SCMI requirements

[vstehle]

On AArch64, when SCMI is implemented, in a table:

• Require minimum protocol versions (as in SCMI 2.0)
• Recommend minimum protocol versions (as in SCMI 3.2)

[xypron]

LGTM

Reviewed-by: @xypron

[vstehle]

This is a bit refined now:

• Broaden the requirements to all cases of transport to encompass the case of a remote processor behind a mailbox.
• Consider all standard protocols; not only the base one.
• Convert to a table for clarity

The requirements correspond to spec version 2.0 B. They are supported by TF-A and (almost) by OP-TEE. They should be aligned with the (upcoming) Arm SystemReady SRS 3.1 and should be sufficient for Linux, which is very permissive.

The recommendations correspond to spec version 3.2 E. They should benefit to recent Linux kernels, which default to those versions and can negotiate to some extend.

[xypron]

LGTM

[vstehle]

Updated the pull request as per the EBBR call of Sep 24:

• Reference SCMI spec v3.2, do not mention the letter, remove the telemetry protocol from the table
• Mention v1.0 in all table fields where necessary

While at it, transform the note after the table on version negotiation into a footnote.

[vstehle]

Rebased, and in the mean time added one missing comma to the SCMI reference.

[xypron]

I think this is good to merge. But I have not checked all the version numbers.

[vstehle]

Accepted during the EBBR call of Nov 5.