linux-kernel - Re: [PATCH] ACPI: Adjust the return value of

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Date:	Mon, 16 Mar 2015 16:21:51 -0700
From:	Jason Ekstrand <jason@...kstrand.net>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Jason Ekstrand <jason@...kstrand.net>
Subject: Re: [PATCH] ACPI: Adjust the return value of _REV on x86

On Sat, Mar 14, 2015 at 12:58 PM, Jason Ekstrand <jason@...kstrand.net> wrote:
> From: Jason Ekstrand <jason@...kstrand.net>
>
> On Wed, 11 Mar 2015 22:50:47, Matthew Garrett <mjg59@...f.ucam.org> wrote:
>> The ACPI spec describes _REV as:
>>
>> "This predefined object evaluates to the revision of the ACPI Specification
>>  that the specified \_OS implements"
>>
>> We've been assuming that this should increment as ACPICA gains support for
>> new versions of the spec. Unfortunately, Windows always reports "2" for this
>> value and vendors are now using this as a means to tell whether a system is
>> running Windows or Linux. From an HP Envy 15:
>>
>> If (LOr (LEqual (_REV, 0x03), LEqual (_REV, 0x05)))
>>
>> From a Dell XPS 13:
>>
>> If ((_REV == 0x05))
>
> I can confirm that on my 2015 Dell XPS 13, this patch fixes both the audio
> and suspend/resume.  I'm running a build of Linus' master branch as of some
> time this morning.  Without this patch applied I have no audio and suspend
> fails leaving the system unresponsive but the keyboard light still on and
> the fan running indefinitely.  With this patch, I get audio and
> suspend/resume works normally.

A quick update on the Dell XPS 13 for those of you who are following
this discussion but aren't aware of the XPS 13-specific discussions.
The "problem" triggered by _REV=5 is not a *real* problem.  The reason
they special-cased it for the XPS 13 is that the sound card is
dual-mode and can run over either HDA and I2S.  Since the I2S support
on Linux isn't great at the moment, they special-cased linux to run it
in HDA mode which has good support.  The problem is that, in the A01
bios update where they changed this, they made a mistake that left the
sound card in an invalid state.  A one-line change to the DSDT table
in the bios puts it into HDA mode properly and fixes both the audio
and suspend/resume issues.  They should be coming out with a bios
update shortly to fix this.

I'm not knowledgeable enough to weigh in on the philosophical issues
here, but I thought it was worth explaining the reason for the linux
special-casing.  In the case of the new XPS 13, Dell was doing
something useful with their special-casing, they just made a mistake.
If we did start advertising _REV=2 this would cause the laptop (with
the fixed bios) to load the sound card in I2S mode and it would be
less reliable.
--Jason Ekstrand
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