Support for all devices under `/sys/class/backlight`

[bugabinga]

If I understand the sysfs backlight API correctly, the various drivers expose their functionality under /sys/class/backlight/<device_name>/ {brightness,max_brightness}.

There are more files, but only brightness and max_brightness are relevant for kira.

kira should enumerate all devices into a vector.
If a human specifies a device, use that one, otherwise pick the first in the vector.
Devices can be specified by giving their name after the brightness value.

$ kira 44 <device_name>

The device name is fuzzily matched.
No match or an ambiguous match results in an error.

Assuming a machine with 3 backlight devices:

1. /sys/class/backlight/intel_backlight/
2. /sys/class/backlight/acpi_backlight/
3. /sys/class/backlight/acpi_super_backlight/

kira 33 intel uses 1.
kira 33 acpi is an error.
kira 33 acpi_b uses 2.
kira 33 ab uses 2.
kira 33 abs uses 3.

Questions

• Are the value ranges (min,max) consistent between drivers?
• What does it mean when there are multiple devices under /sys/class/backlight?
• Why have I never seen this functionality on desktops? Laptops only?