linux-kernel - Re: [PATCH v5 1/5] powerpc/85xx: implement hardware timebase sync

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Date:	Wed, 6 Jun 2012 17:31:42 +0800
From:	Zhao Chenhui <chenhui.zhao@...escale.com>
To:	Scott Wood <scottwood@...escale.com>
CC:	<linuxppc-dev@...ts.ozlabs.org>, <linux-kernel@...r.kernel.org>,
	<galak@...nel.crashing.org>, <leoli@...escale.com>,
	Matthew McClintock <msm@...escale.com>
Subject: Re: [PATCH v5 1/5] powerpc/85xx: implement hardware timebase sync

On Tue, Jun 05, 2012 at 11:07:41AM -0500, Scott Wood wrote:
> On 06/05/2012 04:08 AM, Zhao Chenhui wrote:
> > On Fri, Jun 01, 2012 at 10:40:00AM -0500, Scott Wood wrote:
> >> I know you say this is for dual-core chips only, but it would be nice if
> >> you'd write this in a way that doesn't assume that (even if the
> >> corenet-specific timebase freezing comes later).
> > 
> > At this point, I have not thought about how to implement the cornet-specific timebase freezing.
> 
> I wasn't asking you to.  I was asking you to not have logic that breaks
> with more than 2 CPUs.

These routines only called in the dual-core case. 

> 
> >> Do we need an isync after setting the timebase, to ensure it's happened
> >> before we enable the timebase?  Likewise, do we need a readback after
> >> disabling the timebase to ensure it's disabled before we read the
> >> timebase in give_timebase?
> > 
> > I checked the e500 core manual (Chapter 2.16 Synchronization Requirements for SPRs).
> > Only some SPR registers need an isync. The timebase registers do not.
> 
> I don't trust that, and the consequences of having the sync be imperfect
> are too unpleasant to chance it.
> 
> > I did a readback in mpc85xx_timebase_freeze().
> 
> Sorry, missed that somehow.
> 
> >>> +#ifdef CONFIG_KEXEC
> >>> +	np = of_find_matching_node(NULL, guts_ids);
> >>> +	if (np) {
> >>> +		guts = of_iomap(np, 0);
> >>> +		smp_85xx_ops.give_timebase = mpc85xx_give_timebase;
> >>> +		smp_85xx_ops.take_timebase = mpc85xx_take_timebase;
> >>> +		of_node_put(np);
> >>> +	} else {
> >>> +		smp_85xx_ops.give_timebase = smp_generic_give_timebase;
> >>> +		smp_85xx_ops.take_timebase = smp_generic_take_timebase;
> >>> +	}
> >>
> >> Do not use smp_generic_give/take_timebase, ever.  If you don't have the
> >> guts node, then just assume the timebase is already synced.
> >>
> >> -Scott
> > 
> > smp_generic_give/take_timebase is the default in KEXEC before.
> 
> That was a mistake.
> 
> > If do not set them, it may make KEXEC fail on other platforms.
> 
> What platforms?
> 
> -Scott

Such as P4080, P3041, etc.

-Chenhui


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