Re: [PATCH] acpi-video: Add disable_native_backlight quirk for Dell XPS15 L521X

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 12-01-15 03:44, Matthew Garrett wrote:
> On Sun, Jan 11, 2015 at 11:37:52AM +0100, Hans de Goede wrote:
>
>> Userspace is writing to the intel_backlight device because it is the
>> only one present under /sys/class/backlight, as we disable acpi-video
>> on win8 ready laptops by default now.
>
> Why is nouveau not providing a backlight device?

I do not know.

>> The commit message may not be 100% accurate, although I believe this is
>> related to the laptop having nvidia graphics, but the code / fix is correct
>> regardless.
>
> I don't think we have enough evidence to know whether this fix is
> correct.

This fix fixes a regression introduced from kernel 3.15 to 3.16, when we
switched video.use_native_brightness's default from 0 to 1, all this patch
does is restore the previous behaviour on this specific model as the new
behaviour is broken.

So this is in essence a model targeted "revert" fixing a regression and as
such most certainly is correct, given the clear no regressions policy the
kernel has.

Might it be good to dive deeper as to why the intel_backlight interface
does not work, yes certainly.

Regards,

Hans