docs: sbsa: document board to firmware interface

We plan to add more hardware information into DeviceTree to limit amount
of hardcoded values in firmware.

Signed-off-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Message-id: 20230531171834.236569-1-marcin.juszkiewicz@linaro.org
[PMM: fix format nits, add text about platform version fields from
 a comment in the C source file]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

docs/system/arm/sbsa.rst

@@ -6,20 +6,15 @@ any real hardware the ``sbsa-ref`` board intends to look like real
 hardware. The `Server Base System Architecture
 <https://developer.arm.com/documentation/den0029/latest>`_ defines a
 minimum base line of hardware support and importantly how the firmware
-reports that to any operating system. It is a static system that
-reports a very minimal DT to the firmware for non-discoverable
-information about components affected by the qemu command line (i.e.
-cpus and memory). As a result it must have a firmware specifically
-built to expect a certain hardware layout (as you would in a real
-machine).
+reports that to any operating system.
 
 It is intended to be a machine for developing firmware and testing
 standards compliance with operating systems.
 
 Supported devices
 """""""""""""""""
 
-The sbsa-ref board supports:
+The ``sbsa-ref`` board supports:
 
   - A configurable number of AArch64 CPUs
   - GIC version 3
@@ -30,3 +25,32 @@ The sbsa-ref board supports:
   - Bochs display adapter on PCIe bus
   - A generic SBSA watchdog device
 
+
+Board to firmware interface
+"""""""""""""""""""""""""""
+
+``sbsa-ref`` is a static system that reports a very minimal devicetree to the
+firmware for non-discoverable information about system components. This
+includes both internal hardware and parts affected by the qemu command line
+(i.e. CPUs and memory). As a result it must have a firmware specifically built
+to expect a certain hardware layout (as you would in a real machine).
+
+DeviceTree information
+''''''''''''''''''''''
+
+The devicetree provided by the board model to the firmware is not intended
+to be a complete compliant DT. It currently reports:
+
+   - CPUs
+   - memory
+   - platform version
+   - GIC addresses
+
+The platform version is only for informing platform firmware about
+what kind of ``sbsa-ref`` board it is running on. It is neither
+a QEMU versioned machine type nor a reflection of the level of the
+SBSA/SystemReady SR support provided.
+
+The ``machine-version-major`` value is updated when changes breaking
+fw compatibility are introduced. The ``machine-version-minor`` value
+is updated when features are added that don't break fw compatibility.