..
 # 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-hosted Linux instances. The Primary Compute firmware stack of
Trusted Firmware-A, U-Boot, and systemd-boot is also aligned 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 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
`EWAOL`_ distribution, is available in baremetal architecture. `EWAOL`_
provides `Bluechi`_ as a service controller for orchestration.

The high level CSS-Aspen Architecture is shown below.

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

|

.. _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:

  * Arm SystemReady Devicetree-aligned software stack
  * Reliability, Availability, and Serviceability

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.

Arm SystemReady Devicetree validation
=====================================

Arm SystemReady is a compliance certification program based on a set of
hardware and firmware standards that enable interoperability with generic
off-the-shelf operating systems and hypervisors.

See :ref:`rd-aspen_design_systemready_devicetree` for more information.

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

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

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.

See :ref:`rd-aspen_design_ras` for more information.

********************
Repository structure
********************

The Arm Automotive Solutions repository (|arm auto solutions repository|) is
structured as follows:

  * ``arm-auto-solutions``:

    * ``yocto``

      Directory implementing the ``meta-arm-auto-solutions``,
      ``meta-arm-bsp-extras``, and ``meta-arm-safety-island`` Yocto layers as
      well as kas build configuration files.

    * ``documentation``

      Directory which contains documentation sources, defined in
      ReStructuredText (``.rst``) format for building via ``sphinx``.