Input: add keyboard notifier documentation

Document the keyboard notifier.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
SIgned-off-by: Dmitry Torokhov <dtor@mail.ru>

diff --git a/Documentation/input/notifier.txt b/Documentation/input/notifier.txt
new file mode 100644 (file)
index 0000000..95172ca
--- /dev/null
+++ b/Documentation/input/notifier.txt
@@ -0,0 +1,52 @@
+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:
+
+- '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_*;
+- '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.
+
+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.