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:

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On Fri, Jun 01, 2012 at 10:40:00AM -0500, Scott Wood wrote:

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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. 

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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.

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I did a readback in mpc85xx_timebase_freeze().

Sorry, missed that somehow.

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+#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.

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If do not set them, it may make KEXEC fail on other platforms.

What platforms?

-Scott

Such as P4080, P3041, etc.

-Chenhui