xserver and splitting ultra-wide monitors

[Michael Wyraz]

Hello Xorg developers,

about a year ago I investigated in how to split ultra-wide monitors on
linux into multiple virtual monitors. While this is basically possible
with xserver and xrandr, a combination of different issues stops from
using this feature on most desktops. Unfortunately, the issues still
exist. I'd like to start another attempt to get this finally work.

The first step would be xserver and xrandr.


It is easily possible to split a monitor with xrandr into two:

  xrandr --setmonitor VIRTUAL-LEFT 2560/0x1440/1+0+0 DP-4
  xrandr --setmonitor VIRTUAL-RIGHT 2560/1x1440/1+2560+0 none

For the second monitor, the output must be set to "none" which is
obviously wrong since it is connected to a device. The reason why it is
set to "none" is some code in xserver that removes an monitor if another
one is added to the same output:

https://gitlab.freedesktop.org/mwyraz/xserver/-/merge_requests/1/diffs

I believe, this code should be removed entirely for two reasons. The
first is, that two virtual monitors on one output is perfectly valid.
The other is, that the function is about adding monitor. It should not
be silently delete another monitor.

If there is a reason to delete one monitor if another is added, such
functionality should be part of the tooling (like in xrandr) - this
would also allow to implement different behaviours (e.g. a warning or an
option to enforce to have multiple monitors on one output).

What do you think about this? Could you help to get this solved?

If so and this could be fixed, my next step would be to address the
related issue in gtk (https://gitlab.gnome.org/GNOME/gtk/-/issues/2013)
so that gtk based desktops would properly work on a splitted ultra-wide
screen.

Kind regards,

Michael.

[Keith Packard]

Michael Wyraz <mic...@wyraz.de> writes:

> For the second monitor, the output must be set to "none" which is
> obviously wrong since it is connected to a device. The reason why it is
> set to "none" is some code in xserver that removes an monitor if another
> one is added to the same output:

That's actually required in the RandR spec:

For each output in 'info.outputs, each one is removed from all
pre-existing Monitors. If removing the output causes the list of
outputs for that Monitor to become empty, then that Monitor will
be deleted as if RRDeleteMonitor were called.

The notion of splitting one physical output into multiple virtual
monitors was not considered when this extension was defined, which is
why it doesn't work. I don't see any particular reason for *not*
supporting your use case.

However, there are subtleties here. We want to remove any automatically
created 'Monitor' objects when mapping user-specified monitors to
them, and we want to re-generate automatically generated 'Monitors' when
all virtual monitors associated with an output are removed.

I think what we want is:

* If no user-specified Monitors map to a particular Output, then automatically
create a Monitor for that Output

* If any user-specified Monitors map to a particular Output, then
remove the automatically generated Monitor for that Output.

In the current spec, there's no real separation between user-specified
and automatically-generated Monitors, I think that would be necessary to
make this work?

--
-keith

[Michael Wyraz]

Hello Keith,

I'm glad that you have a look on this. I first tried to contact you
directly using the intel address in
https://gitlab.freedesktop.org/xorg/doc/xorg-docs/-/blob/master/MAINTAINERS
but this seems not be valid anymore.

I absolutely agree with what you wrote about the user-specified vs
automatically created monitors. But the code I'd like to remove does not
deal with these. It's only about having 2 monitors on one output.

https://gitlab.freedesktop.org/xorg/xserver/-/blob/master/randr/rrmonitor.c

Lines 526-529 should not be touched. If a monitor with the same name
will be added elsewhere, the existing should be removed (avoids to have
multiple monitors with the same name)

Lines 537-551 sould be removed. It's only about having not more than one
monitor on one output. I removed these on my local xserver and did the
following test:

# xrandr --listmonitors
Monitors: 1
 0: +*DisplayPort-1 3440/820x1440/346+0+0  DisplayPort-1
# xrandr --setmonitor VIRTUAL-LEFT 1720/0x1440/1+0+0 DisplayPort-1
# xrandr --listmonitors
Monitors: 1
 0: VIRTUAL-LEFT 1720/0x1440/1+0+0  DisplayPort-1
# xrandr --setmonitor VIRTUAL-RIGHT 1720/0x1440/1+1720+0 DisplayPort-1
# xrandr --listmonitors
Monitors: 2
 0: VIRTUAL-LEFT 1720/0x1440/1+0+0  DisplayPort-1
 1: VIRTUAL-RIGHT 1720/0x1440/1+1720+0  DisplayPort-1
# xrandr --delmonitor VIRTUAL-LEFT
# xrandr --delmonitor VIRTUAL-RIGHT
# xrandr --listmonitors
Monitors: 1
 0: +*DisplayPort-1 3440/820x1440/346+0+0  DisplayPort-1

As you can see, all works fine, I can create 2 monitors on the one
output and the handling of the auto-generated monitor still works as
expected.

When I do
# xrandr --fb 3440x1441; xrandr --fb 3440x1440
afterwards (this triggers an update to the display manager), I can use
both monitors on xfce (at least I can drag windows over both and
maximize on the second but that's a different issue within gtk).

Kind regards,

Michael.


Am 29.09.22 um 22:41 schrieb Keith Packard:

[Michael Wyraz]

Hello xorg developers,

I have attached my patch to xserver that removes the
"one-monitor-per-output" restriction. My attempt and the result is
described in
https://gitlab.gnome.org/GNOME/gtk/-/issues/2013#note_1564376 . The
result is amazing, split-screen works flawlessly with all of my desktop
applications, similar as if I had 2 monitors.

I'd be happy if that could make it into xserver. Should I create a PR
from the patch in the gitlab?

Kind regards,

Michael.


Am 29.09.22 um 22:41 schrieb Keith Packard:

[Michael Wyraz]

Hello xorg developers,

I have created a MR for this feature at
https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/981. What
do you think about it? Could it be merged?

Kind regards,

Michael.

Am 01.10.22 um 09:52 schrieb Michael Wyraz:

[Michael Wyraz]

Hello Keith,

I have verified that the current implementation actually works exactly
as suggested. Initially, an auto-generated monitor exists. Adding a user
defined monitor replaces it. Removing the user defined monitor restores
the auto-generated monitor.

I have refined the randr spec regarding this. I also changed the
definition to allow more than one monitor per output. The change can be
found at

https://gitlab.freedesktop.org/xorg/proto/xorgproto/-/merge_requests/64

After I got some advice from Oliver how to improve my PR, I updated the
description there and added my findings as well as an estimation what
side effects this change could have. This PR to xorg-server is at

https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/981

Please let me know if there is anything missing or if some more
discussion is required to get this merged.

Kind regards,
Michael.