linux-kernel - Re: [RFC PATCH 0/1] dt-bindings: arm: Add scmi

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Date:   Tue, 22 Feb 2022 09:06:25 +0100
From:   Krzysztof Kozlowski <krzk@...nel.org>
To:     Oleksii Moisieiev <Oleksii_Moisieiev@...m.com>
Cc:     "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Cristian Marussi <cristian.marussi@....com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 0/1] dt-bindings: arm: Add scmi_devid paramter for

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@...90000
>>> {
>>>     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

2. Does your example work properly? Passes dt_binding_check? Reg looks
different than unit-address.


> 
>> 2. Your schema does is not selected by anything. How is it intended to
>> be used? Nothing is including it, either...
>>
> 
> The idea is to use this parameter to set the device_id for the device in
> the device-tree, which matches to the device mapping in the Firmware, so
> Trusted Agent can use it to the device permissions.
> Please see Sections 4.2.2.10 and 4.2.1 [0] (Link was provided in the
> cover letter).
> 
> I'm currently propose the new feature, called SCI mediator to Xen-devel
> community. Please see link [1] from cover letter for the details.
> In this feature - Xen is the Trusted Agent, which uses scmi_devid
> parameter to set the device permissions.
> We think that this parameter will be useful for other possible SCMI
> implementations, such as other hypervisor or SCMI backend server etc.

We talk about different things, I think. I was asking how is this schema
selected?

I gave it a fast try (dtbs_check) and it confirmed - schema does not
have an effect. It's a noop. You need something like "select: true", see:
Documentation/devicetree/bindings/nvmem/nvmem-consumer.yaml
or this schema should be included by other schemas... but then I would
be happy to see actual usage in this patchset (more commits...).


Best regards,
Krzysztof