linux-kernel - Re: [PATCH 1/3] Powerpc: mpc85xx: refactor the PM operations

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Date:	Thu, 6 Aug 2015 13:02:52 -0500
From:	Scott Wood <scottwood@...escale.com>
To:	Chenhui Zhao <chenhui.zhao@...escale.com>
CC:	<b29983@...escale.com>, <b07421@...escale.com>,
	<linux-kernel@...r.kernel.org>,
	Tang Yuantian <Yuantian.Tang@...escale.com>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH 1/3] Powerpc: mpc85xx: refactor the PM operations

On Thu, 2015-08-06 at 13:54 +0800, Chenhui Zhao wrote:
> On Thu, Aug 6, 2015 at 1:46 PM, Scott Wood <scottwood@...escale.com> 
> wrote:
> > On Thu, 2015-08-06 at 12:20 +0800, Chenhui Zhao wrote:
> > >  On Thu, Aug 6, 2015 at 10:57 AM, Scott Wood 
> > > <scottwood@...escale.com>
> > >  wrote:
> > >  > On Wed, 2015-08-05 at 18:11 +0800, Chenhui Zhao wrote:
> > >  > >  On Tue, Aug 4, 2015 at 4:26 AM, Scott Wood 
> > > <scottwood@...escale.com>
> > >  > >  wrote:
> > >  > >  > On Mon, 2015-08-03 at 19:32 +0800, Chenhui Zhao wrote:
> > >  > >  > >  >
> > >  > >  >
> > >  > >  > >  On Sat, Aug 1, 2015 at 7:59 AM, Scott Wood
> > >  > > <scottwood@...escale.com>
> > >  > >  > >  wrote:
> > >  > >  >
> > >  > >  > >  >
> > >  > >  > >  > Could you explain irq_mask()?  Why would there still be 
> > > IRQs
> > >  > >  > > destined
> > >  > >  > >  > for
> > >  > >  > >  > this CPU at this point?
> > >  > >  > >
> > >  > >  > >  This function just masks irq by setting the registers in 
> > > RCPM
> > >  > > (for
> > >  > >  > >  example, RCPM_CPMIMR, RCPM_CPMCIMR). Actually, all irqs to
> > >  > > this CPU
> > >  > >  > >  have been migrated to other CPUs.
> > >  > >  >
> > >  > >  > So why do we need to set those bits in RCPM?  Is it just 
> > > caution?
> > >  > >
> > >  > >  Setting these bits can mask interrupts signalled to RCPM from 
> > > MPIC
> > >  > > as a
> > >  > >  means of
> > >  > >  waking up from a lower power state. So, cores will not be 
> > > waked up
> > >  > >  unexpectedly.
> > >  >
> > >  > Why would the MPIC be signalling those interrupts if they've been
> > >  > masked at
> > >  > the MPIC?
> > >  >
> > >  > -Scott
> > >  >
> > > 
> > >  The interrupts to RCPM from MPIC are IRQ, Machine Check, NMI and
> > >  Critical interrupts. Some of them didn't be masked in MPIC.
> > 
> > What interrupt could actually happen to a sleeping cpu that this 
> > protects
> > against?
> > 
> > -Scott
> 
> Not sure. Maybe spurious interrupts or hardware exceptions.

Spurious interrupts happen due to race conditions.  They don't happen because 
the MPIC is bored and decides to ring a CPU's doorbell and hide in the bushes.

If by "hardware exceptions" you mean machine checks, how would such a machine 
check be generated by a core that is off?

>  However, setting them make sure dead cpus can not be waked up unexpectedly.

I'm not seeing enough value here to warrant resurrecting the old sleep node 
stuff.

-Scott

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