SRU update for acpi backlight problems

[Stefan Bader]

Stefan Bader wrote:
> https://bugs.launchpad.net/ubuntu/+source/linux/+bug/311716
> 
> SRU justification:
> 

I forgot, this is for the Intrepid update, but Jaunty code is in that state and 
we would have to revert the latest change on both which tries to verify some 
ACPI elements to decide whether to activate acpi generic support or not.

> Impact: Some laptops had issues with the current backlight control. This 
> is because up to then acpi and vendor specific control methods were 
> able/allowed to access the video hardware 
> (https://bugs.launchpad.net/bugs/257827). The fix for this (taken from 
> upstream) fixes this but causes regressions for others. So for stable it 
> should probably be taken back but this would cause an ABI bump.
> 
> Fix: I changed the detection code in a (slightly ugly to keep diff 
> minimal) way to make the old behaviour the default (generic _and_ vendor 
> control at the same time) but leave the infrastructure (no ABI bump) 
> which gives the additional ability for those that have problems with 
> that to force either generic (video) or vendor specific (vendor) by 
> using the module option acpi_backlight.
> 
> Testcase: Boot the kernel and try to adjust backlight levels with 
> FN-keys and/or the gnome backlight applet.
> 
> Note: going over the original acpi patches, it looks like some vendor 
> drivers might have been fixed wrong (eg. sony-laptop.c which bails out 
> on !acpi_backlight_suppoert(). But that is the case when acpi is _not_ 
> active).
> 


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