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

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Message-ID: <alpine.DEB.2.02.1303291804010.4430@kaball.uk.xensource.com>
Date:	Fri, 29 Mar 2013 18:07:11 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
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 <will.deacon@....com>,
	Arnd Bergmann <arnd@...db.de>,
	"marc.zyngier@....com" <marc.zyngier@....com>,
	Russell King - ARM Linux <linux@....linux.org.uk>
Subject: Re: [PATCH v4 2/2] arm: prefer PSCI for SMP bringup

On Fri, 29 Mar 2013, 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.
> 
> That would be more eleguant than adding xen exception hooks in generic 
> code.

I guess I could introduce a Xen-enabled mdesc for all the platforms we intend to
support, but as the list of boards with virtualization extensions grow,
also the list of Xen-enabled mdescs would grow accordingly.
We would end up with dozen (more?) of Xen mdescs.
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