ACPI / bus: Introduce a list of ids for "always present" devices

Several Bay / Cherry Trail devices (all of which ship with Windows 10) hide
the LPSS PWM controller in ACPI, typically the _STA method looks like this:

    Method (_STA, 0, NotSerialized)  // _STA: Status
    {
        If (OSID == One)
        {
            Return (Zero)
        }

        Return (0x0F)
    }

Where OSID is some dark magic seen in all Cherry Trail ACPI tables making
the machine behave differently depending on which OS it *thinks* it is
booting, this gets set in a number of ways which we cannot control, on
some newer machines it simple hardcoded to "One" aka win10.

This causes the PWM controller to get hidden, which means Linux cannot
control the backlight level on cht based tablets / laptops.

Since loading the driver for this does no harm (the only in kernel user
of it is the i915 driver, which will only uses it when it needs it), this
commit makes acpi_bus_get_status() always set status to ACPI_STA_DEFAULT
for the LPSS PWM device, fixing the lack of backlight control.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
[ rjw: Rename the new file to utils.c ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/drivers/acpi/bus.c b/drivers/acpi/bus.c
index 34fbe027e73a..784bda663d16 100644
--- a/drivers/acpi/bus.c
+++ b/drivers/acpi/bus.c
@@ -114,6 +114,11 @@ int acpi_bus_get_status(struct acpi_device *device)
 	acpi_status status;
 	unsigned long long sta;
 
+	if (acpi_device_always_present(device)) {
+		acpi_set_device_status(device, ACPI_STA_DEFAULT);
+		return 0;
+	}
+
 	status = acpi_bus_get_status_handle(device->handle, &sta);
 	if (ACPI_FAILURE(status))
 		return -ENODEV;