[PATCH 2/2] ACPI / video / i915: Remove ACPI backlight if firmware expects Windows 8

[Jani Nikula]

On Wed, 11 Sep 2013, Aaron Lu <aaron.lu at intel.com> wrote:
> It is possible the i915 driver decides not to register a backlight
> interface for the graphics card for some reason(memory allocation failed
> or it knows the native control does not work on this card or whatever),
> so I would prefer let i915 tell ACPI video that it has registered a
> native backlight control interface as Jani has said.
>
> Then together with the video.use_native_backlight, we can register or
> not register ACPI video backlight interface accordingly. Or rather, we
> can simply not register ACPI video backlight interface for Win8 systems
> as long as i915 indicates that it has native backlight control(if the
> native control is broken, i915 should fix it or blacklist it so that
> i915 will not indicate it has native backlight control and ACPI video
> will continue to register its own).
>
> How does this sound?

Sounds good to me.

Before plunging forward, have you observed any difference between the
boot modes? We have reports [1] that the backlight behaviour is
different with UEFI vs. UEFI+CSM or legacy boot. So I'm wondering if the
acpi_gbl_osi_data >= ACPI_OSI_WIN_8 check in patch 2/2 is the whole
story.

Further, if we tell the BIOS we're Windows 8 to use the tested BIOS code
paths, what guarantees do we have of UEFI+CSM or legacy boots working?

BR,
Jani.


[1] https://bugzilla.kernel.org/show_bug.cgi?id=47941#c96


-- 
Jani Nikula, Intel Open Source Technology Center