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

[Aaron Lu <aaron.lu@intel.com>]

On Wed, Sep 11, 2013 at 11:45:19AM +0300, Jani Nikula wrote:
> On Wed, 11 Sep 2013, Aaron Lu <aaron.lu@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

Not yet from ACPI's point of view.

> 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.

This check in patch 2/2 is a policy: for Win8 system, we think the
native backlight control has a better chance of working than the ACPI
video's, so I think the check is enough in ACPI video.

> 
> 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?

I suppose the 'tested BIOS code paths' means the pure UEFI boot mode? I
don't know what guatantees do we have since I don't know what happened
underneath after the backlight register is set in i915 driver, you or
other i915 driver people should know more than I do :-)

BTW, after the backlight register is set in i915, is it that some
find of firmware code will run in response to the setting of the
register(e.g. the BLC_PWM_CTL/BLC_PWM_CPU_CTL/PCI_LBPC reg)?

Thanks,
Aaron