fontconfig service: drop dpi option

Recommend to use services.xserver.dpi option instead. Mention in the
documentation that it's a sledgehammer approach and monitor settings should be
used instead.

Also don't set DPI in fontconfig settings; fontconfig should use Xft settings
by default so let's not override one value in multiple places. For example,
user now can set DPI via ~/.Xresources properly.

nixos/doc/manual/from_md/release-notes/rl-2111.section.xml

@@ -779,6 +779,16 @@ Superuser created successfully.
           group.
         </para>
       </listitem>
+      <listitem>
+        <para>
+          The fontconfig service’s dpi option has been removed.
+          Fontconfig should use Xft settings by default so there’s no
+          need to override one value in multiple places. The user can
+          set DPI via ~/.Xresources properly, or at the system level per
+          monitor, or as a last resort at the system level with
+          <literal>services.xserver.dpi</literal>.
+        </para>
+      </listitem>
       <listitem>
         <para>
           The <literal>yambar</literal> package has been split into

nixos/doc/manual/release-notes/rl-2111.section.md

@@ -223,6 +223,10 @@ subsonic-compatible api. Available as [navidrome](#opt-services.navidrome.enable
 
 - The `openrazer` and `openrazer-daemon` packages as well as the `hardware.openrazer` module now require users to be members of the `openrazer` group instead of `plugdev`. With this change, users no longer need be granted the entire set of `plugdev` group permissions, which can include permissions other than those required by `openrazer`. This is desirable from a security point of view. The setting [`harware.openrazer.users`](options.html#opt-services.hardware.openrazer.users) can be used to add users to the `openrazer` group.
 
+- The fontconfig service's dpi option has been removed.
+  Fontconfig should use Xft settings by default so there's no need to override one value in multiple places.
+  The user can set DPI via ~/.Xresources properly, or at the system level per monitor, or as a last resort at the system level with `services.xserver.dpi`.
+
 - The `yambar` package has been split into `yambar` and `yambar-wayland`, corresponding to the xorg and wayland backend respectively. Please switch to `yambar-wayland` if you are on wayland.
 
 - The `services.minio` module gained an additional option `consoleAddress`, that

nixos/modules/config/fonts/fontconfig.nix

@@ -78,14 +78,6 @@ let
         </edit>
       </match>
 
-      ${optionalString (cfg.dpi != 0) ''
-      <match target="pattern">
-        <edit name="dpi" mode="assign">
-          <double>${toString cfg.dpi}</double>
-        </edit>
-      </match>
-      ''}
-
     </fontconfig>
   '';
 
@@ -237,6 +229,7 @@ in
     (mkRemovedOptionModule [ "fonts" "fontconfig" "hinting" "style" ] "")
     (mkRemovedOptionModule [ "fonts" "fontconfig" "forceAutohint" ] "")
     (mkRemovedOptionModule [ "fonts" "fontconfig" "renderMonoTTFAsBitmap" ] "")
+    (mkRemovedOptionModule [ "fonts" "fontconfig" "dpi" ] "Use display server-specific options")
   ] ++ lib.forEach [ "enable" "substitutions" "preset" ]
      (opt: lib.mkRemovedOptionModule [ "fonts" "fontconfig" "ultimate" "${opt}" ] ''
        The fonts.fontconfig.ultimate module and configuration is obsolete.
@@ -282,15 +275,6 @@ in
           '';
         };
 
-        dpi = mkOption {
-          type = types.int;
-          default = 0;
-          description = ''
-            Force DPI setting. Setting to <literal>0</literal> disables DPI
-            forcing; the DPI detected for the display will be used.
-          '';
-        };
-
         localConf = mkOption {
           type = types.lines;
           default = "";

nixos/modules/services/x11/display-managers/default.nix

@@ -18,7 +18,6 @@ let
 
   fontconfig = config.fonts.fontconfig;
   xresourcesXft = pkgs.writeText "Xresources-Xft" ''
-    ${optionalString (fontconfig.dpi != 0) ''Xft.dpi: ${toString fontconfig.dpi}''}
     Xft.antialias: ${if fontconfig.antialias then "1" else "0"}
     Xft.rgba: ${fontconfig.subpixel.rgba}
     Xft.lcdfilter: lcd${fontconfig.subpixel.lcdfilter}

nixos/modules/services/x11/xserver.nix

@@ -297,7 +297,11 @@ in
       dpi = mkOption {
         type = types.nullOr types.int;
         default = null;
-        description = "DPI resolution to use for X server.";
+        description = ''
+          Force global DPI resolution to use for X server. It's recommended to
+          use this only when DPI is detected incorrectly; also consider using
+          <literal>Monitor</literal> section in configuration file instead.
+        '';
       };
 
       updateDbusEnvironment = mkOption {