Re: [RFC PATCH v3 0/7] xen/arm: scmi: introduce SCI SCMI SMC multi-agent support

[Grygorii Strashko <grygorii_strashko@xxxxxxxx>]

Hi All, Jan, Anthony, Stefano,

On 11.03.25 13:16, Grygorii Strashko wrote:
> Hi,
>
> This is respin of RFCv2 series from Oleksii Moisieiev [1] which was send pretty 
> long time ago (2022),
> with the main intention is to resume this discussion in public and gather more 
> opinions.
>
> Hence the code was previously sent long time ago there are pretty high number 
> of changes,
> including rebase on newer Xen version 4.20.0-rc2, which already contains some 
> basic SCMI SMC driver
> introduced by Andrei Cherechesu, commit 3e322bef8bc0 ("xen/arm: firmware: Add 
> SCMI over SMC calls
> handling layer").
>
> Patch 1 "xen/arm: add generic SCI subsystem"
> - rebased and refactored
> - introduced DEVICE_ARM_SCI DT device class and used for SCI drivers probing 
> instead of custom,
>    linker sections based implementation.
> - added SCI API for Dom0 DT handling, instead of manipulating with ARM arch 
> dom0 code directly.
> - TODO: RFC changes in XEN_DOMCTL_assign_device OP processing
>
> Patch 2 "xen/arm: scmi-smc: update to be used under sci subsystem"
> - update driver introduced by commit 3e322bef8bc0 ("xen/arm: firmware: Add SCMI 
> over SMC calls
> handling layer") be used under sci subsystem.
> - no functional changes in general
>
> Patch 3 "xen/arm: scmi-smc: passthrough SCMI SMC to guest domain
> This is new change which allows passthrough SCMI SMC, single agent interface to 
> guest domain
> cover use case "thin Dom0 with guest domain, which serves as Driver domain".
> See patch commit message for full description.
>
> Patch 4 - xen/arm: scmi: introduce SCI SCMI SMC multi-agent driver
> - added "xen,scmi-secondary-agents" property in "chosen" to inform SCI SCMI 
> multi-agent driver
>    about available agents and their configuration. It defines <agent_id> to 
> <smc-id,scmi_shm> map.
>    This option is Xen specific as Xen is the only one entry in the system which 
> need to know
>    about SCMI multi-agent support and configuration.
> - each guest using SCMI should be configured with SCMI agent_id, so SCMI
>    FW can implement Agent-specific permission policy.
>    -- dom0: dom0_scmi_agent_id=<agent_id> in Xen command line option
>    -- toolstack: arm_sci = "type=scmi_smc_multiagent,agent_id=<agent_id>"
>    -- dom0less: todo: "xen,sci_type", "xen,sci_agent_id" properties in 
> "xen,domain" nodes.
> - factored out SCMI generic definitions (re-usable)
> - factored out SCMI shmem code (re-usable)
> - the SCMI passthrough configuration for guest domains is similar to any other 
> HW passthrough cfg.
>
> Patches 5-7
> - no major changes, except to follow rebase and changes in previous patches
>
> Regarding patches 5-7 I'd like to rise a question and I, personally, feel very 
> skeptical doing any
> kind of SCMI DT nodes generation as from toolstack as from Xen.
> 1) SCMI is no differ as any other HW MFD device, and HW passthrough 
> configuration works for it in
>     the same way.
> 2) if toolstack generates DT then dom0less case might need it also, but this 
> means more code in Xen,
>     so, with certification in mind, it means more overhead requirements, docs 
> and testing.
>     In my opinion if something can be done outside "kernel" - it should.
>     So better invest in tools (imagebuilder, lopper, etc.) instead.
> 3) Hence SCMI DT bindings are pretty complex the corresponding guest DT nodes 
> can't be generated
>     from scratch - the user still need to add scmi node, protocols and 
> protocols subnodes in the
>     partial device tree, at least empty. But stop, not exactly empty - the 
> properties like
>     "#clock-cells" need to be added to avoid DTC warnings. Such behavior is 
> rather confusing than
>     helpful.
> 4) Exposing the Host Device tree in Dom0 is another questionable thing for 
> toolstack SCMI DT
>     generation. It consumes 128K of memory on Renesas r8a779g0-whitehawk.
> 5) No needs for additional public API (XEN_DOMCTL_get_sci_info, 
> GUEST_SCI_SHMEM_BASE..) if dropped

Thanks a lot for your time, review and valuable comments.
Most of comments are clear enough and will be addressed.

Those, required clarification, I'll reply separately.

--
Best regards,
-grygorii