ACPI / video: change acpi-video brightness_switch_enabled default to 0

acpi-video is unique in that it not only generates brightness up/down
keypresses, but also (sometimes) actively changes the brightness itself.

This presents an inconsistent kernel interface to userspace, basically there
are 2 different scenarios, depending on the laptop model:

 1) On some laptops a brightness up/down keypress means: show a brightness osd
 with the current brightness, iow it is a brightness has changed notification.

 2) Where as on (a lot of) other laptops it means a brightness up/down key was
 pressed, deal with it.

Most of the desktop environments interpret any press as in scenario 2, and
change the brightness up / down as a response to the key events, causing it
to be changed twice, once by acpi-video and once by the DE.

With the new default for video.use_native_backlight we will be moving even
more laptops over to behaving as in scenario 2. Making the remaining laptops
even more of a weird exception. Also note that it is hard to detect scenario
1 properly in userspace, and AFAIK none of the DE-s deals with it.

Therefor this commit changes the default of brightness_switch_enabled to 0
making its behavior consistent with all the other backlight drivers.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/kernel-parameters.txt

@@ -3461,7 +3461,7 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			the allocated input device; If set to 0, video driver
 			will only send out the event without touching backlight
 			brightness level.
-			default: 1
+			default: 0
 
 	virtio_mmio.device=
 			[VMMIO] Memory mapped virtio (platform) device.

drivers/acpi/video.c

@@ -68,7 +68,7 @@ MODULE_AUTHOR("Bruno Ducrot");
 MODULE_DESCRIPTION("ACPI Video Driver");
 MODULE_LICENSE("GPL");
 
-static bool brightness_switch_enabled = 1;
+static bool brightness_switch_enabled;
 module_param(brightness_switch_enabled, bool, 0644);
 
 /*