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<b><a class="bz_bug_link
bz_status_NEEDINFO "
title="NEEDINFO --- - Dual onboard LCD (eDP1 and DP1) cannot turn on DP1 (no backlight), no backlight control but can see screen in reflected light"
href="https://bugs.freedesktop.org/show_bug.cgi?id=73156#c14">Comment # 14</a>
on <a class="bz_bug_link
bz_status_NEEDINFO "
title="NEEDINFO --- - Dual onboard LCD (eDP1 and DP1) cannot turn on DP1 (no backlight), no backlight control but can see screen in reflected light"
href="https://bugs.freedesktop.org/show_bug.cgi?id=73156">bug 73156</a>
from <span class="vcard"><a class="email" href="mailto:jani.nikula@intel.com" title="Jani Nikula <jani.nikula@intel.com>"> <span class="fn">Jani Nikula</span></a>
</span></b>
<pre>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] <a href="http://en.wikipedia.org/wiki/Display_Data_Channel">http://en.wikipedia.org/wiki/Display_Data_Channel</a>
[2] <a href="http://ddccontrol.sourceforge.net/">http://ddccontrol.sourceforge.net/</a></pre>
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