ACPI / video: Run _BCL before deciding registering backlight

The ASUS K53SM's ACPI table queries _OSI("Windows 2012") in the video
output device's _BCL and _BCM control method instead of the usual _INI
functions of the _SB or PCI host bridge PCI0 devices. This made our
video module thought this is a pre-Win8 system when deciding if we
should register a backlight interface for it and the end result is that
a non-working acpi_video interface is registered and user is unable to
control backlight from GUI. Solve this problem by evaluating _BCL control
method before doing the decision.

Note that for some Thinkpad systems, the _BCL is also required to be
evaluated for the hotkey event to be generated no matter if we will
register an ACPI video backlight interface for it or not. Since the
thinkpad_acpi module will do this anyway we didn't add such a thing in
the video module previously. But now with this change here, the
thinkpad_acpi module is no more necessary for those systems regarding
backlight functionality.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=85051Reported-and-tested-by: Ralf Jung <post+kernel@ralfj.de>
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/video.c

@@ -1680,6 +1680,19 @@ static void acpi_video_dev_register_backlight(struct acpi_video_device *device)
 		printk(KERN_ERR PREFIX "Create sysfs link\n");
 }
 
+static void acpi_video_run_bcl_for_osi(struct acpi_video_bus *video)
+{
+	struct acpi_video_device *dev;
+	union acpi_object *levels;
+
+	mutex_lock(&video->device_list_lock);
+	list_for_each_entry(dev, &video->video_device_list, entry) {
+		if (!acpi_video_device_lcd_query_levels(dev, &levels))
+			kfree(levels);
+	}
+	mutex_unlock(&video->device_list_lock);
+}
+
 static int acpi_video_bus_register_backlight(struct acpi_video_bus *video)
 {
 	struct acpi_video_device *dev;
@@ -1687,6 +1700,8 @@ static int acpi_video_bus_register_backlight(struct acpi_video_bus *video)
 	if (video->backlight_registered)
 		return 0;
 
+	acpi_video_run_bcl_for_osi(video);
+
 	if (!acpi_video_verify_backlight_support())
 		return 0;