Status changed to 'Confirmed' because the bug affects multiple users.
** Changed in: mutter (Ubuntu)
Status: New => Confirmed
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https://bugs.launchpad.net/bugs/1942653
Title:
RandR fractional scaling patch interferes with manual RandR output
scaling
Status in mutter package in Ubuntu:
Confirmed
Bug description:
$ lsb_release -rd
Description: Ubuntu 20.04.3 LTS
Release: 20.04
$ apt-cache policy mutter
mutter:
Installed: 3.36.9-0ubuntu0.20.04.1
Candidate: 3.36.9-0ubuntu0.20.04.1
Version table:
*** 3.36.9-0ubuntu0.20.04.1 500
500 http://us.archive.ubuntu.com/ubuntu focal-updates/main amd64
Packages
100 /var/lib/dpkg/status
3.36.1-3ubuntu3 500
500 http://us.archive.ubuntu.com/ubuntu focal/main amd64 Packages
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As an example with one output, scaling it with e.g. 'xrandr --output
<output> --mode 3840x2160 --scale-from 1920x1080', would usually
result in a screen size of 1920x1080, as xrandr sets the screen size
to the bounding box of the configured crtcs.
Ubuntu carries an out-of-tree patch for Mutter:
x11-Add-support-for-fractional-scaling-using-Randr.patch, attached. It adds a
new function, meta_monitor_manager_xrandr_update_screen_size_derived(), that in
response to RandR configuration changes will attempt to set its own screen
size. When calculating the new screen size, it applies a scaling factor to each
crtc that effectively reverses the requested one. As a result, in the above
example the resulting screen size is 3840x2160, despite the fact that there is
only a 1920x1080 viewport into it.
With the modesetting driver, this isn't a huge issue. The resulting
screen size is larger than you can actually interact with, wasting
memory, but it doesn't cause any visible functional issues.
The NVIDIA driver automatically configures panning when the requested
outputs' viewports are smaller than the screen size. This increases
crtc size, which impacts Mutter's screen size calculation. When Mutter
sets the larger-than-outputs screen size, it triggers the NVIDIA
driver to automatically configure panning. Mutter reacts to the RandR
configuration change by again recalculating the screen size, this time
growing it even larger due to the larger crtcs. In response, the
NVIDIA driver sets up the new, larger panning configuration, causing
the crtcs to be even larger, and causing Mutter to once again
recalculate the screen size. This goes on until all memory is consumed
and gnome-shell is killed by the OOM killer, or the screen reaches
maximum size. In the meantime, the screen is black with only a cursor
while gnome-shell spins.
It appears that
meta_monitor_manager_xrandr_update_screen_size_derived() is intended
for GNOME fractional scaling, adjusting the screen size to
accommodate. In this case we are manually configuring output scaling
with RandR, but this code still runs, causing unintended consequences.
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