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

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Message-ID: <20150216152948.GH8994@leverpostej>
Date:	Mon, 16 Feb 2015 15:29:48 +0000
From:	Mark Rutland <mark.rutland@....com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Magnus Damm <magnus.damm@...il.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Simon Horman <horms@...ge.net.au>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Nicolas Pitre <nico@...aro.org>,
	Dave P Martin <Dave.Martin@....com>,
	SH-Linux <linux-sh@...r.kernel.org>
Subject: Re: [PATCH v2] ARM: smp: Only expose /sys/.../cpuX/online if
 hotpluggable

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.

Thanks,
Mark.
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