Re: [PATCH] acpi-video: Add use native backlight quirk for the ThinkPad W530

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

Hi,

On 05/15/2014 10:56 AM, Hans de Goede wrote:

<snip>

>>> So maybe we should simply drop the backlight_device_registered(raw) check?
>>
>> Unfortunately, there are indeed systems that with Intel GFX do not have
>> a GPU backlight control interface:
>>
>> commit c675949ec58ca50d5a3ae3c757892f1560f6e896
>> Author: Jani Nikula <jani.nikula@intel.com>
>> Date:   Wed Apr 9 11:31:37 2014 +0300
>>
>>     drm/i915: do not setup backlight if not available according to VBT
>>
>> And I remembered last time when we push the use_native default to 1
>> without checking if a raw interface is available, there are people
>> complaining about no backlight interface is created on his system(and
>> the only working interface is acpi_video on his Win8 system). So simply
>> dropping this check doesn't seem like a good idea.
> 
> Hmm, ok. So any smart ideas how to deal with the ordering problem we've
> here ?
> 
> Note this also plays into the proposal I'm about to send to unify and
> simplify backlight control selection. Which besides just trying to
> clean things up also tries to get rid of various module load ordering
> issues.
> 
> ... <this represent me thinking for half an hour trying to come up with a clever solution>
> 
> So I think we really need some clean and generic way to deal with this,
> which is not prone to module loading ordering issues, any suggestions?

Ok, after yet more thinking about this I think I've what is likely going
to be the best solution for this:

1) Add a callback to the backlight core which allows interested parties
to get notified if a backlight device gets registered / unregistered

2) make acpi/video.c listen to these events and on these events re-check
acpi_video_verify_backlight_support, and if it returns something
different then the current situation (un)register the acpi_video#
backlight devices

This means that we will have ping-ponging of backlight interfaces which
I really wanted to avoid, but given all the interdepencies that
seems unavoidable. This will also simplify my cleanup proposal since
if we accept the ping-ponging all the quirks can stay in the vendor
specific firmware backlight control drivers.

So, good or bad idea ?

Regards,

Hans