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

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Message-ID: <20220222085532.GA439897@EPUAKYIW015D>
Date:   Tue, 22 Feb 2022 08:55:33 +0000
From:   Oleksii Moisieiev <Oleksii_Moisieiev@...m.com>
To:     Krzysztof Kozlowski <krzk@...nel.org>
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

Hi Krzysztof,

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@...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

That's a good advice. arm,scmi-devid looks good for me.

> 
> 2. Does your example work properly? Passes dt_binding_check? Reg looks
> different than unit-address.
> 
dt_bindings_check passes without errors. Also I've checked this file
explicitly by using command:
yamllint -c Documentation/devicetree/bindings/.yamllint Documentation/devicetree/bindings/firmware/arm,scmi-devid.yaml

Reg value, if you mean reg parameter from an Example, was taken from
r8a77961.dtsi file.

> 
> > 
> >> 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...).
> 

I think select: true will work for me. I'll do dtbs_check and
dt_bindings_check after making all changes and prepare v2 if there will
be no further comments.

Also what do you think about maintainers: field? Is it correct? I'm not
sure if I used it correctly.

> 
> Best regards,
> Krzysztof