docs: sbsa: update specs, add dt note
Hardware of sbsa-ref board is nowadays defined by both BSA and SBSA
specifications. Then BBR defines firmware interface.
Added note about DeviceTree data passed from QEMU to firmware. It is
very minimal and provides only data we use in firmware.
Added NUMA information to list of things reported by DeviceTree.
Signed-off-by: Marcin Juszkiewicz <marcin.juszkiewicz@linaro.org>
Message-id: 20240328163851.1386176-1-marcin.juszkiewicz@linaro.org
Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/docs/system/arm/sbsa.rst b/docs/system/arm/sbsa.rst
index bca6160..2bf22a1 100644
--- a/docs/system/arm/sbsa.rst
+++ b/docs/system/arm/sbsa.rst
@@ -1,12 +1,16 @@
Arm Server Base System Architecture Reference board (``sbsa-ref``)
==================================================================
-While the ``virt`` board is a generic board platform that doesn't match
-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.
+The ``sbsa-ref`` board intends to look like real hardware (while the ``virt``
+board is a generic board platform that doesn't match any real hardware).
+
+The hardware part is defined by two specifications:
+
+ - `Base System Architecture <https://developer.arm.com/documentation/den0094/>`__ (BSA)
+ - `Server Base System Architecture <https://developer.arm.com/documentation/den0029/>`__ (SBSA)
+
+The `Arm Base Boot Requirements <https://developer.arm.com/documentation/den0044/>`__ (BBR)
+specification defines how the firmware reports that to any operating system.
It is intended to be a machine for developing firmware and testing
standards compliance with operating systems.
@@ -35,16 +39,29 @@
(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).
+Note
+''''
+
+QEMU provides the guest EL3 firmware with minimal information about hardware
+platform using minimalistic devicetree. This is not a Linux devicetree. It is
+not even a firmware devicetree.
+
+It is information passed from QEMU to describe the information a hardware
+platform would have other mechanisms to discover at runtime, that are affected
+by the QEMU command line.
+
+Ultimately this devicetree may be replaced by IPC calls to an emulated SCP.
+
DeviceTree information
''''''''''''''''''''''
-The devicetree provided by the board model to the firmware is not intended
-to be a complete compliant DT. It currently reports:
+The devicetree reports:
- CPUs
- memory
- platform version
- GIC addresses
+ - NUMA node id for CPUs and memory
Platform version
''''''''''''''''
@@ -70,4 +87,4 @@
GIC ITS information is present in devicetree.
0.3
- The USB controller is an XHCI device, not EHCI
+ The USB controller is an XHCI device, not EHCI.
