No direct scanout in fullscreen games with dash-to-dock enabled

[an9949an]

System

Fedora 37 GNOME 43.5-1, but I also checked on Manjaro with 44

dash-to-dock 83

X11 session

Bug summary

Some time ago I started to notice that games (steam/proton) feels bad (increased input lag, no expected responsiveness). After a lot of experiments I understood that there are no tearing in games even with vsync switched off which breaks the gaming experience almost completely, especially for multiplayer games. In my next experiments I noticed that there are no such issue with dash-to-dock disabled (need to relogin to see effect).

Also interesting that if I login with dash-to-dock disabled and then enable it - games work as expected (with tearing).

Steps to reproduce

1. Enable dash-to-dock
2. Logout
3. Login
4. Run any proton game with uncapped framerate and vsync off
5. Check tearing

Actual result

No screen tearing, increased input lag

Expected result

Allowed screen tearing

[3v1n0]

@vanvugt I feel it's your area, it maybe that in some scenarios we should get rid of the dock from the uiGroup... Mhmh.

[vanvugt]

Not necessarily my area. The problem here is just that mutter thinks there's a window on top of the fullscreen game which is preventing direct scanout. So it sounds like a simple dash-to-dock bug (or a mutter bug). Perhaps if we fix #1381 then it will be solved?

[an9949an]

@vanvugt I tried to rollback mutter to 43.0-4 and the issue still there. Also I'm sure that previously it was ok and then broken by update of some component (not sure which one, possibly some compatibility regression).

[an9949an]

@vanvugt

The problem here is just that mutter thinks there's a window on top of the fullscreen game

Also as I told if I login with dash-to-dock disabled and then enable it - everything is fine. And vice versa: if I login with dash-to-dock enabled and then disable it - tearing is still broken. Maybe it will help to investigate.

[vanvugt]

Sounds like it could be an (invisible) window/actor leak. Try Alt+F2, lg, then click the target icon to identify hidden actors.

[an9949an]

Sounds like it could be an (invisible) window/actor leak. Try Alt+F2, lg, then click the target icon to identify hidden actors.

I didn't find anything... But found some mistake in my original description. Screen tearing don't working under the wayland session at all, probably because it still don't implemented on the GNOME side. I will edit it.

[vanvugt]

Yes tearing is actually a bug, not a feature. It's a bug that only exists in X11 and will be fixed in a future Xorg release. So tearing alone should not be a test case.

To treat tearing as a feature, you can:

• Disable the TearFree option in your Xorg conf; and/or
• Discuss the addition of tearing as a feature for Wayland sessions.

[an9949an]

Yes tearing is actually a bug, not a feature. It's a bug that only exists in X11 and will be fixed in a future Xorg release. So tearing alone should not be a test case.

Are you sure that it's related to gaming? Because on the gaming side of things lack of tearing breaks everything completely. So, disabled vsync in the game window is completely expected behavior and gaming is not possible without it. And actually it was working like this all the time on the X11 session. And works even now, but without dash-to-dock.

So, all of this is a good question, but at least dash-to-dock should not affect this behavior I think (previously it didn't affect). Am I wrong?

[vanvugt]

Are you sure that it's related to gaming? Because on the gaming side of things lack of tearing breaks everything completely. So, disabled vsync in the game window is completely expected behavior and gaming is not possible without it. And actually it was working like this all the time on the X11 session. And works even now, but without dash-to-dock.

It's complicated.

In Xorg open source drivers, tearing is a bug because actually the kernel drivers forbid it. Xorg only tears because of its own inefficient rendering doing a linear buffer copy. That's a bug.

In Xorg NVIDIA drivers, tearing is a feature. And you can probably configure it.

In Wayland, tearing would be a bug but I don't think you'll ever see it there. It's forbidden by the kernel interfaces we use, at least for open source drivers. For gaming it might be added as a feature later.

Disabling vsync and tearing are different things. You can disable vsync without tearing using the mailbox algorithm (in any graphics system, not just Vulkan), which is usually what is used when vsync is disabled in an app.

So, all of this is a good question, but at least dash-to-dock should not affect this behavior I think (previously it didn't affect). Am I wrong?

dash-to-dock should not disable tearing if the system was tearing without it. That's a bug in dash-to-dock or gnome-shell, which is why this issue is open.

[an9949an]

@vanvugt Thank you for such a detailed explanation! I hope this weird stuff will be aligned one day :) Also my bad, I forgot to mention that I use a laptop with intel + nvidia hybrid graphics. Nvidia gpu is used for games, intel for the rest. I am not sure how it works on the different configurations but maybe I will have ability to check. So, let me know if it is needed.