Strange effect with i915 backlight controller

[Daniel Mack]

Didn't get any response yet, hence copying LKML for a broader audience.


On 11/04/2011 03:36 PM, Daniel Mack wrote:
> 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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