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

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Message-ID: <5ef2f1ae-f6b2-9905-801a-395c7269bd93@pengutronix.de>
Date:   Wed, 16 Mar 2022 12:18:47 +0100
From:   Ahmad Fatoum <a.fatoum@...gutronix.de>
To:     Etienne Carriere <etienne.carriere@...aro.org>
Cc:     Sudeep Holla <sudeep.holla@....com>, 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 Etienne,

On 08.03.22 11:18, Etienne Carriere wrote:
> Hello Ahmad,
> 
> On Tue, 8 Mar 2022 at 10:51, Ahmad Fatoum <a.fatoum@...gutronix.de> wrote:
>>
>> 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.
> 
> Some SCMI operations in OP-TEE need to execute in a threaded context
> (preemptible, ...).
> There is no SMC function ID defined for an SCMI thread entry in
> OP-TEE. We rather use standard invocation of a TEE service: opening a
> session and invoking commands.
> Invoked commands are executed in an OP-TEE native threaded context.
> The service accessed is referred to as the OP-TEE SCMI PTA.
> 
> As for STM32MP15x, one willing to extend resources assigned to secure
> world may also need to move mp15 SCMI from SMC transport to optee
> transport.

Yes. Makes sense.

Thanks again for explaining,
Ahmad

> 
> Regards,
> Etienne
> 
>>
>> Thanks again!
>> Ahmad
>>
>> --
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> 
etienn

-- 
Pengutronix e.K.                           |                             |
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