linux-kernel - Re: [PATCH v3] [RFC] arm: use PSCI if available

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Message-ID: <51559D01.9070607@gmail.com>
Date:	Fri, 29 Mar 2013 08:54:09 -0500
From:	Rob Herring <robherring2@...il.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
CC:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"arnd@...db.de" <arnd@...db.de>,
	Marc Zyngier <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 v3] [RFC] arm: use PSCI if available

On 03/29/2013 08:22 AM, Stefano Stabellini wrote:
> On Thu, 28 Mar 2013, Rob Herring wrote:
>> On 03/28/2013 10:39 AM, Nicolas Pitre wrote:
>>> On Thu, 28 Mar 2013, Rob Herring wrote:
>>>
>>>> On 03/28/2013 09:51 AM, Nicolas Pitre wrote:
>>>>> On Thu, 28 Mar 2013, Stefano Stabellini wrote:
>>>>>
>>>>>> - the interface to bring up secondary cpus is different and based on
>>>>>> PSCI, in fact Xen is going to add a PSCI node to the device tree so that
>>>>>> Dom0 can use it.
>>>>>>
>>>>>> Oh wait, Dom0 is not going to use the PSCI interface even if the node is
>>>>>> present on device tree because it's going to prefer the platform smp_ops
>>>>>> instead.
>>>>>
>>>>> Waitaminute...  I must have missed this part.
>>>>>
>>>>> Who said platform specific methods must be used in preference to PSCI?
>>>>
>>>> I did. Specifically, I said the platform should be allowed to provide
>>>> its own smp_ops. A platform may need to do addtional things on top of
>>>> PSCI for example.
>>>
>>> Then the platform should have its special hook that would override the 
>>> default PSCI methods.  But, by *default* the PSCI methods should be used 
>>> if the related DT information is present.
>>
>> Agreed. The special hook to override is setting mach desc smp_ops, right?
> 
> If you consider the mach smp_ops a platform specific override, then
> again PSCI and providing a PSCI node on DT doesn't solve the Xen problem
> at all.
> 
> See above: Xen adds a PSCI node to DT, and Linux still does not use it.

Okay, I see. I wasn't distinguishing Dom0 vs DomU cases. Is this really
the only issue with having a platform run in Dom0? We expect all
platforms to work without any modifications? I would think for more
complex platforms there would be some other work needed.

How is Xen going to really do physical cpu power management if a
platform does not provide PSCI firmware? Are you going to pull all the
platform specific code we have in the kernel now into Xen? If you make
PSCI firmware a requirement for Xen, then you would only be modifying
existing PSCI data to the DTB and the platform would be converted to use
PSCI already.

>>>>> If DT does provide PSCI description, then PSCI should be used.  Doing 
>>>>> otherwise is senseless.  If PSCI is not to be used, then it should not 
>>>>> be present in DT.
>>>>
>>>> You can't assume the DT and kernel are in-sync. For example, I've added
>>>> PSCI in the firmware and DTB (part of the firmware), but the highbank
>>>> kernel may or may not use it depending if I convert it.
>>>
>>> If the kernel does not understand PSCI bindings in the DT, it naturally 
>>> won't use PSCI, right?  Conversely, if the firmware and therefore 
>>> provided DT don't have PSCI, then the PSCI enabled kernel won't use PSCI 
>>> either. So what is the problem?
>>
>> I'm distinguishing the kernel in general is enabled for PSCI and a
>> platform is enabled. The kernel may have PSCI smp_ops and the DTB may
>> have PSCI data, but that alone should not make a platform use the
>> default PSCI smp_ops. The platform has to make the decision and it
>> cannot be just based on the platform's dtb having PSCI data.
> 
> I can see how this would give greater flexibility to firmware
> developers, but on the other hand it would limit the flexibility of the
> kernel.

It limits the flexibility of the kernel too. If PSCI is present in the
DTB, then the kernel must use it and the platform has no say? That's not
flexible.

> 
> In fact, unfortunately, it is diametrically the opposite of what Xen
> needs.
> 
> I would kindly ask the maintainers to let me know what direction I
> should take to move forward.

My argument is somewhat academic. I fully expect to convert highbank
over to PSCI for 3.10 assuming this patch gets sorted out in time. So it
is not really an issue for me. Adding Nico's smp_init function could
give the platform flexibility later if needed.

We're only talking about the behavior of a small portion of the patch,
so I would go ahead with implementing the rest of the feedback.

Rob
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