Re: [RFC PATCH 0/1] dt-bindings: arm: Add scmi_devid paramter for

[Sudeep Holla <sudeep.holla@arm.com>]

On Tue, Feb 22, 2022 at 09:06:25AM +0100, Krzysztof Kozlowski wrote:
> On 21/02/2022 22:39, Oleksii Moisieiev wrote:
> > Hi Krzysztof,
> > 
> > On Mon, Feb 21, 2022 at 10:01:43PM +0100, Krzysztof Kozlowski wrote:
> >> On 21/02/2022 18:26, Oleksii Moisieiev wrote:
> >>> Introducing new parameter called scmi_devid to the device-tree bindings.
> >>> This parameter should be set for the device nodes, which has
> >>> clocks/power-domains/resets working through SCMI.
> >>> Given parameter should set the device_id, needed to set device
> >>> permissions in the Firmware. This feature will be extremely useful for
> >>> the virtualized systems, which has more that one Guests running on the
> >>> system at the same time or for the syestems, which require several
> >>> agents with different permissions. Trusted agent will use scmi_devid to
> >>> set the Device permissions for the Firmware (See Section 4.2.2.10 [0]
> >>> for details).
> >>> Agents concept is described in Section 4.2.1 [0].
> >>>
> >>> scmi_devid in Device-tree node example:
> >>> usb@e6590000
> >>> {
> >>>     scmi_devid = <19>;
> >>>     clocks = <&scmi_clock 3>, <&scmi_clock 2>;
> >>>     resets = <&scmi_reset 10>, <&scmi_reset 9>;
> >>>     power-domains = <&scmi_power 0>;
> >>> };
> >>
> >> And how do you prevent DT overlay adding such devid to any other node
> >> thus allowing any other device to send requests with given devid?
> >>
> > Thank you for the quick response.
> > scmi_devid value will be used only by Trusted Agent when the device
> > permissions are set. Non-trusted agents, which in our case are
> > represented as Guest OS are using scmi drivers, already present in linux
> > kernel, ignores scmi_devid and uses scmi_clocks, scmi_power, scmi_reset
> > nodes to access to SCMI protocol.
> 
> Ah, ok.
> 
> > 
> >> Plus few technicalities:
> >> 1. Hyphen, not underscore in property name, so scmi-devid.
> > 
> > Thanks for the tip, I will change that in v2.
> 
> Few more thoughts:
> 1. This looks specific to ARM SCMI, so you also need vendor prefix, so
> something like:
> arm,scmi-devid
> arm,scmi-device-id
> 

Keeping the other discussion separate, I wanted to comment on this.
I agree with Krzysztof on having vendor specific prefix if we decide to add
this device id thing. However, I prefer not to use "arm,scmi-" here.
It can be "xen,scmi-" as we had plans to introduce some concepts in SCMI
spec that may use looks like this device-id. I would just like to avoid
conflicting with that in the future. It may happen to be same in the future
(i.e. this xen device-id matches 100% with definition of device-id we might
introduce in the spec, but I want to make assumption otherwise and leave
scope for divergence however small/little it can be). No issues even if
they converge and match 100% later in the far future.

-- 
Regards,
Sudeep