Re: how to find out number of brightness levels supported by any given backlit keyboard via code?

[Bastien Nocera <hadess hadess net>]

On Sat, 2017-02-25 at 16:03 -0700, Gabriel M. Elder wrote:
> Hi all,
>
> A question for all the genius devs out there who are familiar with
> the internal guts and plumbing of the gnome architecture and power
> subsystem(s):
>
> What is the preferred and/or most expedient way to find out, via
> code, how many brightness state levels any given keyboard with
> backlighting capabilities will support? Just for clarity, for
> instance, I'm typing on a usb keyboard that has four brightness state
> levels: 3 (100%), 2 (66.6%), 1 (33.3%), and 0 (off, emitting no
> backlight). I'm looking to either utilize or create a way to
> automatically discover or retrieve that information, in code.
>
> I'm not necessarily looking for example code, per se. I just need to
> know what modules to look at, function calls that could be made,
> and/or objects that are responsible for maintaining this information
> (if extant) that could or should be queried. Ideally, some function
> call that returns an integer. Does something that simple or something
> like it exist?
>
> I know there's a keyboard proxy object from somewhere that gets used
> in cc-power-panel.c, but I'm not very familiar with it, and perhaps I
> should be looking elsewhere. If someone could point me in the right
> direction, it would be a great time-saver.

The code is ultimately in UPower (src/up-kbd-backlight.c) and in the
kernel. /sys/class/leds/*kbd_backlight*/max_brightness is what you're
looking for.

Cheers