linux-kernel - Re: [PATCH v8 1/2] arm: introduce psci_smp

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Message-ID: <20130425104508.GG12848@mudshark.cambridge.arm.com>
Date:	Thu, 25 Apr 2013 11:45:08 +0100
From:	Will Deacon <will.deacon@....com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc:	"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>,
	"arnd@...db.de" <arnd@...db.de>,
	"nicolas.pitre@...aro.org" <nicolas.pitre@...aro.org>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"olof@...om.net" <olof@...om.net>
Subject: Re: [PATCH v8 1/2] arm: introduce psci_smp_ops

On Thu, Apr 25, 2013 at 11:12:54AM +0100, Stefano Stabellini wrote:
> On Thu, 25 Apr 2013, Will Deacon wrote:
> > > +/*
> > > + * cpu_suspend   Suspend the execution on a CPU
> > > + * @state        we don't currently describe affinity levels, so just pass 0.
> > > + * @entry_point  the first instruction to be executed on return
> > > + * returns 0  success, < 0 on failure
> > > + *
> > > + * cpu_off       Power down a CPU
> > > + * @state        we don't currently describe affinity levels, so just pass 0.
> > > + * no return on successful call
> > > + *
> > > + * cpu_on        Power up a CPU
> > > + * @cpuid        cpuid of target CPU, as from MPIDR
> > > + * @entry_point  the first instruction to be executed on return
> > > + * returns 0  success, < 0 on failure
> > > + *
> > > + * migrate       Migrate the context to a different CPU
> > > + * @cpuid        cpuid of target CPU, as from MPIDR
> > > + * returns 0  success, < 0 on failure
> > > + *
> > > + */
> > 
> > Can you move these comments into psci-smp.c please? They're really specific
> > to the implementation there, and if we put them in a header we're lying to
> > ourselves about the parameters actually described by the PSCI specification.
> 
> You have a good point about the PSCI spec.
> 
> However from the Linux POV these comments should regard the functions
> exported by psci_operations, not the firmware interface, this is why I
> think it makes sense to keep them in psci.h.
> What we are saying is for example that psci_operations.cpu_on returns 0
> on success and < 0 on failure, and it takes a cpuid and an entry point
> as parameters. We are not saying anything about the firmware interface.

I disagree. You're explicitly stating that we pass the `cpuid of target CPU,
as from MPIDR'. That's simply not true -- the firmware could choose any
numbering scheme to identify the CPUs. For KVM and Xen, it *is* the mpidr,
which is why psci-smp.c works at all, but that's where the comment belongs,
not in this header file.

Will
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