[Bug 73156] Dual onboard LCD (eDP1 and DP1) cannot turn on DP1 (no backlight), no backlight control but can see screen in reflected light

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=73156

--- Comment #14 from Jani Nikula <jani.nikula at intel.com> ---
I'm inclined to believe what Chris says, it's controlled through ddc/ci [1].
Some googling suggests some Windows display drivers support this.

The ddccontrol tool [2] can be used to change brightness (among other things)
through the ddc/ci. It hasn't been maintained for years, but seems to work for
me. If you're daring enough, please probe the monitors using:

$ sudo ddccontrol -p -v

and try to read the brightness using:

$ sudo ddccontrol -r 0x10 dev:/dev/i2c-N

replacing N with the relevant monitor from the ddccontrol probe step. Finally,
to change brightness, use:

$ sudo ddccontrol -r 0x10 -w NN dev:/dev/i2c-N

where NN is the value.

It would be kind of cool to expose a connector specific backlight control using
ddc/ci from the kernel, but it's probably not the right thing to do in kernel.
Userspace could handle this transparently if ddc/ci control was incorporated
into the appropriate components. But in any case this falls near the bottom of
the pile in priority.


[1] http://en.wikipedia.org/wiki/Display_Data_Channel
[2] http://ddccontrol.sourceforge.net/

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