Re: [RFC PATCH v4 6/8] xen/arm: scmi: introduce SCI SCMI SMC multi-agent driver

[Julien Grall <julien@xxxxxxx>]

Hi Oleksii & Stefano,

On 23/05/2025 21:06, Stefano Stabellini wrote:
> > +SCMI SMC multi-agent support
> > +============================
> > +
> > +For enabling the ARM SCMI SMC multi-agent support (enabled by 
> > CONFIG_SCMI_SMC_MA)
> > +the Xen specific SCMI Agent's configuration shell be provided in the Host DT
> > +according to the SCMI compliant EL3 Firmware specification with
> > +ARM SMC/HVC transport using property "xen,scmi-secondary-agents" under
> > +the top-level "chosen" node:
> > +
> > +- xen,scmi-secondary-agents
> > +
> > +    Defines a set of SCMI agents configuration supported by SCMI EL3 FW and
> > +    available for Xen. Each Agent defined as triple consisting of:
> > +    SCMI agent_id,
> > +    SMC/HVC function_id assigned for the agent transport ("arm,smc-id"),
> > +    phandle to SCMI SHM assigned for the agent transport ("arm,scmi-shmem").
> > +
> > +As an example:
> > +
> > +chosen {
> > +    xen,scmi-secondary-agents = <
> > +        1 0x82000003 &scmi_shm_1
> > +        2 0x82000004 &scmi_shm_2
> > +        3 0x82000005 &scmi_shm_3
> > +        4 0x82000006 &scmi_shm_4>;
> > +}
> NIT: it should be };
>
> Looking at scmi_probe, collect_agents, and the following SCMI
> SCMI_BASE_DISCOVER_AGENT request, I wonder: do we actually need this
> information?
>
> It looks like we can discover the agend_ids for every channel, I guess
> what we need to know is the shmem location for every channel? But the
> full list of shmem channel is available below thanks to the scmi-shmem
> nodes.
>
> So, we have the list of scmi-shmem anyway, and we can probe the
> agent_id. The only parameter left is the smc_id/func_id.
>
> Or maybe smc_id/func_id can be calculated from agent_id?
>
> I am asking mostly because if a user is supposed to add this
> xen,scmi-secondary-agents property, where are they supposed to find the
> smc_id/func_id information?
>
> It is important that we write down in this document how the user is
> expected to find out what 1 is 0x82000003 which is scmi_shm_1.
I was actually going to write something similar to what you wrote. I 
think this is error-prone to let the user specify the full SMC ID. It 
would be better if this can be computed from a base + an agent ID.
Cheers,

--
Julien Grall