Hans de Goede - Backlight kernel ABI discussion

Recently 3 issues with the kernel ABI for backlights have come up:

1) It would be nice to have the backlight level as a property of the connector. Problem is mapping a backlight interface to a connector. There are some ideas to let userspace tell the kernel which backlight interface to use (through e.g. udev rules), but otherwise handle this in the kernel

2) What is the meaning of a brightness level of "0" (or min, but troublesome cases all seem to actually have 0 as min), does this mean backlight at minimum or backlight off ? Currently backlight interfaces are not consistent in this. We need to clarify the ABI here (and then fix drivers which don't match the clarified API)

3) Related to 2, for some backlight interfaces the value has a scale of perceived brightness, where for others it controls the raw pwm value (which does not map linearly to perceived brightness)

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