linux-kernel - Re: [PATCH v8 1/2] dt-bindings: arm: Add OP-TEE transport for SCMI

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Message-ID: <2b4442d9-fb10-36ee-585d-4103b76abbbb@pengutronix.de>
Date:   Tue, 8 Mar 2022 10:51:54 +0100
From:   Ahmad Fatoum <a.fatoum@...gutronix.de>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     Etienne Carriere <etienne.carriere@...aro.org>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Cristian Marussi <cristian.marussi@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        devicetree@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Pengutronix Kernel Team <kernel@...gutronix.de>
Subject: Re: [PATCH v8 1/2] dt-bindings: arm: Add OP-TEE transport for SCMI

Hello Sudeep,

On 01.03.22 16:12, Sudeep Holla wrote:
> 
> Hi Ahmad,
> 
> On Mon, Feb 28, 2022 at 05:01:39PM +0100, Ahmad Fatoum wrote:
>> Hello Etienne,
>>
>> On 28.10.21 16:00, Etienne Carriere wrote:
>>> Introduce compatible "linaro,scmi-optee" for SCMI transport channel
>>> based on an OP-TEE service invocation. The compatible mandates a
>>> channel ID defined with property "linaro,optee-channel-id".
>>
> 
> Not sure if Etienne's reply addressed your queries/concerns correctly.
> I thought I will add my view anyways.
> 
>> I just found this thread via the compatible in the STM32MP131 patch set:
>> https://lore.kernel.org/all/20220225133137.813919-1-gabriel.fernandez@foss.st.com/
>>
>> Linux doesn't care whether PSCI is provided by TF-A, OP-TEE or something
>> else, so there is just the arm,psci* compatible.
>>
> 
> Correct, the interface to the kernel is fixed and hence we must be able
> to manage with the standard and fixed sole set of bindings for the same.
> 
>> What's different about SCMI that this is not possible? Why couldn't the
>> existing binding and driver be used to communicate with OP-TEE as secure
>> monitor as well?
>>
> 
> However with SCMI, the spec concentrates and standardises all the aspects
> of the protocol used for the communication while it allows the transport
> used for such a communication to be implementation specific. It does
> address some standard transports like mailbox and PCC(ACPI). However,
> because of the flexibility and also depending on the hardware(or VM),
> different transports have been added to the list. SMC/HVC was the one,
> followed by the virtio and OPTEE. While I agree SMC/HVC and OPTEE seem
> to have lot of common and may have avoided separate bindings.
> 
> However the FIDs for SMC/HVC is vendor defined(the spec doesn't cover this
> and hence we utilised/exploited DT). Some vendors wanted interrupt support
> too which got added. OPTEE eliminates the need for FID and can also provide
> dynamic shared memory info. In short, it does differ in a way that the driver
> needs to understand the difference and act differently with each of the
> unique transports defined in the binding.
> 
> Hope that explains and addresses your concern.

Thanks for the elaborate answer. I see now why it's beneficial to have
an OP-TEE transport in general. I don't yet see the benefit to use it
in the STM32MP13x instead of SMCs like with STM32MP15x, but that a discussion
that I need to have in the aforementioned thread.

Thanks again!
Ahmad

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