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Creating own XKB tweaks
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@@@@@@@@@@@@@@@@@@@@@@@
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:slug: custom-xkb-tweaks
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:date: 2024-04-17 16:40
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:tags: lifehack, software, linux
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:category: technology
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:keywords: xkb, wayland, unprivileged tweaks
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:lang: en
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:translation: false
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:status: published
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Debugging this took me a bit too long, so I want to write about the caveat.
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My problem: My laptop does not have PageUp and PageDown keys, and many other
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keyboards I use have similar deficiencies. And I use various environments and
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various systems, some of which are shared with other people who don't need/want
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my tweaks. IOW: I want something generic, but it must be confined to my user –
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no system-wide daemon, no udev remapping. (I mostly ended up with these
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solutions when I searched for a way to remap keys on Wayland.)
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Requirements: xkbcommon implementation of XKB with utilities. It is quite
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common these days (duh…), but you could probably just compile this yourself if
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you don't have it.
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The tweaking
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============
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The `xkbcommon guide
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<https://xkbcommon.org/doc/current/user-configuration.html>`__ tells us that we
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can inspect the files in [#lazy]_ ``/usr/share/X11/xkb`` for the source files
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and just write our bits to ``~/.config/xkb/symbols/ledoian``. In particular, I
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added this snippet to remap keyboard brightness controls to PageUp/Down::
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partial
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xkb_symbols "qs" {
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key <I238> {[ Prior ]};
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key <I237> {[ Next ]};
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};
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The key identifiers are taken e.g. from ``xkbcli interactive-wayland``.
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However, this is KcCGST [#kccgst-vs-rmlvo]_ description, but layouts are
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configured using RMLVO, so I need to define an option and tell what it should
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do. The guide wants me to create ``~/.config/xkb/rules/evdev`` with::
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! option = symbols
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ledoian:qs = +ledoian(qs)
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! include %S/evdev
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Now I just add ``ledoian:qs`` [#option-vs-symbol]_ to my keyboard configuration
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and… it does not work. For this, at all, but if I remap e.g. the L key, that
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gets applied. The problem? That included file says that the default keyboard
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model always includes ``inet(evdev)`` symbols. Those symbols set the default
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meaning of the keys, but since that got applied later, it overrides my tweak.
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Solution: first include, then add my option.
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How to debug: read stuff that ``xkbcli compile-keymap --verbose`` tells you
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(pass your config as ``--layout``, ``--variant``, ``--options``, …). At the top
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it says what it does::
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xkbcommon: DEBUG: Include path added: /home/ledoian/.config/xkb
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xkbcommon: DEBUG: Include path added: /usr/share/X11/xkb
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xkbcommon: DEBUG: Compiling from RMLVO: rules 'evdev', model 'pc105', layout 'us', variant '(null)', options '(null)'
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xkbcommon: DEBUG: Compiling from KcCGST: keycodes 'evdev+aliases(qwerty)', types 'complete', compat 'complete', symbols 'pc+us+inet(evdev)'
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My option would appear before the ``inet(evdev)`` part.
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A note about X11
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================
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X11 uses a separate implementation of XKB (the original one, in fact), which
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does _not_ look into the user directory, just the system ones. However, you can
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compile the keymap yourself using ``xkbcli compile-keymap [KEYMAP OPTIONS] >
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my_layout.xkb`` and load it into the X server with ``xkbcomp my_layout.xkb $DISPLAY``.
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-------
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.. [#lazy] This is not technically accurate, really the paths reference
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``$XDG_something`` variables. I am lazy and just copied my system, so YMMV
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(probably won't, though).
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.. [#kccgst-vs-rmlvo] There are two levels of describing the keymap: the
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lowlevel one is called KcCGST (short for keycodes, compat, geometry,
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symbols, types) and is considered to be an implementation detail; the
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user-facing one is RMLVO (rules, model, layout, variant, options) and that
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is what you use in the configs, with ``setxkbmap`` &c.
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.. [#option-vs-symbol] While both this example and the `upstream
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<https://www.freedesktop.org/wiki/Software/XKeyboardConfig/>`__ layouts name
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the symbols and options similarly, I think they don't need to be related –
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you should be able to put whatever you want in your options to the left of
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``=``, the right hand side is the name of the symbol file and if a
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non-default layout from that file is used, its name is put in the
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parentheses.
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