[PATCH] HID: apple: Map Fn+Esc to SysRq

[Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>]

Apple keyboards have no SysRq/Print Screen key, so nothing on them
reports KEY_SYSRQ and the magic SysRq key cannot be used from them
without a keymap change.

The documented way for keyboards without SysRq is to remap another key
to KEY_SYSRQ through the keymap (setkeycodes, EVIOCSKEYCODE, a udev
hwdb entry). Compact Apple keyboards have no unused key for that, so
the remap gives up a key. The free combinations are in the Fn layer,
which the keymap cannot reach: it is indexed by scancode, and Fn does
not change the scancode of the key pressed with it. Esc reports usage
0x70029 with and without Fn, so a keymap entry cannot tell Fn+Esc from
Esc.

hid-apple already implements the Fn layer and uses it to provide PC
keys that Apple keyboards lack: Fn+Backspace is Delete, Fn+Enter is
Insert, and Fn+arrow keys are Page Up, Page Down, Home and End. Add
SysRq to it as Fn+Esc, in the table used by the Magic Keyboard 2021 and
2024 models. No table in hid-apple translates Esc, so today Fn+Esc is
just a second Esc key.

The new entry has no flags, so like Fn+Enter it is translated only
while Fn is held, whether the top row defaults to media keys or to
function keys. Fn+Esc then reports KEY_SYSRQ, and the magic SysRq key
is Alt + Fn+Esc + command key.

Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
---
Tested on the internal keyboard of a MacBook Pro 16" (M1 Max) with the
Fedora Asahi Remix 7.1.13 kernel, where hid-apple handles that keyboard
with the same magic_keyboard_2021_and_2024_fn_keys table; the table is
identical to the one in mainline. In evtest, Esc alone still reports
KEY_ESC, and Fn+Esc reports KEY_SYSRQ on press and release, with the
Esc scancode 0x70029. Alt + Fn+Esc + H prints the SysRq help to the
kernel log. In mainline this table serves the Magic Keyboard 2021 and
2024 models over USB and Bluetooth; that path has not been tested on
hardware.

Background: this came up on linux-input in 2007 for an Apple USB
keyboard. A configurable SysRq keycode was proposed, and the advice was
to remap a key of that keyboard to KEY_SYSRQ instead, so that SysRq
keeps working with other keyboards [1]; that keyboard had a spare F13.
A generic alternative SysRq key was proposed again in 2021 and was not
merged; the objection was to adding a new SysRq key regardless of the
keyboard [2]. This patch leaves the sysrq code alone and makes the
keyboard itself report KEY_SYSRQ, on a Fn combination that has no other
function; Apple's list of macOS Fn shortcuts has none with Esc either
[3].

The same entry could also go into the other Apple tables for keyboards
with a physical Esc, for example older Magic Keyboards and the T2
MacBook Pro models with a dedicated Esc key. I kept this patch to the
table I could test.

If this is accepted, I will follow up with a line in
Documentation/admin-guide/sysrq.rst describing the combination.

[1] https://lore.kernel.org/linux-input/d120d5000707160541k64c40d0dgf661226806fd98c8@mail.gmail.com/
[2] https://lore.kernel.org/lkml/YYUxNaDG0DquQvke@kroah.com/T/
[3] https://support.apple.com/en-us/102650

 drivers/hid/hid-apple.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/hid/hid-apple.c b/drivers/hid/hid-apple.c
index 5b32fb8c77f3..b37964451a43 100644
--- a/drivers/hid/hid-apple.c
+++ b/drivers/hid/hid-apple.c
@@ -173,6 +173,7 @@ static const struct apple_key_translation magic_keyboard_2015_fn_keys[] = {
 static const struct apple_key_translation magic_keyboard_2021_and_2024_fn_keys[] = {
 	{ KEY_BACKSPACE, KEY_DELETE },
 	{ KEY_ENTER,	KEY_INSERT },
+	{ KEY_ESC,	KEY_SYSRQ },
 	{ KEY_F1,	KEY_BRIGHTNESSDOWN, APPLE_FLAG_FKEY },
 	{ KEY_F2,	KEY_BRIGHTNESSUP,   APPLE_FLAG_FKEY },
 	{ KEY_F3,	KEY_SCALE,          APPLE_FLAG_FKEY },
-- 
2.56.0