Re: [PATCH v2 7/7] ARM: smp: Add runtime PM support for CPU hotplug

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Fri, Sep 04, 2015 at 10:17:35AM +0100, Russell King - ARM Linux wrote:
> On Thu, Sep 03, 2015 at 01:58:34PM -0600, Lina Iyer wrote:
> > @@ -401,6 +412,11 @@ asmlinkage void secondary_start_kernel(void)
> >  	local_irq_enable();
> >  	local_fiq_enable();
> >  
> > +	/* We are running, enable runtime PM for the CPU. */
> > +	cpu_dev = get_cpu_device(cpu);
> > +	if (cpu_dev)
> > +		pm_runtime_get_sync(cpu_dev);
> > +
> 
> Please no.  This is fragile.
> 
> pm_runtime_get_sync() can sleep, and this path is used when the system
> is fully up and running.  Locks may be held, which cause this to sleep.
> However, we're calling it from a context where we can't sleep - which
> is the general rule for any part of system initialisation where we
> have not entered the idle thread yet (the idle thread is what we run
> when sleeping.)
> 
> NAK on this.

There's another issue here.  This is obviously wrong.  If the CPU is
inside the PM domain, then don't you need the PM domain powered up for
the CPU to start executing instructions?

If the CPU can run instructions with the PM domain that it's allegedly
inside powered down, then the CPU is _not_ part of the PM domain, and
this whole thing is total crap.

Sorry, I'm really not impressed by the "design" here, it seems to be
totally screwed.

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