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

[Jani Nikula]

On Mon, 09 Sep 2013, "Rafael J. Wysocki" <rjw@xxxxxxx> wrote:
> On Monday, September 09, 2013 05:21:18 PM Daniel Vetter wrote:
>> On Mon, Sep 09, 2013 at 02:16:12PM +0200, Rafael J. Wysocki wrote:
>> > Hi,
>> > 
>> > On Monday, September 09, 2013 11:32:10 AM Daniel Vetter wrote:
>> > > Hi Aaaron,
>> > > 
>> > > Have we grown any clue meanwhile about which Intel boxes need this and for
>> > > which we still need to keep the acpi backlight around?
>> > 
>> > First of all, there is a bunch of boxes where ACPI backlight works incorrectly
>> > because of the Win8 compatibility issue.  [In short, if we say we are compatible
>> > with Win8, the backlight AML goes into a special code path that is broken on
>> > those machines. Presumably Win8 uses native backlight control on them and that
>> > AML code path is never executed there.]  The collection of machines with this
>> > problem appears to be kind of random (various models from various vendors), but
>> > I *think* they are machines that originally shipped with Win7 with a Win8
>> > "upgrade" option (which in practice requires the BIOS to be updated anyway).
>> > 
>> > Because of that, last time we tried to switch all of the systems using i915
>> > and telling the BIOS that they are compatible with Win8 over to native backlight
>> > control, but that didn't work for two reasons.  The first reason is that some
>> > user space doesn't know how to use intel_backlight and needs to be taught about
>> > that (so some systems are just not ready for that switch).  The second and more
>> > fundamental reason is that the native backlight control simply doesn't work on
>> > some machines and we don't seem to have any idea why and how to debug this
>> > particular problem (mostly because we don't have enough information and we
>> > don't know what to ask for).
>> > 
>> > > I've grown _very_ reluctant to just adding tons of quirks to our driver for
>> > > the backlight.
>> > >
>> > > Almost all the quirks we have added recently (or that have been proposed
>> > > to be added) are for the backlight. Imo that indicates we get something
>> > > fundamentally wrong ...
>> > 
>> > This thing isn't really a quirk.  It rather is an option for the users of
>> > the systems where ACPI backlight doesn't work to switch over to the native
>> > backlight control using a command line switch.  This way they can at least
>> > *see* if the native backlight control works for them and (hopefully) report
>> > problems if that's not the case.  This gives us a chance to get more
>> > information about what the problem is and possibly to make some progress
>> > without making changes for everyone, reverting those changes when they don't
>> > work etc.
>> > 
>> > An alternative for them is to pass acpi_osi="!Windows 2012" which will probably
>> > make the ACPI backlight work for them again, but this rather is a step back,
>> > because we can't possibly learn anything new from that.
>> 
>> If Win8 is as broken as we are I'm ok with the module option. It just
>> sounded to me like right now we don't know of a way to make all machines
>> somewhat happy, combined with the other pile of random backlight issues
>> the assumption that we do something (maybe something a bit racy) that
>> windows doesn't do isn't too far-fetched. So I'm not wary of the machines
>> where the aml is busted for acpi_os=win8, but for the others where this
>> broke stuff.
>> 
>> Or do I miss something here?
>
> The ACPI video driver doesn't do anything new now.  It only does things that
> did work before we started to tell BIOSes we're compatible with Win8.  The
> reason why they don't work on some machines now is not related to whether or
> not Win8 is broken, but to what is there in the ACPI tables on those machines.
> That *is* pure garbage, but Win8 users don't see that (presumably, because
> Win8 does't execute the AML in question).  We don't see that either with
> acpi_osi="!Windows 2012" (because then we don't execute that AML either).
>
> Whether or not Win8 is broken doesn't matter here.  What matters is that we
> have to deal with broken AML somehow.
>
> One way may be to tell everyone affected by this to pass
> acpi_osi="!Windows 2012" in the kernel command line or possibly create a
> blacklist of those machines in the kernel (which Felipe has been pushing for
> recently) and wash our hands clean of this, but that leaves some exceptionally
> bad taste in my mouth.
>
> The alternative is to try to use native backlight in i915, which we did, but
> that didn't work on some machines.  I don't think we will know why it didn't
> work there before we collect some more information and that's not possible
> without user testing.  So, the idea is to make that testing possible without
> hacking the kernel and that's why we're introducing the new command line
> switch.  And we're going to ask users to try it and report back.

The thing that slightly bugs me with the proposed patches is that
they're adding a module parameter to i915 to tell ACPI video driver
whether to quirk the backlight or not. Before you know, we *will* have
requests to add quirks to i915 to tell ACPI video driver this.

I think the parameter "Does the ACPI backlight interface work or not"
belongs to the ACPI video driver.

Feel free to file this in your bikeshedding folder, but I think i915
should only tell ACPI "I have a native backlight interface". What ACPI
video driver does with that information is its business. The desired
thing to do here would be to check if there's a module parameter or a
quirk to disable the ACPI backlight interface. acpi_backlight=DTRT? ;)

BR,
Jani.


-- 
Jani Nikula, Intel Open Source Technology Center
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