[PATCH v4] HID: appletb-kbd: support layer switching on Fn double press

[Aditya Garg <aditya.garg@linux.dev>]

Holding Fn temporarily switches the Touch Bar between media controls and
function keys. Users who want to keep the alternate layer currently need
to change the mode through sysfs.

An optional double_press_switch_time module parameter makes a double press
persistently switch the default layer. Its value specifies the double-press
interval in milliseconds, while zero leaves the behavior disabled.

Signed-off-by: Aditya Garg <aditya.garg@linux.dev>
Signed-off-by: Andre Eikmeyer <dev@deq.rocks>
---
Changes in v2:
- Added a check to ensure negative fn switch times are ignored.

Changes in v3:
- Renamed the backing variable to match the module parameter.
- Fixed the reported code style issues.

Changes in v4:
- Added missing "break;" in case statements.
- Change author's email to aditya.garg@linux.dev

 drivers/hid/hid-appletb-kbd.c | 56 ++++++++++++++++++++++++++++++-----
 1 file changed, 49 insertions(+), 7 deletions(-)

diff --git a/drivers/hid/hid-appletb-kbd.c b/drivers/hid/hid-appletb-kbd.c
index 5cc27066f..f10d093c2 100644
--- a/drivers/hid/hid-appletb-kbd.c
+++ b/drivers/hid/hid-appletb-kbd.c
@@ -56,6 +56,12 @@ static int appletb_tb_idle_timeout = 15;
 module_param_named(idle_timeout, appletb_tb_idle_timeout, int, 0644);
 MODULE_PARM_DESC(idle_timeout, "Idle timeout in sec");
 
+static int appletb_tb_double_press_switch_time;
+module_param_named(double_press_switch_time,
+		   appletb_tb_double_press_switch_time, int, 0644);
+MODULE_PARM_DESC(double_press_switch_time,
+		 "Fn double-press interval in ms (0 disables layer switching)");
+
 struct appletb_kbd {
 	struct hid_field *mode_field;
 	struct input_handler inp_handler;
@@ -68,6 +74,7 @@ struct appletb_kbd {
 	bool has_turned_off;
 	u8 saved_mode;
 	u8 current_mode;
+	unsigned long last_fn_press;
 };
 
 static const struct key_entry appletb_kbd_keymap[] = {
@@ -243,6 +250,20 @@ static int appletb_kbd_hid_event(struct hid_device *hdev, struct hid_field *fiel
 	return kbd->current_mode == APPLETB_KBD_MODE_OFF;
 }
 
+static u8 appletb_switch_mode(u8 mode)
+{
+	switch (mode) {
+	case APPLETB_KBD_MODE_SPCL:
+		return APPLETB_KBD_MODE_FN;
+		break;
+	case APPLETB_KBD_MODE_FN:
+		return APPLETB_KBD_MODE_SPCL;
+		break;
+	default:
+		return mode;
+	}
+}
+
 static void appletb_kbd_inp_event(struct input_handle *handle, unsigned int type,
 			      unsigned int code, int value)
 {
@@ -250,15 +271,36 @@ static void appletb_kbd_inp_event(struct input_handle *handle, unsigned int type
 
 	reset_inactivity_timer(kbd);
 
-	if (type == EV_KEY && code == KEY_FN && appletb_tb_fn_toggle &&
-		(kbd->current_mode == APPLETB_KBD_MODE_SPCL ||
-		 kbd->current_mode == APPLETB_KBD_MODE_FN)) {
+	if (type == EV_KEY && code == KEY_FN &&
+	    (kbd->current_mode == APPLETB_KBD_MODE_SPCL ||
+	     kbd->current_mode == APPLETB_KBD_MODE_FN)) {
 		if (value == 1) {
-			kbd->saved_mode = kbd->current_mode;
-			appletb_kbd_set_mode(kbd, kbd->current_mode == APPLETB_KBD_MODE_SPCL
-						? APPLETB_KBD_MODE_FN : APPLETB_KBD_MODE_SPCL);
+			if (appletb_tb_double_press_switch_time > 0) {
+				unsigned long now = jiffies;
+
+				if (time_before(now, kbd->last_fn_press +
+					msecs_to_jiffies(appletb_tb_double_press_switch_time))) {
+					appletb_tb_def_mode =
+						appletb_switch_mode(appletb_tb_def_mode);
+
+					appletb_kbd_set_mode(kbd, appletb_tb_def_mode);
+
+					kbd->saved_mode = appletb_tb_def_mode;
+					kbd->last_fn_press = 0;
+
+					return;
+				}
+
+				kbd->last_fn_press = now;
+			}
+			if (appletb_tb_fn_toggle) {
+				kbd->saved_mode = kbd->current_mode;
+
+				appletb_kbd_set_mode(kbd, appletb_switch_mode(kbd->current_mode));
+			}
 		} else if (value == 0) {
-			if (kbd->saved_mode != kbd->current_mode)
+			if (appletb_tb_fn_toggle &&
+			    kbd->saved_mode != kbd->current_mode)
 				appletb_kbd_set_mode(kbd, kbd->saved_mode);
 		}
 	}
-- 
2.53.0