ACPI can be used to enumerate PNP devices, but the code does not
handle this in the right way currently.  Namely, if an ACPI device
object
 1. Has a _CRS method,
 2. Has an identification of
    "three capital characters followed by four hex digits",
 3. Is not in the excluded IDs list,
it will be enumerated to PNP bus (that is, a PNP device object will
be create for it).  This means that, actually, the PNP bus type is
used as the default bus type for enumerating _HID devices in ACPI.

However, more and more _HID devices need to be enumerated to the
platform bus instead (that is, platform device objects need to be
created for them).  As a result, the device ID list in acpi_platform.c
is used to enforce creating platform device objects rather than PNP
device objects for matching devices.  That list has been continuously
growing recently, unfortunately, and it is pretty much guaranteed to
grow even more in the future.

To address that problem it is better to enumerate _HID devices
as platform devices by default.  To this end, change the way of
enumerating PNP devices by adding a PNP ACPI scan handler that
will use a device ID list to create PNP devices for the ACPI
device objects whose device IDs are present in that list.

The initial device ID list in the PNP ACPI scan handler contains
all of the pnp_device_id strings from all the existing PNP drivers,
so this change should be transparent to the PNP core and all of the
PNP drivers.  Still, in the future it should be possible to reduce
its size by converting PNP drivers that need not be PNP for any
technical reasons into platform drivers.
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
[rjw: Rewrote the changelog, modified the PNP ACPI scan handler code]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>