Add backlight subsystem

[manu]

Summary

This is a simple subsystem that allow drivers to register as a backlight.
Each backlight creates a device node under /dev/backlight/backlightX and an alias based on the name provided.

[manu]

Add changes that were added in D26253 by error.

share/man/man9/backlight.9

26

Meh, date isn't good, I'll change it when I commit this anyway :P

[manu]

Add backlight.9 here.

[val_packett.cool]

share/man/man9/backlight.9

49

It could work for not just panels, but also keyboards (e.g. D21746)..

That would work much better if we expose metadata, so userspace could tell which of the backlightN is a display and which is a keyboard.

sys/sys/backlight.h

39

hmm I don't see where that levels[BACKLIGHTMAXLEVELS] field is used.. or how it would be used

[manu]

share/man/man9/backlight.9

49

I'm not sure about the metadata yet but for a case like that there is the name used for the dev alias, so you would have a /dev/backlight/keyboard0 or something like that.

sys/sys/backlight.h

39

See https://reviews.freebsd.org/D26252
I didn't put this under #ifdef FDT but maybe I should.

[val_packett.cool]

share/man/man9/backlight.9

49

Aliases are only okay for manual ad-hoc configurations. I'd like to eventually add support for this in the UPower daemon, which would have to enumerate the devs like

for (file in /dev/backlight):
  metadata = ioctl(give_me_meta, file)
  if (metadata & IS_DISPLAY):
    new Display(file)
  if (metadata & IS_KEYBOARD):
    new Keyboard(file)

In general anything exposed by the kernel should be enumerable and inspectable, writing userspace software is really frustrating when the kernel isn't being helpful.

[manu]

Add GETINFO ioctl to get the backlight name and type.

[0mp]

share/man/man9/backlight.9

67

You can drop .Pa from here.