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

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Message-ID: <alpine.DEB.2.02.1304021522230.5078@kaball.uk.xensource.com>
Date:	Tue, 2 Apr 2013 15:28:18 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	Rob Herring <robherring2@...il.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 Mon, 1 Apr 2013, Nicolas Pitre wrote:
> On Mon, 1 Apr 2013, Stefano Stabellini wrote:
> 
> > 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?
> 
> VExpress for one.  When booting on a big.LITTLE system such as TC2 on 
> VExpress, the MCPM layer needs to arbitrate power management operations 
> on a per cluster basis.  In that case there is a MCPM specific set of 
> SMP ops to be used, even if it may end up calling into PSCI.
> 
> But the important point is that we don't know beforehand what to use, 
> especially with a kernel that can boot on multiple different VExpress 
> configurations.  The decision has to be made at run time, and therefore 
> a static default or mdesc->smp ops doesn't cut it.

I certainly like the principle and I am in favor of anything that moves
the decisions at runtime. I have pulled the patch in the series, it's
going to be in the next version.

However I am concerned that these platform specific operations won't
work with Xen at all.
I am getting increasingly certain that we need a Xen specific check in
setup_arch to bump up of the priority of PSCI over anything else if Xen
is running. 
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