch-all formula for all graphics modes. The formula works on a good day.

Hands off, Cinnamon’s daemon!

My original idea was to turn off all automatic graphics mode setting mechanisms, and stay with a single xrandr command, running from /etc/X11/Xsession.d/ or something. It was a great idea, but it didn’t work: I saw a momentary switch to the mode I wanted, and then it changed to something else. I could have added some kind of daemon of my own, that waits a bit and then changes the mode with xrandr, but that’s just adding another daemon to wrestle with the others.

So this didn’t really help, but I’ll leave it here anyway, in case someone wants to change the display mode without having some daemon change it back. Note that according to this page, using gsettings as shown below works only up to Cinnamon before version 3.4, after which the procedure is different (haven’t tried it however): Copy /etc/xdg/autostart/cinnamon-settings-daemon-xrandr.desktop to $HOME/.config/autostart. Then append the line Hidden=true to the copied file.

In short, YMMV. Here’s how I did it on my system (and then found it’s not good enough, as mentioned above).

Resolution mode settings made with xrandr will be sporadically overridden by cinnamon-settings-daemon, which has a lot of plugins running for different housekeeping tasks. One of them is to keep X-Window’s display resolution in sync with .config/monitors.xml. So disable it.

Following my own post, this is typically the setting for the said plugin:

$ gsettings list-recursively org.gnome.settings-daemon.plugins.xrandr
org.gnome.settings-daemon.plugins.xrandr active true
org.gnome.settings-daemon.plugins.xrandr priority 0
org.gnome.settings-daemon.plugins.xrandr default-monitors-setup 'follow-lid'
org.gnome.settings-daemon.plugins.xrandr default-configuration-file '/etc/gnome-settings-daemon/xrandr/monitors.xml'

So turn it off:

$ gsettings set org.gnome.settings-daemon.plugins.xrandr active false

and then check again with the list-recursively command above.

xrandr output: The full list of modes

Just for reference, these are the modes given by xrandr for the monitor I did all this for:

$ xrandr -d :0 --verbose

[ ... ]

  1360x768 (0x4b) 85.500MHz +HSync +VSync *current +preferred
        h: width  1360 start 1424 end 1536 total 1792 skew    0 clock  47.71KHz
        v: height  768 start  771 end  777 total  795           clock  60.02Hz
  1920x1080i (0x10b) 74.250MHz -HSync -VSync Interlace
        h: width  1920 start 2008 end 2052 total 2200 skew    0 clock  33.75KHz
        v: height 1080 start 1084 end 1094 total 1125           clock  60.00Hz
  1920x1080i (0x10c) 74.250MHz +HSync +VSync Interlace
        h: width  1920 start 2008 end 2052 total 2200 skew    0 clock  33.75KHz
        v: height 1080 start 1084 end 1094 total 1125           clock  60.00Hz
  1920x1080i (0x10d) 74.250MHz +HSync +VSync Interlace
        h: width  1920 start 2448 end 2492 total 2640 skew    0 clock  28.12KHz
        v: height 1080 start 1084 end 1094 total 1125           clock  50.00Hz
  1920x1080i (0x10e) 74.176MHz +HSync +VSync Interlace
        h: width  1920 start 2008 end 2052 total 2200 skew    0 clock  33.72KHz
        v: height 1080 start 1084 end 1094 total 1125           clock  59.94Hz
  1280x720 (0x10f) 74.250MHz -HSync -VSync
        h: width  1280 start 1390 end 1430 total 1650 skew    0 clock  45.00KHz
        v: height  720 start  725 end  730 total  750           clock  60.00Hz
  1280x720 (0x110) 74.250MHz +HSync +VSync
        h: width  1280 start 1390 end 1430 total 1650 skew    0 clock  45.00KHz
        v: height  720 start  725 end  730 total  750           clock  60.00Hz
  1280x720 (0x111) 74.250MHz +HSync +VSync
        h: width  1280 start 1720 end 1760 total 1980 skew    0 clock  37.50KHz
        v: height  720 start  725 end  730 total  750           clock  50.00Hz
  1280x720 (0x112) 74.176MHz +HSync +VSync
        h: width  1280 start 1390 end 1430 total 1650 skew    0 clock  44.96KHz
        v: height  720 start  725 end  730 total  750           clock  59.94Hz
  1024x768 (0x113) 65.000MHz -HSync -VSync
        h: width  1024 start 1048 end 1184 total 1344 skew    0 clock  48.36KHz
        v: height  768 start  771 end  777 total  806           clock  60.00Hz
  800x600 (0x114) 40.000MHz +HSync +VSync
        h: width   800 start  840 end  968 total 1056 skew    0 clock  37.88KHz
        v: height  600 start  601 end  605 total  628           clock  60.32Hz
  720x576 (0x115) 27.000MHz -HSync -VSync
        h: width   720 start  732 end  796 total  864 skew    0 clock  31.25KHz
        v: height  576 start  581 end  586 total  625           clock  50.00Hz
  720x576i (0x116) 13.500MHz -HSync -VSync Interlace
        h: width   720 start  732 end  795 total  864 skew    0 clock  15.62KHz
        v: height  576 start  580 end  586 total  625           clock  50.00Hz
  720x480 (0x117) 27.027MHz -HSync -VSync
        h: width   720 start  736 end  798 total  858 skew    0 clock  31.50KHz
        v: height  480 start  489 end  495 total  525           clock  60.00Hz
  720x480 (0x118) 27.000MHz -HSync -VSync
        h: width   720 start  736 end  798 total  858 skew    0 clock  31.47KHz
        v: height  480 start  489 end  495 total  525           clock  59.94Hz
  720x480i (0x119) 13.514MHz -HSync -VSync Interlace
        h: width   720 start  739 end  801 total  858 skew    0 clock  15.75KHz
        v: height  480 start  488 end  494 total  525           clock  60.00Hz
  720x480i (0x11a) 13.500MHz -HSync -VSync Interlace
        h: width   720 start  739 end  801 total  858 skew    0 clock  15.73KHz
        v: height  480 start  488 end  494 total  525           clock  59.94Hz
  640x480 (0x11b) 25.200MHz -HSync -VSync
        h: width   640 start  656 end  752 total  800 skew    0 clock  31.50KHz
        v: height  480 start  490 end  492 total  525           clock  60.00Hz
  640x480 (0x11c) 25.175MHz -HSync -VSync
        h: width   640 start  656 end  752 total  800 skew    0 clock  31.47KHz
        v: height  480 start  490 end  492 total  525           clock  59.94Hz

The vast majority are standard VESA modes.

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