ACPI / video: Default lcd_only to true on Win8-ready and newer machines

We're seeing a lot of bogus backlight interfaces on newer machines without
a LCD such as desktops, servers and HDMI sticks. This causes userspace to
show a non-functional brightness slider in e.g. the GNOME3 system menu,
which is undesirable. And, in general, we should simply just not register
a non functional backlight interface.

Checking the LCD flag causes the bogus acpi_video backlight interfaces to
go away (on the machines this was tested on).

This change sets the lcd_only option by default on any machines which
are Win8-ready, to fix this.

This is not entirely without a risk of regressions, but video_detect.c
already prefers native-backlight interfaces over the acpi_video one
on Win8-ready machines, calling acpi_video_unregister_backlight() as soon
as a native interface shows up. This is done because the ACPI backlight
interface often is broken on Win8-ready machines, because win8 does not
seem to actually use it.

So in practice we already end up not registering the ACPI backlight
interface on (most) Win8-ready machines with a LCD panel, thus this
change does not change anything for (most) machines with a LCD panel
and on machines without a LCD panel we actually don't want to register
any backlight interfaces.

This has been tested on the following machines and fixes a bogus backlight
interface showing up there:
 - Desktop with an Asrock B150M Pro4S/D3 m.b. using i5-6500 builtin gfx
 - Intel Compute Stick STK1AW32SC
 - Meegopad T08 HDMI stick

Bogus backlight interfaces have also been reported on:
 - Desktop with Asus H87I-Plus m.b.
 - Desktop with ASRock B75M-ITX m.b.
 - Desktop with Gigabyte Z87-D3HP m.b.
 - Dell PowerEdge T20 desktop

Link: https://bugzilla.redhat.com/show_bug.cgi?id=1097436
Link: https://bugzilla.redhat.com/show_bug.cgi?id=1133327
Link: https://bugzilla.redhat.com/show_bug.cgi?id=1133329
Link: https://bugzilla.redhat.com/show_bug.cgi?id=1133646
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/acpi_video.c

@@ -80,8 +80,8 @@ MODULE_PARM_DESC(report_key_events,
 static bool device_id_scheme = false;
 module_param(device_id_scheme, bool, 0444);
 
-static bool only_lcd = false;
-module_param(only_lcd, bool, 0444);
+static int only_lcd = -1;
+module_param(only_lcd, int, 0444);
 
 static int register_count;
 static DEFINE_MUTEX(register_count_mutex);
@@ -2136,6 +2136,16 @@ int acpi_video_register(void)
 		goto leave;
 	}
 
+	/*
+	 * We're seeing a lot of bogus backlight interfaces on newer machines
+	 * without a LCD such as desktops, servers and HDMI sticks. Checking
+	 * the lcd flag fixes this, so enable this on any machines which are
+	 * win8 ready (where we also prefer the native backlight driver, so
+	 * normally the acpi_video code should not register there anyways).
+	 */
+	if (only_lcd == -1)
+		only_lcd = acpi_osi_is_win8();
+
 	dmi_check_system(video_dmi_table);
 
 	ret = acpi_bus_register_driver(&acpi_video_bus);