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

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Message-ID: <4FCE2ECD.4050107@freescale.com>
Date:	Tue, 5 Jun 2012 11:07:41 -0500
From:	Scott Wood <scottwood@...escale.com>
To:	Zhao Chenhui <chenhui.zhao@...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 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.

>> 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

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