336 lines
17 KiB
Markdown
336 lines
17 KiB
Markdown
# gKeyMapper daemon — implementation plan
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Status: draft / proposal
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Scope: a native C/C++ background service that detects G1–G9 (and later M1–M3/MR
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and media keys) on the Logitech G910 and replays the mappings stored by the GUI.
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## 1. Goals and non-goals
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Goals
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- Detect G-key presses in C/C++ and execute the configured action.
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- Run with a very small, stable resource footprint (idle ≈ 0 % CPU, few MB RSS,
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no interpreter, no polling when nothing happens).
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- Reuse the existing `mappings.conf` format as the single source of truth.
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- Let the GUI and the daemon run independently; either can start first.
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- No Python at runtime.
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Non-goals (v1)
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- Profiles (M1–M3/MR), per-locale text typing (`typeout`), embedded scripting.
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- GUI redesign beyond a status indicator / reload / test button.
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## 2. What the Python reference does (and what we keep)
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Reference: <https://github.com/JSubelj/g910-gkey-macro-support>
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- G-keys are **not** exposed as distinct evdev key codes. The driver therefore
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opens the keyboard's **vendor HID interface (USB interface 1) directly via
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libusb**, detaches the kernel driver from that interface, and reads raw
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interrupt-IN reports.
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- On start it sends a control transfer to **disable the firmware F-key→G-key
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binding**, so pressing G1..G9 no longer also emits F1..F9 on the normal
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keyboard interface.
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- Raw report byte patterns are looked up in tables
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(`events.macroKeys`, `memoryKeys`, `mediaKeys`, `releaseEvents`) and turned
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into actions.
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- A **uinput virtual keyboard** replays `shortcut` (press, later release),
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`typeout` (per-char), `uinput` (direct `KEY_*`); `run` uses `/bin/bash -c`.
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- Config is JSON at `~/.config/g910-gkeys/config.json`, reloaded on directory
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change (inotify via `fcntl`), plus `SIGUSR1` reload.
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- Ship: a **systemd --user** service and **udev rules** with `TAG+="uaccess"` for
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the device and `/dev/uinput`, so no root is needed.
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Relevant USB IDs (from the udev rules): `046d:c335`, `c33f`, `c33e`, `c32b`,
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`c24d`, `c22d`. The G910 in this project is `046d:c32b`.
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What we keep: libusb raw HID access, disabling the F-key binding, uinput output,
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config-dir watching. What we change: JSON→our line format, single-threaded
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event-driven loop instead of a Python loop with fixed sleeps, fork/exec reaping,
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and a real IPC channel to the GUI.
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## 3. Architecture overview
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```
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+---------------------+ mappings.conf (shared truth) +----------------------+
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| gkeymapper (GUI) | <------------ write / inotify ---------> | gkeymapper-daemon |
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| GTK4 + GIO/GDBus | | libusb + uinput |
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| | <===== D-Bus (session bus) ============> | GDBus + inotify |
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+---------------------+ status / reload / events +----------+-----------+
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| raw HID (iface 1)
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v
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+----------------------+
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| Logitech G910 |
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+----------+-----------+
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| uinput
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v
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+----------------------+
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| /dev/uinput keyboard |
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+----------------------+
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```
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Key property: the **hot path never crosses IPC**. G-key press → decode → uinput
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happens entirely inside the daemon. IPC (D-Bus) is only used for control, status
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and notifications, so a slow/absent GUI never delays a keypress.
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## 4. Daemon design
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### 4.1 Process model
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- One process, single-threaded, event loop.
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- No threads in v1. `run`/`command` actions are launched with `fork()` +
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`execve()`; children are reaped via a `SIGCHLD`/`signalfd` handler so no
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zombies accumulate (the reference leaks a `Popen` per invocation).
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- `python` type is intentionally dropped.
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### 4.2 Event loop
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Use one `poll()`/`epoll()` loop over:
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- the **libusb** file descriptor (from `libusb_get_pollfds()`; use libusb's async
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API or `libusb_interrupt_transfer` on a worker, see below),
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- an **inotify** fd for the config file/directory,
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- a **signalfd** for `SIGTERM`/`SIGINT`/`SIGUSR1`,
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- the **D-Bus** connection (GDBus integrates with the same `GMainContext`).
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Preferred: drive libusb asynchronously with `libusb_submit_transfer()` and fold
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the reported poll fds into the same loop (libusb has an official "poll with
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another event loop" pattern). That yields exactly **0 wakeups while idle**
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(excluding the kernel's own HID interrupts). If that is too much work for v1,
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fall back to a blocking `libusb_interrupt_transfer(..., timeout=1000)` inside a
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dedicated thread that posts events into the main loop — still cheap (1 wake/s).
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To keep GDBus and the fd loop together, the loop is easiest on the **GLib main
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loop** (`GMainLoop`): GDBus, `GFileMonitor` and `GSocket`/`GSource` cover
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inotify and signalfd, and libusb poll fds can be wrapped in a custom
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`GSource`. GLib is already a dependency of the GUI, so this adds no new library.
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If minimal footprint is the priority over convenience, a raw `epoll` loop is the
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alternative; decision point in §11.
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### 4.3 Device layer (`libusb`)
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- Find the device by vendor/product from the supported-device table; pick
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interface 1, endpoint from the same table.
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- `libusb_detach_kernel_driver()` on interface 1; remember to
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`libusb_attach_kernel_driver()` on shutdown (reference does this).
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- Send the "disable F-key to G-key binding" control transfer
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(`bmRequestType=0x21, bRequest=0x09, wValue=0x0212, wIndex=1`) and consume the
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confirmation reports; ignore the known LED-return patterns
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(`\x11\xff\x0f`, `\x11\xff\x10`, `\x11\xff\xff`).
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- Hot-plug / re-plug: on `LIBUSB_ERROR_NO_DEVICE`/`IO` re-run discovery with a
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small backoff (reference re-inits in place).
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- Optional (later): `libusb_hotplug_register_callback` to react to device
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add/remove without polling.
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The device table in `supported_devices.py` (byte patterns per model) is ported
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to a generated C++ header (`constexpr` tables) — keep it a build-time
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artifact so the runtime does zero parsing.
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### 4.4 Decoding
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- The report is a fixed-length byte blob. Build a **compile-time lookup**:
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a sorted array of `{report bytes, action id}` with binary search, or a small
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open-addressed hash keyed on the first 8 bytes. This replaces the Python
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`dict` + `list(...).index()` O(n) scans with O(1)/O(log n) and no allocation.
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- Categories: `macro` (G1..G9), `memory` (profiles, later), `media` (later),
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`release` (stops a held shortcut), and ignored/known-noise.
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### 4.5 Action layer (uinput)
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- Open `/dev/uinput` once; `UI_SET_EVBIT` for `EV_KEY`, register the key set used
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by the mappings, `UI_DEV_CREATE`. Keep the fd for the daemon's lifetime.
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- Emit events by `write()`-ing `struct input_event` (from
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`<linux/uinput.h>` / `<linux/input-event-codes.h>` — no libevdev needed).
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- `key` mapping = shortcut grammar `ctrl+c,ctrl+v`:
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- single combo: press modifiers+key in order, remember the keys, release them
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when the keyboard's `release` event arrives (mirror the reference's
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`press_release_fifo` stack).
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- comma list: `emit_combo()` each group (press+release).
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- `command` mapping: `fork()`; in the child `setsid()`, redirect stdio to
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`/dev/null`, `execve("/bin/sh", "-c", cmd)`. Reap via `SIGCHLD`.
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- `disabled`: no-op.
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- Give the uinput device a clear name/IDs (e.g. `gkeymapper virtual keyboard`)
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so `libinput`/users can identify it.
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### 4.6 Config
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- Parse the existing `mappings.conf`:
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`G<n>=<type>:<target>` with `type ∈ {key, command, disabled}`.
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- Parse at load and on change only; the runtime holds a small pre-parsed action
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per G-key (enum + fixed target buffer / `std::string`), so no parsing in the
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hot path.
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- Watch the config file (and, for atomic-save editors, its directory) with
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inotify/`GFileMonitor`; reload on `IN_CLOSE_WRITE`/`IN_MOVED_TO`. Debounce
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(e.g. 100 ms) to coalesce editor save bursts.
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- Also reload on `SIGUSR1` (manual/test) and on `Reload()` D-Bus call.
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### 4.7 Privileges and packaging
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- udev rule with `TAG+="uaccess"` for the supported USB IDs and for
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`KERNEL=="uinput"` (mirrors the reference's `60-g910-gkeys.rules`).
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- `uinput` kernel module must be present; ship `/etc/modules-load.d/` fragment
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or document `modprobe uinput` (here it is built in).
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- **systemd --user** unit `gkeymapper-daemon.service`:
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`Type=dbus`, `BusName=…`, `ExecStart=gkeymapper-daemon`,
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`Restart=on-failure`, `Documentation=…`. `BusName` enables D-Bus activation,
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so starting the GUI can start the daemon on demand.
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- Do **not** require root. Commands run with the user's privileges (the
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reference's "executed as root" note is a wiki artifact, not a design goal).
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## 5. GUI ↔ daemon interaction
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Three layers, from loosest to tightest:
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1. **Shared state = the config file** (`~/.config/gKeyMapper/mappings.conf`).
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The GUI is the writer, the daemon is a watcher/reader. This alone is enough
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for basic operation and is robust: whichever side starts first, the other
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converges. The daemon reloads via inotify; no coupling to the GUI process.
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2. **Control/status = D-Bus on the session bus** (GDBus; part of GIO, already
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linked by the GUI). Used for status, explicit reload, and notifications.
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3. **Lifecycle**: the daemon is a `systemd --user` service; the GUI can start it
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implicitly by calling a D-Bus method (activation) and can show whether it is
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running.
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### 5.1 Proposed D-Bus interface
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- Bus name: `<app-id>` — decide once (§11). Suggested: `dev.rager.gkeymapper`
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(replace the current placeholder `com.example.gkeymapper`).
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- Object path: `/dev/rager/gkeymapper/Daemon`
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- Interface: `dev.rager.gkeymapper.Daemon`
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Methods
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| Method | Signature | Purpose |
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|---|---|---|
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| `Reload` | `() -> ()` | Re-read `mappings.conf` now (GUI "Save" also calls this). |
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| `GetStatus` | `() -> (s version, b running, s device, s configPath)` | Status for the GUI footer/indicator. |
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| `TestMapping` | `(u index) -> ()` | Fire mapping for G1..G9 without hardware (GUI "Test" button). |
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| `SetProfile` | `(s profile) -> ()` | Reserved for M-key profiles (later). |
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Signals
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| Signal | Signature | Purpose |
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|---|---|---|
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| `KeyPressed` | `(u index, s action)` | A G-key fired (for GUI highlight/history). |
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| `ConfigReloaded` | `(b ok)` | Daemon finished reloading. |
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| `DeviceChanged` | `(b present, s device)` | Keyboard attached/detached. |
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Notes
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- All are control-plane calls; none is on the keypress path.
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- `Reload` is best-effort: inotify is the backstop if the call is lost.
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- If D-Bus is unavailable, the daemon still works; the GUI degrades to
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"no status" and "save only". Signals `SIGUSR1` (reload) / `SIGTERM` remain for
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scripted use, matching the reference.
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### 5.2 Interaction sequences
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Save in the GUI:
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```
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GUI: write mappings.conf (atomic: temp + rename)
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GUI: call Daemon.Reload() [optional, for immediacy]
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Daemon: inotify IN_MOVED_TO -> (re)parse -> apply
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Daemon: emit ConfigReloaded(ok)
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```
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Key press:
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```
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G910 --raw HID--> Daemon -> decode -> uinput emit / fork+exec
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Daemon: emit KeyPressed(index, action) [GUI only, non-blocking]
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```
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Start-up orders:
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- Daemon first, GUI later: GUI reads status via D-Bus; no config write needed.
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- GUI first, daemon later: GUI writes config; on daemon start it reads the file.
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### 5.3 GUI-side changes (small)
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- On "Save mappings": keep writing the file, then call `Reload()` (ignore error).
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- Add a footer status line: daemon running / device present / last reload, fed by
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`GetStatus` + the `DeviceChanged`/`ConfigReloaded` signals.
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- Optional "Test" button per row → `TestMapping(index)`.
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- The key-name vocabulary must match: the capture dialog already writes
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lowercase GDK key names; move the canonical name list into the shared module
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(§6) so GUI and daemon agree on the same spellings.
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## 6. Code layout
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```
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src/ # GUI (existing)
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daemon/
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main.cpp # event loop, wiring
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device_libusb.{h,cpp} # discovery, detach/attach, read, disable-binding
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decoder.{h,cpp} # report bytes -> action id (compile-time tables)
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uinput_keyboard.{h,cpp}# virtual keyboard, emit / combo / press-release stack
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actions.{h,cpp} # key / command / disabled execution, child reaping
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config.{h,cpp} # mappings.conf parse + watch
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ipc_dbus.{h,cpp} # GDBus service
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generated/devices.inc # ported from supported_devices.py
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shared/
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mapping.{h,cpp} # parse/format mappings.conf (used by GUI and daemon)
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keys.{h,cpp} # canonical names <-> gdk keyval <-> linux KEY_*
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ipc_ids.h # D-Bus name/path/interface constants
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data/
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60-gkeymapper.rules
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gkeymapper-daemon.service
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uinput.conf # modules-load.d
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po/ # daemon strings, reuse the existing catalogs
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```
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Build: the Makefile gets a second target `gkeymapper-daemon` and install rules
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for the unit, udev rule and modules-load fragment. New build dependency:
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`libusb-1.0-0-dev` (GLib/GIO is already required by the GUI).
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## 7. Resource-efficiency measures (summary)
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- No Python interpreter; native binary, tables in `.rodata`.
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- Event-driven loop, **no idle polling** (libusb async + inotify + signalfd; or a
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1 s blocking USB read in the simple variant).
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- O(1)/O(log n) decode via compile-time tables instead of dict/linear scan.
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- One `uinput` fd for the process lifetime; fixed-size event writes; no
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per-event heap allocation.
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- Config parsed only on load/reload; hot path is table lookups + `write()`.
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- `fork`+`exec` for programs, children reaped (no zombies), stdio to `/dev/null`;
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no shell parsing for `key`/`uinput` actions.
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- Target footprint: idle 0 % CPU, wakeups only on real HID/DBus/file events,
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RSS in the low single-digit MB.
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## 8. Milestones
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- **M0 – Shared core.** `shared/mapping` + `shared/keys`; refactor the GUI to use
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them (still file-only). Unit tests for parse/format and key tables.
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- **M1 – Device bring-up.** `daemon` opens the G910 via libusb, disables the
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F-key binding, logs `G1..G9` presses. No output yet.
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- **M2 – Action layer.** uinput keyboard; `key` (shortcut, incl. release) and
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`command` work end-to-end. `disabled` no-op.
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- **M3 – Config & lifecycle.** config parse/reload (inotify, SIGUSR1), signals,
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systemd --user unit + udev rule + modules-load.
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- **M4 – IPC.** GDBus service (`Reload`, `GetStatus`, `TestMapping`, signals);
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GUI status line + Reload-on-save + optional Test button.
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- **M5 – Polish.** Packaging/install rules, docs, translations, hardening
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(hot-plug, debounce, error reporting to the GUI).
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## 9. Testing strategy
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- Unit: mapping parser round-trip, key-name tables, decoder against recorded
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byte sequences from the reference tables.
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- Device: replay captured reports into the decoder (no hardware needed).
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- Integration: `evtest` / `libinput debug-events` to observe the virtual device;
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compare `xev` output for a configured shortcut between reference and daemon.
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- Permission tests: verify the udev rule grants access as a normal user.
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- Resource check: `powertop`/`perf` or simple wakeup counting while idle.
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## 10. Risks / open points
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- **Device table size**: `supported_devices.py` is large; porting/generating the
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byte tables is real work and model-specific.
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- **Interface detach** may momentarily disturb the device; must re-attach on
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exit and handle crashes gracefully.
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- **Permissions**: relies on `TAG+="uaccess"` + a user service; must never fall
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back to requiring root silently.
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- **`command` environment**: `DISPLAY`/`WAYLAND_DISPLAY`/`XAUTHORITY` and
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`PATH` differ between a user service and an interactive shell; decide whether
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to inherit the user manager environment (`systemd --user` import-environment)
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or document `run` snippets setting `DISPLAY=:0` explicitly.
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- **Key-name vocabulary drift** between GUI (GDK names) and daemon (Linux KEY_*)
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is the most likely source of silent mismatches — that is why the shared table
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in M0 is mandatory before the daemon consumes the file.
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- **uinput device identity**: a second keyboard appears; choose stable
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name/vendor/product so the desktop does not confuse it with the real one.
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## 11. Decisions to make
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1. Bus name / app-id: replace `com.example.gkeymapper` with a real reverse-DNS id
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(suggested `dev.rager.gkeymapper`)? Affects the `.desktop` name too.
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2. Event loop: GLib `GMainLoop` (simplest, reuses GIO) vs. raw `epoll` (leanest).
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3. IPC: D-Bus (idiomatic, GIO already present) vs. a Unix socket (no bus
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dependency). Recommendation: D-Bus, with signals as a fallback.
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4. Scope of v1: keep `key`/`command`/`disabled` only, or also port
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`typeout`/`media`?
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