[Bug 47941] [snb] Backlight control broken between 3.6.0-rc1 and 3.6.0-rc4

[bugzilla-daemon at bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=47941

Evan Callicoat <apsu at propter.net> changed:

           What    |Removed                     |Added
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                 CC|                            |apsu at propter.net

--- Comment #96 from Evan Callicoat <apsu at propter.net> ---
The attached patch ( https://bugzilla.kernel.org/attachment.cgi?id=97751 )
doesn't exactly match the actual patch that was applied in 3.10.5. The actual
patch splits the model matches in the intel_quirks struct into two model
entries each with their own comment, and the comment texts are more detailed,
so attempting to revert from the attachment doesn't work.

That said, this patch absolutely breaks the backlight entirely on my Dell XPS
13 Developer Edition FHD (IvyBridge, with the latest BIOS revision A09). I
believe Kamal did not test the IVB he received thoroughly enough, which is not
surprising since there's some very counter-intuitive behavior based on the BIOS
settings and whether you're booting in UEFI or BIOS mode.

There are three basic scenarios:

1) Booting in Legacy (BIOS) mode
2) Booting in UEFI mode with the Legacy Option Rom
3) Booting in UEFI mode without the Legacy Option ROM

The first two didn't have backlight issues before the patch and don't have any
after the patch. They operate the GPU differently somehow. Booting with
drm.debug=0xe I haven't been able to clearly identify the exact differences
yet.

However, the third configuration (which I use) works great without the patch,
and the backlight is completely inoperable with the patch. I am well-aware of
ACPI OSI strings and have tried various combinations and omitting them. I am
also aware of /sys/class/backlight/{acpi_video0,intel_backlight}, and
acpi_backlight=legacy/vendor/[omitted].

No combination of these things will make the backlight work on this specific
machine booted in mode 3 with this patch.

Please revisit this!

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