Linux backlight control

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Backlight control is one of those things that you'd think would be simple, but ha ha this is computing so of course it's an utter disaster and everything is a huge mess. There's three main classes of backlight control in the x86 world, all of which have drawbacks:

* ACPI specifies a mechanism for backlight control, and the majority of modern machines implement it. It has the advantage that the brightness query interface is generally aware of anything else in the system which may have changed the brightness, so it's unlikely to get out of sync with reality if the platform tries to do something odd like change the backlight itself in response to an ambient light sensor or some other event. The main drawback is that there's typically a fairly small number of available backlight values, usually somewhere between 8 and 20.

* A platform-specific mechanism.

rest here