[PATCH 1/7] dt: update PSCI binding documentation for v0.2

[Dave.Martin@arm.com (Dave Martin)]

On Wed, Jul 31, 2013 at 12:49:20PM -0500, Rob Herring wrote:
> On 07/31/2013 12:24 PM, Matt Sealey wrote:
> > On Wed, Jul 31, 2013 at 8:49 AM, Mark Rutland <mark.rutland@arm.com> wrote:
> 
> [snip]
> 
> >>> * An AArch64 guest under an AArch64 hypervisor/sm would use the 64-bit
> >>> convention or the 32-bit convention depending on how the sm is
> >>> written, but it doesn't matter which is used if both can be
> >>> supported.. but you'd only want to use one of them.
> >>
> >> Not all implementation will implement both, so there needs to be a way
> >> to describe that. Which is actually used is up to the OS.
> > 
> > Okay but putting both ways in a single node, and describing two
> > function ids (per property or with two properties) is what I was
> > getting at.. for the one case where you can use both at once, the
> > device tree description is king here.
> > 
> > I am a little concerned that the support for this is going down the
> > route of telling the OS all possible ways to do the same thing instead
> > of trying to get into using a single, preferred way in the common
> > cases.
> > 
> > Putting both call methods in the same node, doubling the function id
> > property lengths, or suffixing function id properties to do it seems
> > like putting information in the tree purely for the sake of it. In
> > what cases would it (uncommon cases only being existing, pre-PCSI
> > pre-SMC-conventions potentially for other things). In the case of PSCI
> > there's an opportunity to be strict about it.. why would it be sane to
> > allow a mixed implementation? Dictate that either support all the
> > 64-bit versions or all the 32-bit versions or both? And if both are
> > supported, dictate in the device tree which the OS should be using.

The DT is about description, not policy.  If multiple usable flavours of
the PSCI interface exist, then they exist, and there's no special reason
to hide one of them from the DT that I can see.

I take the point that we shouldn't describe useless or redundant
information for no good reason, though.

> > What about having two nodes? There is nothing in device trees that
> > says you can't have two psci { compatible="arm,psci-0.2" } nodes, one
> > with method=smc and one with method=smc64 or hvc64 or what have you.
> > Parsing and setup of the PSCI code can be quit early if it's not the
> > "desirable" method for the OS (i.e. 64-bit on a 32-bit kernel). Then
> > each node follows the exact same definition, and the differentiator is
> > the call method and not the property names or complicating their
> > contents.
> 
> +1
> 
> This would certainly be easier for things like a hypervisor to parse and
> update.

Can you elaborate on that?

Wouldn't a hypervisor always rip out and replace PSCI node(s) with
its own?  Allowing the firmware's PSCI interface to "show through" to a
guest VM sounds unwise.

I'm not sure why having multiple nodes makes this easier.

Having two nodes just moves information around.  Does it really simplify
anything?


With the multiple nodes approach, you might actually need 3 nodes if
you are providing backwards compatible support for psci-0.1 callers: one
for psci-0.2 32-bit, one for psci-0.2 64-bit, and one for psci-0.1.
Maybe not though ... I'd have to think about it a bit more.

CHeers
---Dave