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

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.

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 :)

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.

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

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.

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

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.

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

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.

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.

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

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

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

"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.

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.

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