[PATCH 2/3] ARM: mcpm: Implement cpu_kill() to synchronise on powerdown

[Dave.Martin@arm.com (Dave Martin)]

On Mon, Sep 30, 2013 at 03:18:08PM -0400, Nicolas Pitre wrote:
> On Mon, 30 Sep 2013, Dave Martin wrote:
> 
> > CPU hotplug and kexec rely on smp_ops.cpu_kill(), which is supposed
> > to wait for the CPU to park or power down, and perform the last
> > rites (such as disabling clocks etc., where the platform doesn't do
> > this automatically).
> > 
> > kexec in particular is unsafe without performing this
> > synchronisation to park secondaries.  Without it, the secondaries
> > might not be parked when kexec trashes the kernel.
> > 
> > There is no generic way to do this synchronisation, so a new mcpm
> > platform_ops method power_down_finish() is added by this patch.
> > 
> > The new method is mandatory.  A platform which provides no way to
> > detect when CPUs are parked is likely broken.
> > 
> > Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> 
> There is still a problem with this patch.
> 
> > +int mcpm_cpu_power_down_finish(unsigned int cpu, unsigned int cluster)
> > +{
> > +	int ret;
> > +
> > +	if (WARN_ON_ONCE(!platform_ops || !platform_ops->power_down_finish))
> > +		return 0;
> > +
> > +	ret = platform_ops->power_down_finish(cpu, cluster);
> > +	if (!ret)
> > +		pr_warn("%s: cpu %u, cluster %u failed to power down\n",
> > +			__func__, cpu, cluster);
> > +
> > +	return ret;
> > +}
> 
> So 0 is the error case.
> 
> [...]
> > + * mcpm_cpu_power_down_finish - wait for a specified CPU to halt, and
> > + *	make sure it is powered off
> [...]
> > + * @return:
> > + *	- zero if the CPU is in a safely parked state
> > + *	- nonzero otherwise (e.g., timeout)
> 
> But you document it as being the opposite.
> 
> I'd suggest that the return value is either an int where 0 means success 
> and negative error codes are returned otherwise, or it is a bool and 
> true/false means success/failure.
> 
> I see that platform_cpu_kill() is a bit inconsistent in that regard, but 
> we don't have to propagate this down.

The intention was to be consistent with platform_cpu_kill(), though that
is indeed counterintuitive (hence the snafu in the comments).

I'll go with your suggestion and switch to a zero-on-success convention
internally.

Cheers
---Dave