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

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Date:	Thu, 25 Apr 2013 12:11:25 +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 12:08:02PM +0100, Stefano Stabellini wrote:
> On Thu, 25 Apr 2013, Will Deacon wrote:
> > On Thu, Apr 25, 2013 at 11:12:54AM +0100, Stefano Stabellini wrote:
> > > 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.
> 
> I see, you want to keep psci_operations true to the firmware interface
> while explaining that psci_smp makes some assumptions about it.

Precisely! :)

> So the comment should be something like:
> 
> /*
>  * psci_smp assumes that the following is true about PSCI:
>  * 
>  * 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
>  *
>  */

That's certainly better, but I'd still rather see the comment with the
implementation as there's a greater potential for confusion having it here.

Will
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