[Intel-gfx] [REGRESSION BISECTED] backlight control stops workin with 3.14 and later

[Bertrik Sikken]

>> On Tue, Jul 22, 2014 at 8:42 AM, Hans de Goede <hdegoede at redhat.com>
> Bertrik, can you try blacklisting the samsung-laptop module, then
> check /sys/class/backlight, the samsung_laptop dir should be gone
> replaced by an acpi_video0 (or some such) dir. Please try if that
> works. If that does not work, try booting with both the module
> blacklisted and "acpi_backlight=vendor" on the kernel commandline,
> then you should see only the intel-backlight under /sys/class/backlight
> and things should work.

With linux kernel v3.14 and the samsung_laptop module blacklisted,
I indeed see a /sys/class/backlight/acpi_video0, but changing brightness
by writing (as root) to the brightness file does not have an effect.

Additionally, with the acpi_backlight=vendor kernel command line option,
there is only an intel_backlight directory in /sys/class/backlight.
Brightness does work using "brightness up and down" function keys,
with 'udevadm monitor' I can see it uses the intel_backlight:

bertrik at netbook:~$ udevadm monitor
monitor will print the received events for:
UDEV - the event which udev sends out after rule processing
KERNEL - the kernel uevent

KERNEL[40.375921] change  
/devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight
(backlight)
UDEV  [40.503704] change  
/devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight
(backlight)

With kind regards,
Bertrik Sikken