Intel i915 backlight

[Xīcò]

Dear freebsd-x11,

I am running /stable/10 (with i915kms and newcons) on a Vaio VPCZ21
with a SandyBridge.  Unfortunately backlight is not working out of the
box.

I played a bit with the existing acpi modules (including acpi_sony and
acpi_video), and wrote a bit of code to modify the backlight level,
through sony acpi, but it does not seem quite effective. Basically, the
(sony) acpi reports 16 backlight levels, iirc, but modifying them does
not influence the actual brightness.

On linux, backlight level is controlled through the graphic card. So,
basically I am investigating how to do so on FreeBSD. I am quite unsure
of how it works, probably xbacklight relies on xf86-video-intel. There
is some code in sys/dev/drm2/i915/intel_panel.c which looks appropriate
to handle the backlight, but I fail to understand how it is called by
xorg, for instance.

Well, I assume someone here will have a better understanding of all
that, and possible on how the intel backlight works, and whether it is
only a pwm. For now, I am relying on software xrandr --brightness to set
an adequate luminosity, before further investigating the issue.

Best,

--
Xīcò