Re: [PATCH] ACPI : Disable the C2C3_FFH access mode if MWAIT is unsupported

[Shaohua Li <shaohua.li@intel.com>]

On Thu, 2008-06-05 at 13:35 +0800, Len Brown wrote:
> 
> 
> On Thu, 5 Jun 2008, Zhao Yakui wrote:
> 
> > Subject: ACPI: Disable the C2C3_FFH access mode if MWAIT is
> unsupported
> > From: Zhao Yakui <yakui.zhao@intel.com>
> >
> >    After the following commit is merged, C2C3_FFH mode will be
> enabled and
> > passed into the _PDC object. But because MWAIT is unsupported on
> some laptops,
> > the different power states will be returned by the _CST object.
> > Maybe it will be better to disable C2C3_FFH access mode if MWAIT is
> unsupported.
> > 
> > commit 991528d7348667924176f3e29addea0675298944
> > Author: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
> > Date:   Mon Sep 25 16:28:13 2006 -0700
> >
> >     ACPI: Processor native C-states using MWAIT
> 
> > http://bugzilla.kernel.org/show_bug.cgi?id=10807
> 
> Interesting bug report.
> Unfortunately, it is a BIOS bug that the BIOS exposes C3
> when the OS doesn't support MWAIT, but neglects to expose
> C3 when the OS does support MWAIT.
> 
> This patch will have no effect on the system in the bug report.
> Indeed, it would benefit only a system with a BIOS that thinks
> it has MWAIT support but a CPU that thinks it doesn't...
> 
> probably good hygine to do this, but I don't think it will
> help anything.
> 
> I think the fix for the machine in bug 10807 is to
> have a bootparam that disables mwait support,
> say "idle=!mwait" and a DMI entry for that box
> that invokes it to work around the BIOS bug.
The patch isn't related to the bug, but we got the idea when debugging
the bug. Yakui should write a new description which doesn't mention the
bugzilla. I thought we need this patch to avoid buggy BIOS.

Thanks,
Shaohua