[PATCH 1/2] dt-bindings: firmware: arm,scmi: Document arm,poll-transport property

[Marek Vasut]

On 12/2/25 7:55 PM, Sudeep Holla wrote:

Hello Sudeep,

>> How do you imagine this -- a transport shared with other components, one
>> which does generate IRQs and one which does not -- would look like ? Can you
>> think of an example ?
>>
> 
> Consider a system where a mailbox controller is present and one channel is
> used for SCMI communication, while another channel is used for an unrelated
> purpose. If both channels share the same interrupt line, and the other use
> case enables interrupt mode on its channel, what would be the impact on the
> SCMI-specific channel?

None, SCMI kernel driver and SCMI server side would still do polling on 
their respective SHMEM areas, while whatever kernel driver needs to 
receive the interrupt notifications would subscribe to them using 
request_irq(), right ?

> I am aware of systems that implement such sharing, which is why I prefer to be
> explicit that this type of design is challenging to support within this
> binding. The intent is to support only minimal, constrained cases - essentially
> systems that are already somewhat broken. I do not see value in broadening the
> binding to cover every conceivable scenario.
> 
>>> Clearly defining these constraints would be helpful. It may also be useful to
>>> note that this is primarily intended for mailbox transports, if that’s
>>> accurate. Alternatively, we could keep the DT binding definition broader but
>>> emit warnings when a transport other than mailbox is used. That approach might
>>> make it easier to move forward.
>>
>> DEN0056F refers to this polling mode in Shared memory based transports, that
>> can be other than mailbox transports, it includes e.g. SMC or OPTEE
>> transports.
>>
> 
> However, polling does not make sense in the context of SMC. Once control
> returns from an SMC call, the command has completed. What form of polling in
> an SMC workflow do you have in mind?

I think the polling happens on the SHMEM and the SMC transport is 
capable of that too, see :

drivers/firmware/arm_scmi/transports/smc.c

175         /*
176          * If there is an interrupt named "a2p", then the service and
177          * completion of a message is signaled by an interrupt 
rather than by
178          * the return of the SMC call.
179          */
180         scmi_info->irq = of_irq_get_byname(cdev->of_node, "a2p");

> I believe the same applies to OP-TEE.
> While OP-TEE now provides a notification mechanism that could, in theory,
> allow synchronous commands to be treated in a quasi-asynchronous manner, I
> strongly doubt that the current SCMI-over-OP-TEE implementation behaves this
> way, given that it ultimately reaches the secure side via an SMC call.
> 
>> I don't think a warning is justified, if the behavior follows the
>> specification. But I do agree the behavior is ... suboptimal.
>>
> 
> The specification does not address SMC or OP-TEE transports, placing them
> outside its scope and likewise these DT bindings.

I believe the shmem transport includes the SMC and OPTEE ones, right ?

> Consequently, what we
> decide here in this discussion effectively defines the expected behavior in
> this context, in my view. So I would like to start with minimal possible
> coverage, why do you think that is not a good idea here ?

I would argue the current implementation covers pretty much every 
transport which could ever need to do polling on shmem, so the 
implementation is generic and inline with the specification. Also, the 
current implementation is some 20 lines, so I think it is minimalistic?

What would you propose we do here ?

-- 
Best regards,
Marek Vasut