@abbradar, thanks for your PR! By analyzing the history of the files in this pull request, we identified @edolstra, @aszlig and @nbp to be potential reviewers.

I don't think it's meaningful to have multiple DPI settings. Perhaps have just one nixos option and change the rest into alias names.

Pushed fixed version -- I cherry-picked from the wrong branch, sorry.

@vcunat My thoughts too. There are also other usecases for DPI value (for example perhaps console fonts). IMHO more generic name of those two is fonts.fontconfig.dpi (at least it's about fonts, not about X server). I'll prepare the patch.

Hmm, choosing the default console font depending on DPI... that would be possible, maybe even not too magical.

@vcunat I expect something like:

listOfFonts :: [(DPI, FontName)]

snd (last (takeUntil (\(dpi, _) -> dpi > cfg.dpi) listOfFonts))

BTW while reading docs on -dpi option I discovered that it has the highest priority of all DPI settings -- i.e. it overrides DPI settings in xorg.conf and also overrides what the monitor says itself. Perhaps it's not a good idea to set it from fonts.fontconfig.dpi after all; instead let's advise to use it only when DPI is not detected properly.

I'm still going deeper in the rabbit hole of Linux DPI settings and want to write up a small overview when finished...

So, to sum up what I found out:

There are three distinct sources of DPI information on Linux: fontconfig, Xresources (Xft.dpi) and Xrandr. Xrandr information is also exposed via DisplayHeight/DisplayHeightMM. Xresources may be screen-specific. fontconfig is a separate source which is normally used on per-font basis.

• GTK2 uses Xresources and falls back to DisplayHeight API. I read through the source code a bit and it seems that GTK2 is not only multi-monitor-aware but actually uses monitor settings for the monitor it considers its "main". I'm not yet sure if that's not just always the primary monitor; I'll experiment a bit later;
• GTK3 used to follow GTK2's route but recently stopped using DisplayHeight as per firefox: Not respecting 'services.xserver.dpi' after moving to GTK3 #25023;
• Qt 5 uses Xft.dpi and falls back to Xrandr (much like GTK2 but using a differnet API). They mention that they consider DPI forced when Xft.dpi is set. That seems reasonable to me;
• libXft also uses settings from fontconfig on per-font basis and if nothing is set uses just the same code GTK2 has to calculate DPI using DisplayHeight API;

There are many other places I haven't examined; for example setting DPI in xfce4-appearance-settings magically resizes fonts in all applications -- I'm not yet sure how that works (not via Xresources or Xrandr at least).

Also notice that we already set Xft.dpi from fonts.fontconfig.dpi value globally (for all screens).

Overall it seems to me that we should promote usage of Xrandr/Xresources with this:

1. Revert GTK 3 patch that stops usage of DisplayHeight API;
2. Explicitly mention that services.xserver.dpi is only for overriding invalid monitor DPI and encourage to add Monitor sections instead or use xrandr -dpi;
3. Discourage usage of fonts.fontconfig.dpi as a last resort option; one should set proper DPI per monitor instead.

cc @rbasso as one who may be knowledgeable about DPI -- maybe you have something to add/correct?

Also cc @ttuegel as the current fontconfig person :D

Currently we have fonts.fontconfig.dpi and services.xserver.dpi options. Are there any cases where they can differ? Maybe we want to drop one of them to make this uniform?

Ouch! We should drop fonts.fontconfig.dpi because that is not a font-specific setting. The Fontconfig files we generate can just use the DPI from services.xserver.dpi if that is set.

@ttuegel Yeah, after more thought I think we should just merge them (but also recommend people to set proper DPI for their monitors instead and not use that option). I'll prepare the patch a bit later.

I've pushed the second version of the patchset which implements what we've outlined above:

1. Revert GTK3 patch which breaks DPI autodetection;
2. Add global Xresources option (for convenience);
3. Drop DPI settings from fontconfig (rename the option; also don't set dpi options in fontconfig configuration at all -- it should use Xft values by default);
4. Set Xft.dpi when services.xserver.dpi is set;
5. Mention in the documentation that one is recommended to avoid services.xserver.dpi if possible (it forces the same value for all monitors).

So, any comments on this? I'm not completely sure all changes are a good idea.

I like all the changes — just one thing to add, which is that I think the X resources should be loaded before user sessions to make sure that they apply to the display manager too. I'm not sure if there's a way to do that in one place for all the display managers, considering that (AFAIU) it's the display manager's responsibility to start the X server.

@lheckemann Actually I wanted to do it this very way but I don't know if it's even possible :D IIUC this would require some kind of support in a display manager.

I suppose that's just another reason not to recommend the setting, preferring instead Monitor sections. I approve of the changes overall, for what my opinion's worth :)

Hm, actually we need to test that DPI works properly with display managers. It should be, but still... I'll do this tomorrow if noone beats me.

I'm not sure if there's a way to do that in one place for all the display managers, considering that (AFAIU) it's the display manager's responsibility to start the X server.

I can't speak for other display managers, but SDDM allows to specify a script to setup the X server. At the moment, I think we use the default script shipped by upstream, but we could easily change that. I don't know what other display managers do, but I expect something similar because each distribution has different requirements for starting the server.

I suppose that's just another reason not to recommend the setting, preferring instead Monitor sections.

Is there a way to set individual Monitor sections in NixOS? I was not aware of this. If that is so, I would strongly recommend doing that instead of overriding DPI for the whole server. From the perspective of font rendering, using a DPI that does not match the display is likely to produce really horrible results.

Between services.xserver.monitorSection and services.xserver.config, it is indeed possible to specify arbitrary Monitor sections.

Argh, sorry, life got into the way and I delayed this greatly. DPI and monitor sections seem to work with display managers. As for X resources -- can we somehow load them in our X wrapper? I haven't immediately found a way to do that.

*bump*

AFAICT, we don't have an X wrapper. I think that it would be nice to get this change merged, even if it doesn't affect the display manager in all cases yet.

This will need a release-notes update for the NixOS revision it lands into. If done quickly, and there are no objections, this could land in 18.09 I imagine.

If I understand this correctly; part of the PR is already included in master since November 2017, we're left with the removal of the font-specific DPI hacks, right?

If so, I (personally) think this can be merged 👍 (once release notes written, or arrangements to write them are made).

Any updates on this pull request, please?

Ah, completely forgot about this one. I'll do a writeup for the release notes and then it should be ready.

@abbradar Could you please rebase your PR and add an entry to the next release notes https://github.com/NixOS/nixpkgs/blob/master/nixos/doc/manual/release-notes/rl-2003.xml ?

@abbradar Ping?

I marked this as stale due to inactivity. → More info

This is a good old PR I forgot about and found. Rebased and added release notes.

@Ericson2314 there was a conflict in the release notes. Inserting in a random place mitigates those, fwiw.

OK, let the merge conflict guide me to that random spot :D

I just updated from 21.05 and found that fonts.fontconfig.dpi was removed and that I leapfrogged over the deprecation warning:)
Somehow, fonts.fontconfig.dpi was the only option I found on my laptop to get a consistent dpi across all of gtk, qt, and lightdm system-wide on my laptop (without the need for a dozen of variables like GDK_SCALE or need for per-user Xresources). Using services.xserver.dpi instead gets the correct scale in e.g. alacritty, but scales things down in e.g. firefox.

Meanwhile, I also have DisplaySize set in xrandrHeads, but it doesn't help