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Date:   Wed, 23 Mar 2022 10:54:22 +0000
From:   Sudeep Holla <sudeep.holla@arm.com>
To:     Oleksii Moisieiev <Oleksii_Moisieiev@epam.com>
Cc:     Stefano Stabellini <sstabellini@kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        Rob Herring <robh@kernel.org>,
        Sudeep Holla <sudeep.holla@arm.com>,
        Souvik Chakravarty <Souvik.Chakravarty@arm.com>,
        Cristian Marussi <cristian.marussi@arm.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 2/2] dt-bindings: xen: Add xen,scmi-devid property
 description for SCMI
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On Tue, Mar 22, 2022 at 07:21:47PM +0000, Oleksii Moisieiev wrote:
> Hi Sudeep,
> 
> On Tue, Mar 22, 2022 at 11:12:21AM +0000, Sudeep Holla wrote:
> > On Fri, Mar 18, 2022 at 04:53:20PM -0700, Stefano Stabellini wrote:
> > > On Wed, 16 Mar 2022, Sudeep Holla wrote:
> > > > On Wed, Mar 16, 2022 at 04:46:20PM +0000, Oleksii Moisieiev wrote:
> > > > > 
> > > > > > + The reason I want to keep it xen specific at the moment as we had some
> > > > > > plan to extended the device-id usage in the spec which hasn't progressed
> > > > > > a bit(I must admit that before you ask), and this addition should not be
> > > > > > obstruct that future development. If we align with what we define xen
> > > > > > specific as part of $subject work, we can always define generic binding
> > > > > > in the future and slowly make the other obsolete over the time.
> > > > > 
> > > > > IIUC you have some plans to provide device_id support to the device-tree
> > > > > bindings from your side. Maybe we can discuss some of your plans here
> > > > > and we can come up with the generic device-id binding?
> > > > > So I will have something to base on in Xen.
> > > > > 
> > > > 
> > > > Sorry if I wasn't clear in earlier emails. What I mentioned was that I would
> > > > like to reserve the generic namespace(i.e. just device-id) for generic SCMI
> > > > usage. Since we haven't defined it clearly in the spec, I don't want to
> > > > introduce the generic definition and binding now.
> > > > 
> > > > As mentioned earlier, though Xen definition and generic once may be exactly
> > > > same, but we won't know until then. So keep the xen usage and namespace
> > > > separate for now to avoid any future conflicts.
> > > 
> > > 
> > > Hi Sudeep,
> > > 
> > > I thought the specification already covered this device id, it simply
> > > delegated the description of it to Device Tree or ACPI, which is common
> > > behavior in ARM specs. What is missing in the SCMI spec from your point
> > > of view?
> > >
> > 
> > While you can say so, but for me it isn't to an extent that we can support
> > software around it. I did share my feedback with spec author but as you
> > know it was needed for virtualisation use-case like Xen and was rushed
> > into the spec. All it states is "Device identifier" identifies the device
> > and the enumeration is not part of the spec. It defers to DT/ACPI.
> > 
> > Since I didn't have to use that in OSPM, I hadn't given much thought/review
> > on that.
> > 
> > >
> > > Or would you like this scmi-devid Device Tree property (or similar) to
> > > be described in the SCMI specification itself?
> > >
> > 
> > Spec doesn't cover that in general but do carry some recommendations
> > sometimes.
> > 
> > > Typically Device Tree and ACPI descriptions are delegated to Device Tree
> > > and ACPI respectively. Also specification updates are typically slow
> > > (for good reason.) We might be waiting for a long time. It is usually
> > > not a matter of days.
> > 
> > I agree.
> > 
> > As I said, there were thoughts about adding device protocol to make
> > all the other protocols centered around the device. The idea is as below:
> > 
> > Today a device A is associated with clock domain X, reset domain Y,
> > voltage domain Z, perf domain P, power domain Q, ...and so on.
> > Especially this would get complex with lots of devices and for virtual
> > machines.
> > 
> > Instead let all these different operations use the device identifier A
> > in the above case to drive clock, reset, perf, power, voltage,...etc.
> 
> So, IIUC - the idea is to provide new device based protocol
> which will allow agents to control different domains by using ony device
> id? Does it mean that scmi drivers for agents should be also changed and there will
> be no back compatibility with previous versions of SCMI protocol?

The idea is it is discoverable and if the platform advertises new protocol,
then only it will be used. Otherwise we must continue to use the existing
and advertised protocols. Anyways I realised that we need not even consider
these new changes for the discussion here.

> If yes - we probably can add scmi-devid property for current SCMI
> version, such as scmi-v3,device-id (because current DEN0056D document
> has version 3.1) and say that this property should be
> used for SCMI versions, which doesn't support device protocol.
> What do you think about this idea?

The main idea we had is to re-use the generic definition of device ID
Linux might need for other purposes like device assignments. We would
like to avoid a mapping from the generic device ID Linux might need and
define to the one in scmi context. So as Rob mentioned, it is better to
define one in a generic Linux/OS context and see how we can make use of
that in SCMI context. We could get some recommendations added to the spec
if needed based on what gets added/supported in the kernel.

So better to start addressing or responding to Rob's comments(not sure
if it was this version or the previous) if you want a generic device
ID definition. We are not adding anything SCMI specific as that might
end up conflicting with the one that Linux kernel might add.

Hope this helps.

-- 
Regards,
Sudeep