X display flickering

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I am running 21.11, and bspwm as my window manager. I recently made an edit in home.nix to my polybar configuration. After running home-manager switch, my X display began flickering, randomly, constantly, about 1-2 times every 1-2 seconds. It appears like a black window with no decorations is being drawn over the entire display except a row maybe 20 pixels along the top border.

I have confirmed that it is X, because if I drop down to a TTY, there is no flickering. Also, if I connect an external display, that display is fine, but my internal laptop display continues to flicker.

I don’t think it is related to my polybar config, because I stopped it with a systemctl --user stop polybar.service and the flickering continues.

edit: I switched back one generation (2022-03-03) and the problem persisted. I switched back one more generation (2022-02-07) and the problem cleared up. How can I determine which package messed things up.

p.s. During boot, in the systemd boot loader, I switched to generation 151. But when I run `sudo nix-env --list-generations --profile /nix/var/nix/profiles/system’, it shows generation 152 as current. Why is that?

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If you switch into a non-default generation, it won’t match… with the default generation (called “current” by nix-env). EDIT: perhaps it’s unclear that the choice is the boot menu is not a persistent one.

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Can you diff the generations with diff-closures to narrow things down?

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Is there something I need to install to run nix store? My nix doesn’t have a store command.

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I expect that your nix is too old.

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nix (Nix) 2.3.16. Too old?

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There’s a known issue with picom and nvidia cards. I know I have to disable picom if my system suspends.

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I am running picom, but on Irix Xe Graphics.

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Nix 2.3 does not have the nix store sub-command. 2.4 does.

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Apparently, there is an issue with Iris Xe as well. I just stopped my picom service and the flickering problem has gone away. Thanks for the tip. What is the issue? Is there a solution?

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Follow-up question: If I select a non-default generation at boot, how can I query the system to tell me which generation was used at boot? Since --list-generation shows me the default, and not the currently loaded one. Terminilogy here leads to confusion obviously.

Second follow-up question: Running sudo nixos-rebuild --upgrade switch updates my channel, updates packages and switches to the new generation, but is there a way to list the packages that will be updated before commiting? Also, is there a way to query the channel to see if there are any package updates?

edit: spelling

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I find this useful:

$ ls -l /run/{booted,current}-system

EDIT: note that both can differ from the “current” generation. You can use nixos-rebuild test or activate any system version manually.

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Is there a way to associate those store paths to the nix-env generation numbering?

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I know nothing nice. But if you

$ ls -l /nix/var/nix/profiles/system*

you could grep the hash and see from which number(s) it gets linked.

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The issue is clearly “solved”, but for reference and completeness, the nix subcommands exist since nix 2.4. They’re experimental and behind a feature flag by default (experimental-features = nix-command flakes in nix.extraOptions).

Sorry for sending you down a wild goose chase there, I’m usually more careful about pointing at experimental features. Guess I was just a bit excited upon realizing how useful that command is for bisecting :slight_smile:

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No problem. Actually the problem has been identified as an issue with picom. I don’t know the particulars, don’t know if it is Nix specific, and don’t know the solution other than to temporarily disable picom.

Then, for fun, we went down a bunch of unrelated rabbit holes.

Good leaning, and appreciate the replies.

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