Strange effect with i915 backlight controller

[Daniel Mack]

Hi,

I'm facing a bug on a Samsung X20 notebook which features an i915 
chipset (output of 'lspci -v' attached).

The effect is that setting the backlight to odd values causes the value 
to be misinterpreted. Harald Hoyer (cc:) had the same thing on a Netbook 
(I don't recall which model it was).

So this will turn the backlight to full brightness:

# cat /sys/class/backlight/intel_backlight/max_brightness
29750
# echo 29750 > /sys/class/backlight/intel_backlight/brightness

However, writing 29749 will turn the display backlight off, and 29748 
appears to be the next valid lower value.

It seems like the IS_PINEVIEW() branch in 
drivers/gpu/drm/i915/intel_panel.c: intel_panel_actually_set_backlight() 
could do the right thing, but this code is written for an entirely 
different model, right?

I can reproduce this on 3.0 and 3.1 vanilla as well as with the current 
mainline git.

Let me know if there is any patch that I can test.


Thanks,
Daniel
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