RFC: changing acpi-video brightness_switch_enabled default to 0

[Hans de Goede <hdegoede@redhat.com>]

Hi All,

I was already considering asking for $subject for a while, but I did not see
any bug reports caused by it so I didn't ask. Until today I discovered that
I'm not seeing any bugs because Fedora is carrying a kernel patch
changing the default.

A quick google search for brightness_switch_enabled OTOH reveals that for
other distros it is a serious problem, see ie:
https://bugs.launchpad.net/gnome-settings-daemon/+bug/527157
http://askubuntu.com/questions/173921/why-does-my-thinkpad-brightness-control-skip-steps

The problem is that 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 deasls with it.

Therefor I would like to propose to change the brightness_switch_enabled
default to 0.

Regards,

Hans