linux-kernel - Re: [PATCH v6 1/4] arm: introduce psci_smp

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Message-ID: <alpine.DEB.2.02.1304241913280.4180@kaball.uk.xensource.com>
Date:	Wed, 24 Apr 2013 19:25:29 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"will.deacon@....com" <will.deacon@....com>,
	"arnd@...db.de" <arnd@...db.de>,
	"nicolas.pitre@...aro.org" <nicolas.pitre@...aro.org>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	"marc.zyngier@....com" <marc.zyngier@....com>,
	"nico@...aro.org" <nico@...aro.org>
Subject: Re: [PATCH v6 1/4] arm: introduce psci_smp_ops

On Mon, 22 Apr 2013, Russell King - ARM Linux wrote:
> > > Now, if the psci stuff can't be relied upon to provide the correct
> > > functionality, then that's a separate problem which needs addressing
> > > differently.
> > > 
> > > This should allow the Xen problem to be resolved, because Xen will
> > > provide the PSCI operations, and it's correct in that case to override
> > > the platform's SMP operations.
> > 
> > Yes, increasing the priority of PSCI helps Xen a lot.
> > In order to completely solve the issue for Xen though, another patch is
> > needed (http://marc.info/?l=linux-kernel&m=136630106201968&w=2) because
> > of the introduction of smp_init.
> 
> Given the way the code in setup.c will work:
> 
> +static bool __init xen_smp_init(void)
> +{
> +#ifdef CONFIG_SMP
> +	/* If we are running on Xen, use PSCI if available.
> +	 * In any case do not try to use the native smp_ops. */
> +	if (psci_smp_available())
> +		smp_set_ops(&psci_smp_ops);
> +#endif
> +	return true;
> +}
> 
> Doesn't this just need to return false, and then we'll drop down to
> using PSCI if those operations are available?

If we are running on Xen, we are sure that the only available mechanism
to boot secondary cpus is PSCI; there is no need to try anything else if
that one is not available. This is way xen_smp_init is returning true
unconditionally.

However I am going to drop the "xen/arm: introduce xen_early_init, use
PSCI on xen" patch linked above entirely, because I'll just assume that
if we are running on Xen smp_init is going to fail and return false. The
next option on the list is PSCI so in theory we won't have to do
anything special to make it work.
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