acpi_backlight=vendor no longer working around buggy laptop from hell

["Cristian Rodríguez" <crrodriguez@opensuse.org>]

Hi:

I have a DELL XPS 15z laptop with the latest BIOS (A12).

This piece of crap have never worked properly without special boot 
parameters. Up to kernel 3.10, passing acpi_backlight=vendor was enough 
to allow the machine to complete boot and be usable.

Now with 3.11 rc2 (also with today's linus tree) unbootable machine 
problem is back.

In the past I spent many hours figuring out workarounds and they go 
_now_ as the following.

Booting with:

"nox2apic" --> machine works, flawless.

"x2apic_phys acpi_backlight=vendor" --> machine works, except for the 
big problem that the laptop keyboard and touchpad are unresponsive, I 
have to plug USB keyboard/mouse to get anything done.

With no boot parameters the machine hangs at random points in the boot 
sequence either:

-Very early at boot and there is no way to retrieve any useful debug 
info. [OR]

- Somehow it looses "connection" with the ATA controller and the 
filesystems cannot be mounted. [OR]

- In very few ocassions, the boot proceeds with no keyboard or touchpad 
then the iwlwifi wireless card does something stupid (firmware error...)

The only possible hint in the logs I could ever find is:

[    3.844586] [Firmware Bug]: ACPI(PEGP) defines _DOD but not _DOS


I tried in the past to obtain any debug information, with no luck.

Help is appreciated to get to the bottom of the problem. ;)