Bug 63931

Created attachment 78487 [details]
vbios.dump

Your backlight is controlled through the acpi driver, not intel.

(In reply to comment #2)
> Your backlight is controlled through the acpi driver, not intel.
But Ajax actually said that it was a bug in the intel-gfx driver, not that it was
fixed.
Please, watch comment.. https://bugzilla.gnome.org/show_bug.cgi?id=698271#c28

It is not a bug in -intel or i915.ko, but in the acpi driver for your backlight.

(In reply to comment #4)
> It is not a bug in -intel or i915.ko, but in the acpi driver for your
> backlight.

The backlight works fine when you hit the sysfs node directly, but the backlight returned through XRandR (and xbacklight) always returns 100%. It's possible that the driver always exposes the backlight interface, and returns 100 when it can't actually access the sysfs node for whatever reason as per the commit below.

http://cgit.freedesktop.org/xorg/driver/xf86-video-intel/commit/?id=d7f0df27edb20b052ad39beb26a0b1924f432618

No.

(In reply to comment #6)
> No.

No what?

xbacklight always returns 100% when the sysfs value underneath changes. How is this a bug in the kernel? Isn't the XRandR interface and its backlight property exposed by the driver?

I'd expect a little more explanation than a one-syllable answer.

(In reply to comment #4)
> It is not a bug in -intel or i915.ko, but in the acpi driver for your
> backlight.

Yes the backlight is controller by acpi video driver, but video driver works correctly. The breaking part is xbacklight always return 100 while actual brightness level is not. I wonder how report information to xbacklight?

(In reply to comment #8)
> (In reply to comment #4)
> > It is not a bug in -intel or i915.ko, but in the acpi driver for your
> > backlight.
> 
> Yes the backlight is controller by acpi video driver, but video driver works
> correctly. The breaking part is xbacklight always return 100 while actual
> brightness level is not. I wonder how report information to xbacklight?

I mean, who report information to xbacklight?

(In reply to comment #9)
> (In reply to comment #8)
> > (In reply to comment #4)
> > > It is not a bug in -intel or i915.ko, but in the acpi driver for your
> > > backlight.
> > 
> > Yes the backlight is controller by acpi video driver, but video driver works
> > correctly. The breaking part is xbacklight always return 100 while actual
> > brightness level is not. I wonder how report information to xbacklight?
> 
> I mean, who report information to xbacklight?

The driver, through the backlight property of XRandR. So if the backlight reporting is broken in the driver's XRandR implementation, but works using the kernel directly, the driver has a problem.

Created attachment 78560 [details]
Logfile from test xrandr

I wrote little script for test xrandr.
#!/bin/bash
LOGFILE=test_xrandr.log
echo -n > $LOGFILE
for b in {1..100}
do
  xrandr --output LVDS1 --set Backlight $b
  echo "Test $b value:" >> $LOGFILE
  xrandr --verbose | grep -i backlight  >> $LOGFILE
done

I tested again..
# echo 95 > /sys/class/backlight/acpi_video0/brightness
But in real life brightness of my monitor not change..
$ cat /sys/class/backlight/acpi_video0/brightness
95
$ xbacklight
100
$ cat /sys/class/backlight/acpi_video0/brightness
100
This happens with all the values ​​that are specified in the above attachment.

Created attachment 78573 [details]
test xrandr from lenovo z580 (hd4000)

Created attachment 78574 [details]
test xrandr from acer extensa 5220 (gma x3100)

In lenovo z580 and acer 5220 backlight regulate correctly.
Bug in ACPI ?

kernel bug:
https://bugzilla.kernel.org/show_bug.cgi?id=51231