hw/arm: add versioning to sbsa-ref machine DT

The sbsa-ref machine is continuously evolving. Some of the changes we
want to make in the near future, to align with real components (e.g.
the GIC-700), will break compatibility for existing firmware.

Introduce two new properties to the DT generated on machine generation:
- machine-version-major
  To be incremented when a platform change makes the machine
  incompatible with existing firmware.
- machine-version-minor
  To be incremented when functionality is added to the machine
  without causing incompatibility with existing firmware.
  to be reset to 0 when machine-version-major is incremented.

This versioning scheme is *neither*:
- A QEMU versioned machine type; a given version of QEMU will emulate
  a given version of the platform.
- A reflection of level of SBSA (now SystemReady SR) support provided.

The version will increment on guest-visible functional changes only,
akin to a revision ID register found on a physical platform.

These properties are both introduced with the value 0.
(Hence, a machine where the DT is lacking these nodes is equivalent
to version 0.0.)

Signed-off-by: Leif Lindholm <quic_llindhol@quicinc.com>
Message-id: 20220505113947.75714-1-quic_llindhol@quicinc.com
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Radoslaw Biernacki <rad@semihalf.com>
Cc: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/arm/sbsa-ref.c

@@ -192,6 +192,20 @@ static void create_fdt(SBSAMachineState *sms)
     qemu_fdt_setprop_cell(fdt, "/", "#address-cells", 0x2);
     qemu_fdt_setprop_cell(fdt, "/", "#size-cells", 0x2);
 
+    /*
+     * This versioning scheme is for informing platform fw only. It is neither:
+     * - A QEMU versioned machine type; a given version of QEMU will emulate
+     *   a given version of the platform.
+     * - A reflection of level of SBSA (now SystemReady SR) support provided.
+     *
+     * machine-version-major: updated when changes breaking fw compatibility
+     *                        are introduced.
+     * machine-version-minor: updated when features are added that don't break
+     *                        fw compatibility.
+     */
+    qemu_fdt_setprop_cell(fdt, "/", "machine-version-major", 0);
+    qemu_fdt_setprop_cell(fdt, "/", "machine-version-minor", 0);
+
     if (ms->numa_state->have_numa_distance) {
         int size = nb_numa_nodes * nb_numa_nodes * 3 * sizeof(uint32_t);
         uint32_t *matrix = g_malloc0(size);