[modeset] xrandr transformations cause significant visible tearing in Xorg sessions

[Dean Henrichsmeyer]

On my laptop, fractional scaling seems fine, including watching a YouTube video. It has a quad HD display.

Enabling fractional scaling on my desktop which is attached to a single 4k display, fractional scaling isn't usable. The video appears to be constantly refreshing and not using the same acceleration as if I just run at 100% or 200%.

ProblemType: Bug
DistroRelease: Ubuntu 19.10
Package: gnome-shell 3.34.0-1ubuntu1
ProcVersionSignature: Ubuntu 5.3.0-13.14-generic 5.3.0
Uname: Linux 5.3.0-13-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu7
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Wed Oct 2 14:23:33 2019
DisplayManager: gdm3
InstallationDate: Installed on 2018-07-24 (435 days ago)
InstallationMedia: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180724)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
RelatedPackageVersions: mutter-common 3.34.0-3ubuntu1
SourcePackage: gnome-shell
UpgradeStatus: Upgraded to eoan on 2019-07-19 (74 days ago)

[Daniel van Vugt (vanvugt)]

I can't yet tell which issue this is, but it sounds likely to be one of
https://discourse.ubuntu.com/t/x11-hidpi-scaling-available-for-testing-on-disco/10293/3

If you need a fractional scale value then you should find the implementation for Wayland sessions performs better and is more complete. Because it doesn't have to deal with legacy Xorg.

If you need Xorg then using whole scale values (n * 100%) works better as you mentioned, because it avoids the Xorg fractional scaling path.

[Daniel van Vugt (vanvugt)]

At least I *hope* that Wayland would perform better. If you're using Wayland with multiple monitors then for some people it might not, due to bug 1730460: https://gitlab.gnome.org/GNOME/mutter/issues/3

My goal is to have a fix for that landed in GNOME next year. Which was on one of the slides I missed in my performance talk :)

[Daniel van Vugt (vanvugt)]

We're probably not going to rewrite the Xorg fractional scaling to work around shortcomings in the Xorg feature we rely on. It maybe isn't even possible. More likely we're just waiting on fixes to Xorg itself:

https://gitlab.freedesktop.org/xorg/xserver/merge_requests/24

Or similarly likely Wayland will become the default where fractional scaling works better already.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Dean Henrichsmeyer (dean)]

Just to provide more information - my initial report was incorrect. I did in fact see the tearing on the laptop display. It seems whenever you're using non-integer scaling with Xorg, that's what happens.

[Daniel van Vugt (vanvugt)]

Also discussed in https://trello.com/c/pobEN0Ek

[Matt Austin (mattaustin)]

Thanks for the feedback, but I don't believe I am using fractional scaling. My scaling is set to 100%, but I have changed the font scaling to 1.25.

If I set font scaling to 1.0, then I see the same tearing.

[Daniel van Vugt (vanvugt)]

Sorry Matt, I have now reopened bug 1853094 for you.

[Mert Can Demir (validatedev)]

Well, Wayland won’t be usable with fractional scaling until fixing the blur with XWayland applications. Many apps are doesn’t support native Wayland, unfortunately.

[Daniel van Vugt (vanvugt)]

Maybe upstream bug https://gitlab.freedesktop.org/xorg/xserver/-/issues/996 is more appropriate here, but still refer to bug https://gitlab.freedesktop.org/xorg/xserver/issues/244 for the general problem.

[Daniel van Vugt (vanvugt)]

Retitled again because this includes screen rotation, not just scaling.

[Daniel van Vugt (vanvugt)]

New fix proposed upstream:
https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1006

[Jeffrey Knockel (jeff250)]

The proposed fix has been merged to the xserver master branch.

The author of the fix also has a patch set against a recent xserver stable release here:

https://github.com/kerneltoast/xserver/tree/new-tearfree-21.1.5

[Daniel van Vugt (vanvugt)]

"fixed-in-xorg-22" is probably wrong because the last release was 21.x and the year is already 2023. Fixed in the next major release anyway. I don't yet see it in the 21.x series.

[Jeffrey Knockel (jeff250)]

For testing I've created a PPA with the patches applied to 21.1.6:

https://launchpad.net/%7Ejeff250/+archive/ubuntu/xserver-xorg-with-tearfree-modesetting

Since using it I haven't seen any tearing, and I haven't seen any new issues (yet!) either.

[Daniel van Vugt (vanvugt)]

Just a reminder that most Ubuntu users can ignore this bug now because most are getting Wayland by default instead.