# The input module: broadcasting input events

A keyboard driver has one keystroke and *many* programs that might want it — a shell, a
window server, a logger. None of them owns the hardware, and the driver should not know
who is listening. So between the drivers and the listeners sits the **input service**
(`system/services/input/`): drivers **publish** events to it, programs **subscribe**, and
it fans each event out to every interested subscriber. It is an ordinary ring-3 process
reached over IPC, like the [VFS server](../system/services/vfs/vfs.zig) — no kernel knows
what a key is.

## One service, several device classes

The service carries three device classes today — **keyboard**, **mouse**, and
**joystick/gamepad** — and is built to take more
([protocol.zig](../system/services/input/protocol.zig)). Each class has its own typed
event:

- `KeyEvent` — `key_down`/`key_up` (physical make/break) and `key_press` (a character was
  produced, carrying the Unicode scalar); plus a layout-independent `keycode` and a
  `modifiers` bitmask.
- `MouseEvent` — relative `motion` (`dx`/`dy`), `button_down`/`button_up`, and `scroll`.
- `JoystickEvent` — `axis` moves (a signed value on a `control` index) and
  `button_down`/`button_up`.

All three travel in one **`InputEvent` envelope** tagged with a `DeviceKind`, so the
fan-out is a single code path and a subscriber can take a mix of classes on one stream.
Decode an envelope with `asKeyboard()` / `asMouse()` / `asJoystick()` (each returns null
unless the tag matches). A subscriber names the classes it wants with a **`device_mask`**,
and the service routes each event only to subscribers whose mask includes its class — so a
mouse-only listener never wakes for keystrokes.

## Why this needed a new kernel primitive

The interesting part is delivery, and it runs straight into the shape of danos IPC.
[ipc.md](ipc.md) describes a **synchronous rendezvous**: a server holds exactly one
pending reply (`Task.ipc_client`) and *must* answer it on its next `replyWait`. Two
consequences decide the whole design:

1. **You cannot block N subscribers waiting for "the next event".** A server can hold only
   one caller at a time, so the natural "subscriber calls `next_event()` and blocks" API
   is impossible for more than one subscriber. Delivery therefore has to be **push** — the
   service reaching out to subscribers — not pull.

2. **A synchronous push can hang the whole service.** If the service delivered with
   `ipc_call`, it would block until each subscriber replied. `ipc_call` has no timeout, and
   the kernel does **not** wake a caller parked on a *dead* peer's endpoint (it only fails a
   peer that was mid-reply — see [process.zig](../system/kernel/process.zig)
   `releaseTaskResourcesLocked`). One subscriber that exits mid-delivery would wedge input
   for everyone. That is the opposite of the resilience the microkernel is for.

The fix is the asynchronous send that [ipc.md](ipc.md) had already earmarked as future
work ("asynchronous / buffered send … for notifications between servers"):

```
ipc_send(handle, message_ptr, message_len) -> 0 / -errno
```

`ipc_send` copies a small payload into the endpoint's **bounded queue** and wakes a
receiver, then returns immediately — it never blocks and so can never hang on a dead or
slow subscriber. The receiver picks it up through the same `replyWait` it already runs:
the wake arrives as a **buffered message** — `notify_badge_bit | notify_message_bit` set in
the badge (distinguishing it from a bare IRQ/child-exit notification), the sender's task id
in the low bits, and the payload in the receive buffer, with no reply owed. The queue holds
16 messages per endpoint; a full queue **drops the oldest**, because a buffered message is
discrete data, not a coalescing "level" like an interrupt. See
[ipc-synchronous.zig](../system/kernel/ipc-synchronous.zig) (`sendLocked`, `popPost`, and
the `replyWait` receive loop).

This is the async counterpart of `ipc_call`, and the input service is its first consumer.

## How the pieces fit

```
  keyboard/mouse driver, input-source       input service               subscriber(s)
  -----------------------------------       -------------               -------------
  connectSource(); loop:                    replyWait:                  subscribeKeyboard()/…All:
    publishKeyboardEvent(k) ─ ipc_call ─▶  publish → broadcast:            createIpcEndpoint()
    publishMouseEvent(m)                     for each sub whose            callCap(subscribe,
    publishJoystickEvent(j)                   mask matches event.device:     send_cap = ep,
                                               ipc_send(sub_ep) ──────▶       device_mask)
                                       reply ok                          loop: next()
                                     subscribe → store {ep cap,          └─ replyWait(ep)
                                       task id, device_mask}                → InputEvent
```

- A **subscriber** calls `input.subscribe(mask)` — or a typed helper: `subscribeKeyboard()`,
  `subscribeMouse()`, `subscribeJoystick()` (one class, `next()` returns the decoded event),
  or `subscribeAll()` (every class, `next()` returns a tagged `InputEvent`)
  ([library/runtime/input.zig](../library/runtime/input.zig)). It creates its own endpoint
  and hands it to the service as a **capability** (M13 capability passing — the input
  service is that feature's first real user), along with its `device_mask`. Then it loops on
  `next()`, a `replyWait` on that endpoint returning each pushed event.
- A **source** (a keyboard, mouse, or joystick driver) calls `input.connectSource()` and the
  method for its class: `publishKeyboardEvent`, `publishMouseEvent`, or
  `publishJoystickEvent`. Publishing is a short synchronous `ipc_call` the service answers at
  once; the service's own fan-out is asynchronous, so publishing never blocks on a slow
  subscriber.
- The **service** ([input.zig](../system/services/input/input.zig)) keeps a small subscriber
  table (endpoint handle + owning task id + `device_mask`). On `publish` it `ipc_send`s the
  event to every subscriber whose mask includes the event's device class. On `subscribe` it
  stores the passed capability and mask and, as housekeeping, prunes any slot whose owning
  process has exited (checked against `process_enumerate`) — not for correctness (an async
  send to an orphaned endpoint is harmless) but to reclaim the slot.

Publisher and subscriber must be **separate processes**: a single thread that both
published and serviced its own subscription would deadlock (its `publish` call blocks until
the service delivers to its endpoint, which only the same thread could receive).

## Status and follow-ups

- **The keyboard is real.** The `ps2-bus` driver owns PNP0303, which carries *both* the
  0x60/0x64 ports and IRQ1, so reading the hardware lives in the bus, not in
  [keyboard.zig](../system/drivers/ps2-bus/keyboard.zig): the bus binds IRQ1 and, on each
  interrupt, drains port 0x60, routing every byte by the status register's
  auxiliary-output bit to whichever child driver **attached** for that device (an
  `AttachRequest` to the well-known `ps2_bus` service, carrying the child's endpoint as a
  capability; the bytes then arrive as asynchronous `ForwardedByte` messages, so the IRQ
  path never blocks on a child). The keyboard driver decodes the stream — scancode **set 2**,
  what the keyboard sends with the 8042's legacy translation off, decoded by
  [scancode.zig](../system/drivers/ps2-bus/scancode.zig) into USB HID usage keycodes with
  make/break, typematic-repeat, and modifier tracking (host-tested under `zig build test`) —
  and publishes real `key_down`/`key_press`/`key_up` events.
- **Keycode → character** is wired in: the keyboard driver fills a `key_press` event's
  `character` through [`library/xkeyboard-config`](../library/xkeyboard-config/README.md)
  (`xkb.map(layout, keycode, mods)` → keysym + Unicode character), synthesizing the ASCII
  control characters for Enter/Tab/Backspace/Escape, whose keysyms map to no Unicode. The
  layout defaults to `us`; the bus can pass another as the driver's argv[2] — the seam for
  a future settings source.
- **The mouse is real too.** IRQ12 is enumerated on the auxiliary device's own ACPI node
  (PNP0F13), so the bus claims that node alongside the controller and routes both IRQs to
  its one endpoint, acking whichever line the notification's badge names.
  [mouse.zig](../system/drivers/ps2-bus/mouse.zig) attaches the way the keyboard does and
  assembles the forwarded bytes with
  [mouse-packet.zig](../system/drivers/ps2-bus/mouse-packet.zig) (three-byte stream-mode
  packets: sync/overflow handling, nine-bit movement, screen-convention `dy` — host-tested
  under `zig build test`) into `button_down`/`button_up` transitions and `motion` events.
  **Follow-up:** the IntelliMouse magic-knock for a scroll wheel (four-byte packets) and
  `scroll` events. The hardware-free `input-source` still rotates through all three classes
  synthetically (including a joystick, which has no driver yet) via the
  `input.synthetic*Event` helpers.
- **Drop-oldest under overflow** is a defined loss; the 16-slot ring absorbs normal bursts.
  Real backpressure/flow-control is future work.
- **`publish` is unauthenticated** — any process may publish, consistent with the current
  bring-up trust model (see [driver-model.md](driver-model.md)). A source capability is
  future work.

## Verifying it

The `input` case (`python3 test/qemu_test.py input`, in
[tests.zig](../system/kernel/tests.zig) `inputTest`) boots the real kernel and spawns the
service, the synthetic source (which cycles keyboard, mouse, and joystick events), and a
subscriber that took all three classes. It passes only when the subscriber heartbeats
`input-test: ok` — proof that an event travelled source → service → subscriber over IPC,
exercising `ipc_send`, capability-passing subscription, and per-device routing. Each
serial line names the class received, so the log shows all three arriving on one stream.

## See also

- [ipc.md](ipc.md) — the synchronous rendezvous and the notification path `ipc_send` extends.
- [syscall.md](syscall.md) — the system-call surface, including `ipc_send`.
- [driver-model.md](driver-model.md) — class drivers, capability passing (M13), the trust model.
