Keyboard notifier

Keyboard notifier parameter와 event 종류

`register_keyboard_notifier`를 사용하면 keyboard event에서 callback을 받을 수 있습니다. 자세한 발생 지점은 `kbd_keycode()`를 참고하며 전달 구조체는 `<linux/keyboard.h>`의 `keyboard_notifier_param`입니다.

마지막 `KBD_POST_KEYSYM`을 제외한 각 event에서 callback은 `NOTIFY_STOP`을 반환해 event를 소비할 수 있습니다. 그러면 notify loop가 멈추고 keyboard event가 drop됩니다. Post event는 결과 LED 등을 검사하는 단계입니다.

=================
Keyboard notifier
=================

One can use register_keyboard_notifier to get called back on keyboard
events (see kbd_keycode() function for details).  The passed structure is
keyboard_notifier_param (see <linux/keyboard.h>):

- 'vc' always provide the VC for which the keyboard event applies;
- 'down' is 1 for a key press event, 0 for a key release;
- 'shift' is the current modifier state, mask bit indexes are KG_*;
- 'ledstate' is the current LED state;
- 'value' depends on the type of event.

- KBD_KEYCODE events are always sent before other events, value is the keycode.
- KBD_UNBOUND_KEYCODE events are sent if the keycode is not bound to a keysym.
  value is the keycode.
- KBD_UNICODE events are sent if the keycode -> keysym translation produced a
  unicode character. value is the unicode value.
- KBD_KEYSYM events are sent if the keycode -> keysym translation produced a
  non-unicode character. value is the keysym.
- KBD_POST_KEYSYM events are sent after the treatment of non-unicode keysyms.
  That permits one to inspect the resulting LEDs for instance.

For each kind of event but the last, the callback may return NOTIFY_STOP in
order to "eat" the event: the notify loop is stopped and the keyboard event is
dropped.

`kbd_keycode()` notifier 제어 흐름

개략 C 흐름에서 먼저 `params.value=keycode`로 두고 `KBD_KEYCODE` chain을 호출합니다. Callback이 `NOTIFY_STOP`을 반환하거나 keycode가 unbound이면 `KBD_UNBOUND_KEYCODE` chain을 호출하고 반환합니다.

Unicode translation이면 `param.value=unicode`로 바꾸고 `KBD_UNICODE` chain을 호출합니다. Stop이면 반환하고 그렇지 않으면 Unicode를 emit한 뒤 반환합니다.

Non-Unicode keysym이면 `params.value=keysym`으로 설정해 `KBD_KEYSYM` chain을 호출합니다. Stop이면 반환하고, 아니면 keysym을 적용한 뒤 `KBD_POST_KEYSYM` chain을 호출합니다.

이 notifier는 보통 interrupt context에서 호출됩니다. Callback은 그 문맥에 맞게 sleep하지 않고 빠르게 끝나도록 작성해야 합니다.

In a rough C snippet, we have::

    kbd_keycode(keycode) {
        ...
        params.value = keycode;
        if (notifier_call_chain(KBD_KEYCODE,&params) == NOTIFY_STOP)
            || !bound) {
                notifier_call_chain(KBD_UNBOUND_KEYCODE,&params);
                return;
        }

        if (unicode) {
                param.value = unicode;
                if (notifier_call_chain(KBD_UNICODE,&params) == NOTIFY_STOP)
                        return;
                emit unicode;
                return;
        }

        params.value = keysym;
        if (notifier_call_chain(KBD_KEYSYM,&params) == NOTIFY_STOP)
                return;
        apply keysym;
        notifier_call_chain(KBD_POST_KEYSYM,&params);
    }

.. note:: This notifier is usually called from interrupt context.