drvs/intel/gma/acpi: Add methods to use MBOX3

* Add Mailbox 3 driver
* Request brightness change through Mailbox 3
* Return Ones on error or if unsupported
* Mark existing code as legacy
  (still required if no GMA driver is running)
* Call legacy code if Mailbox 3 is unsupported, on error or
  if gma driver isn't running

Tested on Lenovo T430:
* Brightness control still works
* Brightness is the same on S3 resume

Change-Id: I51554c819148336b204d28972cbf775a10c3fb8a
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/27711
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-by: Nathaniel Roach <nroach44@gmail.com>
Reviewed-by: Nicola Corna <nicola@corna.info>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>

1 files changed, 8 insertions, 2 deletions

diff --git a/src/drivers/intel/gma/acpi/common.asl b/src/drivers/intel/gma/acpi/common.asl
index 7415859d5c..009e3a54cd 100644
--- a/src/drivers/intel/gma/acpi/common.asl
+++ b/src/drivers/intel/gma/acpi/common.asl
@@ -44,8 +44,11 @@
 		Store (And(Arg0, 7), DSEN)
 	}
 
-	/* Using Notify is the right way. But Windows doesn't handle
-	   it well. So use both method in a way to avoid double action.
+	/*
+	 * Decrement display brightness.
+	 *
+	 * Using Notify is the right way. But Windows doesn't handle
+	 * it well. So use both method in a way to avoid double action.
 	 */
 	Method (DECB, 0, NotSerialized)
 	{
@@ -62,6 +65,9 @@
 		}
 	}
 
+	/*
+	 * Increment display brightness.
+	 */
 	Method (INCB, 0, NotSerialized)
 	{
 		If (BRCT)