On 03/29/2013 08:22 AM, Stefano Stabellini wrote:

On Thu, 28 Mar 2013, Rob Herring wrote:

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On 03/28/2013 10:39 AM, Nicolas Pitre wrote:

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On Thu, 28 Mar 2013, Rob Herring wrote:

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On 03/28/2013 09:51 AM, Nicolas Pitre wrote:

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On Thu, 28 Mar 2013, Stefano Stabellini wrote:

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- 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.

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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