linux-kernel - Re: [PATCH v4 2/2] arm: prefer PSCI for SMP bringup

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Message-ID: <alpine.DEB.2.02.1304011541010.5078@kaball.uk.xensource.com>
Date:	Mon, 1 Apr 2013 15:42:18 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Rob Herring <robherring2@...il.com>
CC:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Arnd Bergmann <arnd@...db.de>,
	"marc.zyngier@....com" <marc.zyngier@....com>,
	Will Deacon <will.deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v4 2/2] arm: prefer PSCI for SMP bringup

On Fri, 29 Mar 2013, Rob Herring wrote:
> On 03/29/2013 12:53 PM, Nicolas Pitre wrote:
> > On Fri, 29 Mar 2013, Stefano Stabellini wrote:
> > 
> >> On Fri, 29 Mar 2013, Nicolas Pitre wrote:
> >>> On Fri, 29 Mar 2013, Stefano Stabellini wrote:
> >>>
> >>>> If PSCI initializes correctly and PSCI SMP operations are available, use them.
> >>>> This is required for SMP support in Dom0 on Xen.
> >>>>
> >>>> Signed-off-by: Stefano Stabellini <stefano.stabellini@...citrix.com>
> >>>> CC: will.deacon@....com
> >>>> CC: arnd@...db.de
> >>>> CC: marc.zyngier@....com
> >>>> CC: linux@....linux.org.uk
> >>>> CC: nico@...aro.org
> >>>
> >>> I'd suggest you also include in your series the patch I posted earlier 
> >>> providing a runtime mdesc->smp_init method as well.
> >>
> >> OK.
> >>
> >>
> >>> This way the 
> >>> priority order would be:
> >>>
> >>> - If mdesc->smp_init is non null then use that.
> >>>
> >>> - Otherwise, if PSCI is available then use that.
> >>>
> >>> - Otherwise use mdesc->smp.
> >>>
> >>> This way, if the PSCI default has to be overriden (like in the MCPM case 
> >>> because it needs to wrap PSCI itself, or to cover Rob's concern) then 
> >>> this can be achieved at run time on a per mdesc basis.
> >>
> >> Actually that's not a bad idea, it could make everybody happy.
> >> What about the following, in this precise order:
> >>
> >> - if a xen hypervisor node is present on device tree, use PSCI;
> >> - otherwise if mdesc->smp_init is non null then use it;
> >> - otherwise if PSCI is available then use it;
> >> - otherwise use mdesc->smp.
> >>
> >> It's the most practical solution to satisfy everybody's needs.
> > 
> > Maybe I'm missing something obvious, but why can't xen declare a mdesc 
> > of its own?  Given it is going to tweak the DT passed to the kernel 
> > anyway that shouldn't be a problem.
> 
> Xen does have it's own mdesc. It is (or will be) mach-virt, but that is
> only for DomU guests. For Dom0, you still need all the platform specific
> code except smp_ops. However, I'm doubtful this would work without other
> changes on more complicated platforms like OMAP.
> 
> I would say wait to add this until you have platforms that actually need
> the first case.

OK, that is not unreasonable.

What are the platforms that are going to use smp_init? Do we know how do
they intend to use it?
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