Re: [PATCH] ACPI: Adjust the return value of _REV on x86

From: Jason Ekstrand
Date: Mon Mar 16 2015 - 19:21:58 EST

On Sat, Mar 14, 2015 at 12:58 PM, Jason Ekstrand <jason@xxxxxxxxxxxxxx> wrote:
> From: Jason Ekstrand <jason@xxxxxxxxxxxxxx>
> On Wed, 11 Mar 2015 22:50:47, Matthew Garrett <mjg59@xxxxxxxxxxxxx> 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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