Haswell-based Fujitsu laptops (Lifebook E734/E744/E754) have a touchpad
toggle hotkey (Fn+F4) which is handled transparently to the operating
system: while an ACPI notification is sent to FUJ02B1 when Fn+F4 is
pressed, touchpad state is properly toggled without any explicit support
for this operation in fujitsu-laptop.

Skylake-based models (Lifebook E736/E746/E756) also have that hotkey,
but the touchpad is not toggled transparently to the operating system.
When Fn+F4 is pressed, an ACPI notification is sent to FUJ02E3.  A
subsequent call to S000 (FUNC_RFKILL) can be used to determine whether
the touchpad toggle hotkey was pressed so that an input event can be
sent to userspace.

Relevant ACPI code:

    Method (_L21, 0, NotSerialized)
    {
        ...
        If (AHKF)
        {
            Notify (\_SB.FEXT, 0x80)
        }
        ...
    }

    Method (S000, 3, Serialized)
    {
        Name (_T_0, Zero)
        Local0 = Zero
        While (One)
        {
            _T_0 = Arg0
            If (_T_0 == Zero)
            {
                Local0 |= 0x04000000
                Local0 |= 0x02000000
                Local0 |= 0x00020000
                Local0 |= 0x0200
                Local0 |= 0x0100
                Local0 |= 0x20
            }
            ElseIf (_T_0 == One)
            {
                ...
                If (AHKF & 0x08)
                {
                Local0 |= 0x04000000
                AHKF ^= 0x08
                }
                ...
            } ...
            Break
        }
        Return (Local0)
    }

Pressing Fn+F4 raises GPE 0x21 and sets bit 3 in AHKF.  This in turn
results in bit 26 being set in the value returned by FUNC_RFKILL called
with 1 as its first argument.  On Skylake-based models, bit 26 is also
set in the value returned by FUNC_RFKILL called with 0 as its first
argument (this value is saved in fujitsu_hotkey->rfkill_supported upon
module initialization), which suggests that this bit is set on models
which do not handle touchpad toggling transparently to the operating
system.

Note that bit 3 is cleared in AHKF once FUNC_RFKILL is called with 1 as
its first argument, which requires fujitsu-laptop to handle this hotkey
in a different manner than the other, GIRB-based hotkeys: two input
events (press and release) are immediately sent once Fn+F4 is pressed.
Reported-and-tested-by: Jan-Marek Glogowski <glogow@fbihome.de>
Signed-off-by: Michał Kępień <kernel@kempniu.pl>
Acked-by: Jonathan Woithe <jwoithe@just42.net>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>