180 lines
6.3 KiB
Markdown
180 lines
6.3 KiB
Markdown
# 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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When you hit a key (like your "lock" key), roughly this happens:
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``` 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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```
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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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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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([USB Implementers Forum](https://usb.org/sites/default/files/hut1_21.pdf?utm_source=chatgpt.com))
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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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([d1.amobbs.com](https://d1.amobbs.com/bbs_upload782111/files_47/ourdev_692986N5FAHU.pdf?utm_source=chatgpt.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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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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([Linux Kernel Documentation](https://docs.kernel.org/input/input.html?utm_source=chatgpt.com))
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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<sub>KEY</sub>=, `EV_MSC`, `EV_REL`, `EV_SYN`)
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- `code` (e.g. =KEY<sub>L</sub>=, `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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([Linux Kernel Documentation](https://docs.kernel.org/input/input.html?utm_source=chatgpt.com))
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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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([Kernel.org](https://www.kernel.org/doc/Documentation/input/event-codes.txt?utm_source=chatgpt.com))
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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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([GitHub](https://raw.githubusercontent.com/torvalds/linux/master/include/uapi/linux/input-event-codes.h?utm_source=chatgpt.com))
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When `evtest` prints:
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``` 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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```
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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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Above the kernel, you've got an extra mapping layer that says:
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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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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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([X.Org](https://www.x.org/releases/X11R7.7/doc/kbproto/xkbproto.html?utm_source=chatgpt.com))
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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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([ArchWiki](https://wiki.archlinux.org/title/X_keyboard_extension?utm_source=chatgpt.com))
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- A nice "practical" walkthrough of XKB concepts:
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[[https://medium.com/\[cite/t:@damko/a-simple-humble-but-comprehensive-guide-to-xkb-for-linux-6f1ad5e13450](https://medium.com/%5Bcite/t:@damko/a-simple-humble-but-comprehensive-guide-to-xkb-for-linux-6f1ad5e13450)](https://medium.com/@damko/a-simple-humble-but-comprehensive-guide-to-xkb-for-linux-6f1ad5e13450)\]
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([Medium](https://medium.com/%40damko/a-simple-humble-but-comprehensive-guide-to-xkb-for-linux-6f1ad5e13450?utm_source=chatgpt.com))
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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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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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<!-- end list -->
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``` ini
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bind = SUPER, L, exec, hyprlock
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```
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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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- `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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([python-evdev.readthedocs.io](https://python-evdev.readthedocs.io/?utm_source=chatgpt.com))
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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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([Linux Kernel Documentation](https://docs.kernel.org/driver-api/input.html?utm_source=chatgpt.com))
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