..
 # SPDX-FileCopyrightText: <text>Copyright 2024-2025 Arm Limited and/or its
 # affiliates <open-source-office@arm.com></text>
 #
 # SPDX-License-Identifier: MIT

########
Overview
########

************
Introduction
************

The **Arm Zena CSS** introduces the concept of a high-performance
|Cortex|-A720AE Application Processor (Primary Compute) system augmented with an
|Cortex|-R82AE
based Safety Island and real-time domain for scenarios where additional system
safety monitoring and real-time services are required. The system additionally
includes a Runtime Security Engine (RSE) used for the secure boot of the system
elements and the runtime Secure Services.

Throughout the following documentation, the names "CSS-Aspen" and "RD-Aspen"
are used in place of Arm Zena CSS due to historical reasons.
For more information, visit :link_subs:`rd-aspen:arm_zena_css`.

A Fixed Virtual Platform (FVP) is available as part of the **Arm Zena CSS**
Reference Design Package.
Further information on FVPs, including expected runtime performance and other
capabilities, can be found at :link_subs:`arm-ecosystem-fvps`.

This documentation, together with the FVP, allows for the exploration of
baremetal and virtualization hosted Linux instances.
The Primary Compute firmware stack of Trusted Firmware-A, U-Boot, and
systemd-boot aims to align with the
technologies and goals of the Arm SystemReady Devicetree program.

*********************************
Arm Automotive Solutions overview
*********************************

The CSS-Aspen is composed of multiple Open Source components,
including:

  * The :link_subs:`rd-aspen:tf-m-doc`, running an instance of Trusted
    Firmware-M, which offers secure boot and pre-SCP platform initialization.

  * The Safety Island subsystem, running a single instance of
    :link_subs:`rd-aspen:scp-firmware`. The CFG2 variant contains one additional
    instance, running the :link_subs:`rd-aspen:zephyr-repo`.

  * The firmware for the Primary Compute, using :link_subs:`rd-aspen:tf-a-doc`
    , :link_subs:`rd-aspen:u-boot` and :link_subs:`rd-aspen:systemd-boot-doc`.
    These are configured to be aligned with
    :link_subs:`rd-aspen:arm-systemready-devicetree`.

The platform consists of the following CPU cores IP:

.. table:: Arm Automotive Solutions platform
   :widths: auto
   :align: center

   =================== ================================
   Component           CSS-Aspen
   =================== ================================
   Primary Compute     Cortex-A720AE Armv9.2-A
   Safety Island       Cortex-R82AE Armv8-R AArch64
   RSE                 Cortex-M55 Armv8.1-M
   =================== ================================

The remaining software in the Primary Compute subsystem, based on the
:link_subs:`common:ewaol` distribution, is available in two main architectures:
baremetal and virtualization.
:link_subs:`common:ewaol` provides :link_subs:`common:bluechi` as a service
controller for containerized workload orchestration.

**Baremetal Architecture**

  The Primary Compute boots a single rich operating system (real-time Linux with
  PREEMPT_RT patches).

.. figure:: images/aspen_css_high_level_arch.*
   :align: center
   :alt: Arm Automotive Solutions High-Level Diagram - Pre Platform Architecture

   Arm Automotive Solutions High-Level Diagram - Pre Platform Architecture

|

**Virtualization Architecture**

    The Primary Compute boots into a type-1 hypervisor (Xen) using Arm’s
    hardware virtualization support. There are three isolated, resource-managed
    virtual machines: Dom0 (privileged domain) and DomU1 and DomU2 (unprivileged
    domains).

.. figure:: images/aspen-virtualization-arch.*
   :align: center
   :alt: Virtualization Architecture Diagram

   Virtualization Architecture Diagram

|

.. _rd-aspen_overview_use_cases:

*********
Use cases
*********

Arm Automotive Solutions demonstrates how the following features can be
used to enhance the overall functional safety level of a high-performance
compute platform:

  * PSA APIs Tests in Primary Compute
  * Safety Status Unit (SSU) integration test
  * Fault Management Unit (FMU) integration test
  * Software Built-In Self-Test Controller (SBISTC) integration test
  * Platform Fault Detection Interface (PFDI) Integration
  * Reliability, Availability, and Serviceability (RAS) error processing validation
  * Arm Cryptographic Extension demo
  * Mission-based power profile demo
  * Virtualization demo
  * Linux Distribution Installation
  * Firmware aiming to align with Arm SystemReady Devicetree
 
The :ref:`Reproduce <rd-aspen/user_guide/reproduce:Reproduce>` section of the
User Guide contains all the instructions necessary to fetch and build the source
as well as to download the required FVP and launch the use cases.

Following are the main use cases implemented by the Reference Software Stack.

PSA APIs Tests in Primary Compute
=================================

Demonstrates implementation of the secure services on Application Processor.

Safety Status Unit (SSU) integration test
=========================================

This test demonstrates the working of the Safety Status Unit.

Fault Management Unit (FMU) integration test
============================================

It validates that an FMU integrates and behaves correctly within the target
system or simulation environment.

Software Built-In Self-Test Controller (SBISTC) integration test
================================================================

It performs self-diagnostics of hardware blocks (like CPUs, memories, or logic clusters)
under software control.

Platform Fault Detection Interface (PFDI) Integration
=====================================================

It is used to detect hardware faults by registering appropriate firmware test libraries.

Reliability, Availability, and Serviceability (RAS) error processing validation
===============================================================================

Reliability, Availability, and Serviceability (RAS) aims to increase the
robustness of a system by detecting hardware errors, recording them and
correcting them where possible.

Arm Cryptographic Extension demo
================================

The demo demonstrates how the Arm Cryptographic Extension can be used to offload
cryptographic work to hardware instructions.

Mission-based power profile demo
================================

This demo is used to demonstrate system power and performance settings for different
mission modes.

Virtualization demo
===================

Demonstrates Xen hypervisor boot with two guest domains.

Linux Distribution Installation
===============================

Demonstrates the installation of three unmodified generic UEFI distribution
images, Debian, openSUSE and Fedora.

Firmware aiming to align with Arm SystemReady Devicetree
========================================================

The Arm SystemReady Devicetree Firmware Build option just builds the Arm SystemReady
Devicetree firmware.