Re: [PATCH v2] ARM: smp: Only expose /sys/.../cpuX/online if hotpluggable

From: Mark Rutland
Date: Mon Feb 16 2015 - 10:30:28 EST

On Fri, Feb 13, 2015 at 11:01:37PM +0000, Russell King - ARM Linux wrote:
> On Fri, Feb 13, 2015 at 09:44:50PM +0000, Magnus Damm wrote:
> > Also, based on the comment in mcpm_cpu_can_disable() it looks like the
> > PSCI hook may be executed once only with your change in place?
> > Hopefully PSCI is OK not being invoked for every CPU shutdown.
> This is why I've said (in the parent thread) that I'm not happy to
> apply this patch. Mark Rutland has indicated that he has MCPM cases
> where the CPUs which can be disabled changes dynamically according
> to the secure firmware requirements, and ripping out todays
> infrastructure in light of that, only to have to add it back again
> later makes no sense.

To clarify, PSCI and MCPM are unrelated. It was originally conceived
that MCPM would use PSCI as a backend, but it turns out that they're
effectively mutually exclusive, and are handled separately. I still want
to add support for MIGRATE in the PSCI client code, but this is
independent of MCPM.

In some cases it's possible to determine at boot time that a CPU cannot
be hotplugged (e.g. in PSCI if the TOS is non-migrateable), so having
separate hooks for determining that static and dynamic ability to
hotplug a CPU sounds reasonable to me.

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