Since commit fdde0ff8 ("ACPI: PM: s2idle: Prevent spurious SCIs from
waking up the system") the SCI triggering without there being a wakeup
cause recognized by the ACPI sleep code will no longer wakeup the system.

This works as intended, but this is a problem for devices where the SCI
is shared with another device which is also a wakeup source.

In the past these, from the pov of the ACPI sleep code, spurious SCIs
would still cause a wakeup so the wakeup from the device sharing the
interrupt would actually wakeup the system. This now no longer works.

This is a problem on e.g. Bay Trail-T and Cherry Trail devices where
some peripherals (typically the XHCI controller) can signal a
Power Management Event (PME) to the Power Management Controller (PMC)
to wakeup the system, this uses the same interrupt as the SCI.
These wakeups are handled through a special INT0002 ACPI device which
checks for events in the GPE0a_STS for this and takes care of acking
the PME so that the shared interrupt stops triggering.

The change to the ACPI sleep code to ignore the spurious SCI, causes
the system to no longer wakeup on these PME events. To make things
worse this means that the INT0002 device driver interrupt handler will
no longer run, causing the PME to not get cleared and resulting in the
system hanging. Trying to wakeup the system after such a PME through e.g.
the power button no longer works.

Add an acpi_register_wakeup_handler() function which registers
a handler to be called from acpi_s2idle_wake() and when the handler
returns true, return true from acpi_s2idle_wake().

The INT0002 driver will use this mechanism to check the GPE0a_STS
register from acpi_s2idle_wake() and to tell the system to wakeup
if a PME is signaled in the register.

Fixes: fdde0ff8

 ("ACPI: PM: s2idle: Prevent spurious SCIs from waking up the system")
Cc: 5.4+ <stable@vger.kernel.org> # 5.4+
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>