:PROPERTIES:
:ID:       0217f537-442a-4593-8c69-d481f0d1f2a8
:END:
#+title: keyboard
#+filetags: :hardware:linux:

* Keyboard inputs

This all comes from the fact that a "key press" isn't one thing --- it's
a stack of specs and translations from *keyboard hardware → USB/HID →
Linux kernel → userspace (XKB/Wayland) → Hyprland binds*.


** 1. Big picture: what happens when you press a key?
:PROPERTIES:
:CUSTOM_ID: big-picture-what-happens-when-you-press-a-key
:END:
When you hit a key (like your "lock" key), roughly this happens:

#+begin_src text
Physical switch on keyboard
        ↓
Keyboard firmware → USB HID "usage" (e.g. 0x07:0xE3 = Left GUI)
        ↓
Linux kernel input subsystem (evdev) → KEY_LEFTMETA (code 125)
        ↓
libinput / xkbcommon → keysyms (like Super_L, L, etc.)
        ↓
Hyprland → your `bind = SUPER, L, exec, ...`
#+end_src

The "topic" we've been poking at is basically *understanding each layer
in that stack*.

--------------

** 2. USB HID: where =700e3= comes from
:PROPERTIES:
:CUSTOM_ID: usb-hid-where-700e3-comes-from
:END:
Modern USB keyboards follow the *USB HID (Human Interface Device)
specification*. This spec defines *Usage Tables*: numerical codes for
things like keys, buttons, axes, etc.

- The keyboard page is *Usage Page 0x07 (Keyboard/Keypad)*.

- In your =evtest= output, =MSC_SCAN value 700e3= means:

  - =0x07= (Keyboard page)
  - =0xE3= (Left GUI / "Windows/Super" key)

You can see the official tables here (PDF):

- *HID Usage Tables (includes Keyboard/Keypad page 0x07)*
  [[https://usb.org/sites/default/files/hut1_21.pdf]]
  ([[https://usb.org/sites/default/files/hut1_21.pdf?utm_source=chatgpt.com][USB
  Implementers Forum]])

If you scroll to the *Keyboard/Keypad Page (0x07)* section, it lists all
the key usages: A, B, C, modifiers, function keys, etc. There's also a
standalone Keyboard/Keypad-page extract people mirror, like this PDF
snippet:
([[https://d1.amobbs.com/bbs_upload782111/files_47/ourdev_692986N5FAHU.pdf?utm_source=chatgpt.com][d1.amobbs.com]])

So:

- =700e3= = Page =0x07=, Usage =0xE3= → /Keyboard Left GUI/
- =7000f= = Page =0x07=, Usage =0x0F= → /Keyboard L/ key

That's how we knew your lock key was sending *Super + L*.

--------------

** 3. Linux input subsystem: =EV_MSC=, =EV_KEY=, =KEY_LEFTMETA=
:PROPERTIES:
:CUSTOM_ID: linux-input-subsystem-ev_msc-ev_key-key_leftmeta
:END:
Linux has a dedicated *input subsystem* in the kernel (=drivers/input=,
=drivers/hid= etc.) that takes those HID usages and turns them into a
unified stream of *input events*.
([[https://docs.kernel.org/input/input.html?utm_source=chatgpt.com][Linux
Kernel Documentation]])

Key ideas:

- Devices expose =/dev/input/eventX= nodes.

- Each event is a struct with:

  - =type= (e.g. =EV_KEY=, =EV_MSC=, =EV_REL=, =EV_SYN=)
  - =code= (e.g. =KEY_L=, =KEY_LEFTMETA=)
  - =value= (pressed = 1, released = 0, repeat = 2)

Docs worth bookmarking:

- *Linux input subsystem overview*
  [[https://docs.kernel.org/input/input.html]]
  ([[https://docs.kernel.org/input/input.html?utm_source=chatgpt.com][Linux
  Kernel Documentation]])

- *Input event types and codes (=event-codes.txt=)*
  [[https://www.kernel.org/doc/Documentation/input/event-codes.txt]]
  ([[https://www.kernel.org/doc/Documentation/input/event-codes.txt?utm_source=chatgpt.com][Kernel.org]])

The *keycode definitions* (=KEY_L=, =KEY_LEFTMETA=, etc.) live in:

- =include/uapi/linux/input-event-codes.h= in the kernel source Example
  mirror:
  [[https://raw.githubusercontent.com/torvalds/linux/master/include/uapi/linux/input-event-codes.h]]
  ([[https://raw.githubusercontent.com/torvalds/linux/master/include/uapi/linux/input-event-codes.h?utm_source=chatgpt.com][GitHub]])

When =evtest= prints:

#+begin_src text
type 4 (EV_MSC), code 4 (MSC_SCAN), value 700e3
type 1 (EV_KEY), code 125 (KEY_LEFTMETA), value 0
#+end_src

that means:

- =EV_MSC / MSC_SCAN= → "Here is the raw hardware scancode" (from USB
  HID).
- =EV_KEY / KEY_LEFTMETA= → "Linux mapped that scancode to logical key
  LEFTMETA".

--------------

** 4. XKB / xkbcommon: mapping to actual characters & modifiers
:PROPERTIES:
:CUSTOM_ID: xkb-xkbcommon-mapping-to-actual-characters-modifiers
:END:
Above the kernel, you've got an extra mapping layer that says:

#+begin_quote
For keycode N, with this layout, when Shift is held, produce this
character/symbol.
#+end_quote

On X11 this is handled by *XKB (X Keyboard Extension)*; on Wayland
compositors (including Hyprland) the same ideas are implemented via
*xkbcommon*.

Docs:

- *X Keyboard Extension (XKB) protocol spec*
  [[https://www.x.org/releases/X11R7.7/doc/kbproto/xkbproto.html]]
  ([[https://www.x.org/releases/X11R7.7/doc/kbproto/xkbproto.html?utm_source=chatgpt.com][X.Org]])

- *Arch Wiki: X keyboard extension* (good high-level intro)
  [[https://wiki.archlinux.org/title/X_keyboard_extension]]
  ([[https://wiki.archlinux.org/title/X_keyboard_extension?utm_source=chatgpt.com][ArchWiki]])

- A nice "practical" walkthrough of XKB concepts:
  [[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]]]
  ([[https://medium.com/%40damko/a-simple-humble-but-comprehensive-guide-to-xkb-for-linux-6f1ad5e13450?utm_source=chatgpt.com][Medium]])

Hyprland, Sway, etc. all use libinput + xkbcommon under the hood to
interpret those keycodes and translate them into keysyms and modifiers.

--------------

** 5. Hyprland / your config: where the bindings fit in
:PROPERTIES:
:CUSTOM_ID: hyprland-your-config-where-the-bindings-fit-in
:END:
Hyprland sits at the top of this stack:

- It listens to the input events (via libinput).
- It sees keysyms/modifiers (e.g. =Super=, =L=).
- It matches them against your config:

#+begin_src ini
bind = SUPER, L, exec, hyprlock
#+end_src

So your keyboard's "lock" key:

1. Firmware sends HID usages =0xE3= (Left GUI) and =0x0F= (L).
2. Linux maps them to =KEY_LEFTMETA= (125) and =KEY_L= (38).
3. xkbcommon maps that to =Super= + =L=.
4. Hyprland says: "Ah, SUPER+L → run =hyprlock=".

The "topic" you stumbled into is just *peeling back each abstraction
layer*.

--------------

** 6. Handy tools & libraries if you want to go deeper
:PROPERTIES:
:CUSTOM_ID: handy-tools-libraries-if-you-want-to-go-deeper
:END:
- =evtest=, =libinput debug-events=, =showkey= → To watch what your
  keyboard is actually sending.

- *Python-evdev* -- Python bindings to read =/dev/input/event*=
  yourself: [[https://python-evdev.readthedocs.io/]]
  ([[https://python-evdev.readthedocs.io/?utm_source=chatgpt.com][python-evdev.readthedocs.io]])

- Linux input subsystem docs again:
  [[https://docs.kernel.org/driver-api/input.html]]
  ([[https://docs.kernel.org/driver-api/input.html?utm_source=chatgpt.com][Linux
  Kernel Documentation]])
 
