linux-kernel - RE: acpi_idle: Very idle Core i7 machine never enters C3

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Message-ID: <CE761E84DADF2947A4AF22FB8D97A4731E0366B5@shsmsx501.ccr.corp.intel.com>
Date:	Tue, 25 May 2010 13:59:39 +0800
From:	"Yu, Luming" <luming.yu@...el.com>
To:	Len Brown <lenb@...nel.org>, Philip Langdale <philipl@...rt.org>
CC:	Matthew Garrett <mjg59@...f.ucam.org>,
	Jeff Garrett <jeff@...rrett.org>,
	Andi Kleen <andi@...stfloor.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"venki@...gle.com" <venki@...gle.com>
Subject: RE: acpi_idle: Very idle Core i7 machine never enters C3



> -----Original Message-----
> From: Len Brown [mailto:lenb@...nel.org]
> Sent: Tuesday, May 25, 2010 1:43 PM
> To: Philip Langdale
> Cc: Matthew Garrett; Jeff Garrett; Andi Kleen; Linux Kernel Mailing
> List; linux-acpi@...r.kernel.org; Yu, Luming; venki@...gle.com
> Subject: Re: acpi_idle: Very idle Core i7 machine never enters C3
> 
> > >> I am hopeful that the "right thing to do" is to not look at bm-
> status
> > >> and that perhaps there is a bug where we are looking at it
> > >> "by mistake".
> > >
> > > https://patchwork.kernel.org/patch/58962/ - it seems to be a win.
> >
> > Indeed. This patch does solve the C6 problem. I'm not in a position
> to
> > speak about whether there's any undesirable I/O latency, but it
> > passes the basic sanity check.
> >
> > I have filed https://bugzilla.kernel.org/show_bug.cgi?id=15886 with
> > my acpi dump - assuming that's still useful.
> 
> Luming's patch above basically deletes acpi_idle_bm_check() --
> the BM_STS check -- from the C3 path on all Intel SMP boxes.
> This is effectively the same as my test patch
> https://patchwork.kernel.org/patch/77370/
> that made acpi_idle_bm_check() do nothing.
> 
> I'm told by the hardware guys that BM_STS is _not_ always
> a NOP, and so we're not supposed to simply ignore it on C3 --
> though it should be extremely rare that we see it set.

On some platforms like NHM-EX, I was told that it's a NOP,
But I might be given wrong information at that time when I wrote that patch.

IIRC, acpi spec just say it's optional..

Thanks,
Luming
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