chapter3: require SCMI 2.x, with x >= 1 recommended

On AArch64, when SCMI is implemented, require an SCMI Base protocol
version 2.x, recommend a minimum version of 2.1, and warn that this
minimum version will be required in the future.

Signed-off-by: Vincent Stehlé <[email protected]>

Author: vstehle

source/chapter3-secureworld.rst

@@ -56,6 +56,26 @@ version compatible with caller version 1.2. [#FFANote]_
    with x >= 2, are compatible with caller versions 1.0 to 1.x, which includes
    caller version 1.2.
 
+AArch64 System Control and Management
+=====================================
+
+On AArch64 platforms, when privileged or secure firmware implements the System
+Control and Management Interface (SCMI), it must conform to the Arm System
+Control and Management Interface specification [SCMI]_ and it must implement a
+Base protocol version 2.x, where x >= 1 is recommended. [#SCMINote]_
+
+.. [#SCMINote] SCMI is a set of software interfaces defined by Arm, used in
+   the system management of power domains, performances, clocks, sensors, reset,
+   voltage domains, power and pins.
+   SCMI v2.1 introduced the `NEGOTIATE_PROTOCOL_VERSION` command, which eases
+   agents and platforms interoperability.
+   As per the SCMI specification § 4.1.3 Protocol discovery, all agents relying
+   on version 2.x should interoperate with platforms implementing version 2.y,
+   with y >= x, even without a negotiation.
+
+.. warning:: A future version of this specification will require a minimum SCMI
+   Base protocol version of 2.1.
+
 RISC-V Multiprocessor Startup Protocol
 ======================================