204 lines
6.8 KiB
Org Mode
204 lines
6.8 KiB
Org Mode
:PROPERTIES:
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:ID: 0217f537-442a-4593-8c69-d481f0d1f2a8
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:END:
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#+title: keyboard
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#+filetags: :hardware:linux:
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* Keyboard inputs
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This all comes from the fact that a "key press" isn't one thing --- it's
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a stack of specs and translations from *keyboard hardware → USB/HID →
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Linux kernel → userspace (XKB/Wayland) → Hyprland binds*.
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** 1. Big picture: what happens when you press a key?
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:PROPERTIES:
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:CUSTOM_ID: big-picture-what-happens-when-you-press-a-key
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:END:
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When you hit a key (like your "lock" key), roughly this happens:
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#+begin_src text
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Physical switch on keyboard
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↓
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Keyboard firmware → USB HID "usage" (e.g. 0x07:0xE3 = Left GUI)
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↓
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Linux kernel input subsystem (evdev) → KEY_LEFTMETA (code 125)
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↓
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libinput / xkbcommon → keysyms (like Super_L, L, etc.)
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↓
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Hyprland → your `bind = SUPER, L, exec, ...`
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#+end_src
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The "topic" we've been poking at is basically *understanding each layer
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in that stack*.
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--------------
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** 2. USB HID: where =700e3= comes from
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:PROPERTIES:
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:CUSTOM_ID: usb-hid-where-700e3-comes-from
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:END:
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Modern USB keyboards follow the *USB HID (Human Interface Device)
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specification*. This spec defines *Usage Tables*: numerical codes for
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things like keys, buttons, axes, etc.
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- The keyboard page is *Usage Page 0x07 (Keyboard/Keypad)*.
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- In your =evtest= output, =MSC_SCAN value 700e3= means:
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- =0x07= (Keyboard page)
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- =0xE3= (Left GUI / "Windows/Super" key)
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You can see the official tables here (PDF):
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- *HID Usage Tables (includes Keyboard/Keypad page 0x07)*
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[[https://usb.org/sites/default/files/hut1_21.pdf]]
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([[https://usb.org/sites/default/files/hut1_21.pdf?utm_source=chatgpt.com][USB
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Implementers Forum]])
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If you scroll to the *Keyboard/Keypad Page (0x07)* section, it lists all
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the key usages: A, B, C, modifiers, function keys, etc. There's also a
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standalone Keyboard/Keypad-page extract people mirror, like this PDF
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snippet:
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([[https://d1.amobbs.com/bbs_upload782111/files_47/ourdev_692986N5FAHU.pdf?utm_source=chatgpt.com][d1.amobbs.com]])
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So:
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- =700e3= = Page =0x07=, Usage =0xE3= → /Keyboard Left GUI/
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- =7000f= = Page =0x07=, Usage =0x0F= → /Keyboard L/ key
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That's how we knew your lock key was sending *Super + L*.
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--------------
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** 3. Linux input subsystem: =EV_MSC=, =EV_KEY=, =KEY_LEFTMETA=
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:PROPERTIES:
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:CUSTOM_ID: linux-input-subsystem-ev_msc-ev_key-key_leftmeta
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:END:
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Linux has a dedicated *input subsystem* in the kernel (=drivers/input=,
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=drivers/hid= etc.) that takes those HID usages and turns them into a
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unified stream of *input events*.
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([[https://docs.kernel.org/input/input.html?utm_source=chatgpt.com][Linux
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Kernel Documentation]])
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Key ideas:
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- Devices expose =/dev/input/eventX= nodes.
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- Each event is a struct with:
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- =type= (e.g. =EV_KEY=, =EV_MSC=, =EV_REL=, =EV_SYN=)
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- =code= (e.g. =KEY_L=, =KEY_LEFTMETA=)
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- =value= (pressed = 1, released = 0, repeat = 2)
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Docs worth bookmarking:
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- *Linux input subsystem overview*
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[[https://docs.kernel.org/input/input.html]]
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([[https://docs.kernel.org/input/input.html?utm_source=chatgpt.com][Linux
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Kernel Documentation]])
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- *Input event types and codes (=event-codes.txt=)*
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[[https://www.kernel.org/doc/Documentation/input/event-codes.txt]]
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([[https://www.kernel.org/doc/Documentation/input/event-codes.txt?utm_source=chatgpt.com][Kernel.org]])
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The *keycode definitions* (=KEY_L=, =KEY_LEFTMETA=, etc.) live in:
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- =include/uapi/linux/input-event-codes.h= in the kernel source Example
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mirror:
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[[https://raw.githubusercontent.com/torvalds/linux/master/include/uapi/linux/input-event-codes.h]]
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([[https://raw.githubusercontent.com/torvalds/linux/master/include/uapi/linux/input-event-codes.h?utm_source=chatgpt.com][GitHub]])
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When =evtest= prints:
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#+begin_src text
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type 4 (EV_MSC), code 4 (MSC_SCAN), value 700e3
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type 1 (EV_KEY), code 125 (KEY_LEFTMETA), value 0
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#+end_src
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that means:
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- =EV_MSC / MSC_SCAN= → "Here is the raw hardware scancode" (from USB
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HID).
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- =EV_KEY / KEY_LEFTMETA= → "Linux mapped that scancode to logical key
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LEFTMETA".
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--------------
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** 4. XKB / xkbcommon: mapping to actual characters & modifiers
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:PROPERTIES:
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:CUSTOM_ID: xkb-xkbcommon-mapping-to-actual-characters-modifiers
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:END:
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Above the kernel, you've got an extra mapping layer that says:
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#+begin_quote
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For keycode N, with this layout, when Shift is held, produce this
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character/symbol.
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#+end_quote
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On X11 this is handled by *XKB (X Keyboard Extension)*; on Wayland
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compositors (including Hyprland) the same ideas are implemented via
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*xkbcommon*.
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Docs:
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- *X Keyboard Extension (XKB) protocol spec*
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[[https://www.x.org/releases/X11R7.7/doc/kbproto/xkbproto.html]]
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([[https://www.x.org/releases/X11R7.7/doc/kbproto/xkbproto.html?utm_source=chatgpt.com][X.Org]])
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- *Arch Wiki: X keyboard extension* (good high-level intro)
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[[https://wiki.archlinux.org/title/X_keyboard_extension]]
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([[https://wiki.archlinux.org/title/X_keyboard_extension?utm_source=chatgpt.com][ArchWiki]])
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- A nice "practical" walkthrough of XKB concepts:
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[[https://medium.com/@damko/a-simple-humble-but-comprehensive-guide-to-xkb-for-linux-6f1ad5e13450][https://medium.com/[cite/t:@damko/a-simple-humble-but-comprehensive-guide-to-xkb-for-linux-6f1ad5e13450]]]
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([[https://medium.com/%40damko/a-simple-humble-but-comprehensive-guide-to-xkb-for-linux-6f1ad5e13450?utm_source=chatgpt.com][Medium]])
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Hyprland, Sway, etc. all use libinput + xkbcommon under the hood to
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interpret those keycodes and translate them into keysyms and modifiers.
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--------------
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** 5. Hyprland / your config: where the bindings fit in
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:PROPERTIES:
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:CUSTOM_ID: hyprland-your-config-where-the-bindings-fit-in
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:END:
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Hyprland sits at the top of this stack:
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- It listens to the input events (via libinput).
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- It sees keysyms/modifiers (e.g. =Super=, =L=).
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- It matches them against your config:
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#+begin_src ini
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bind = SUPER, L, exec, hyprlock
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#+end_src
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So your keyboard's "lock" key:
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1. Firmware sends HID usages =0xE3= (Left GUI) and =0x0F= (L).
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2. Linux maps them to =KEY_LEFTMETA= (125) and =KEY_L= (38).
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3. xkbcommon maps that to =Super= + =L=.
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4. Hyprland says: "Ah, SUPER+L → run =hyprlock=".
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The "topic" you stumbled into is just *peeling back each abstraction
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layer*.
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--------------
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** 6. Handy tools & libraries if you want to go deeper
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:PROPERTIES:
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:CUSTOM_ID: handy-tools-libraries-if-you-want-to-go-deeper
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:END:
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- =evtest=, =libinput debug-events=, =showkey= → To watch what your
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keyboard is actually sending.
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- *Python-evdev* -- Python bindings to read =/dev/input/event*=
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yourself: [[https://python-evdev.readthedocs.io/]]
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([[https://python-evdev.readthedocs.io/?utm_source=chatgpt.com][python-evdev.readthedocs.io]])
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- Linux input subsystem docs again:
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[[https://docs.kernel.org/driver-api/input.html]]
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([[https://docs.kernel.org/driver-api/input.html?utm_source=chatgpt.com][Linux
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Kernel Documentation]])
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