docs/design_documents/hardware_abstraction_layer.rst

##########################
Hardware Abstraction Layer
##########################

:Organization: Arm Limited
:Contact: tf-m@lists.trustedfirmware.org

:API Version: 0.9

************
Introduction
************
:term:`TF-M` :term:`HAL` abstracts the hardware-oriented and platform specific
operations on the :term:`SPE` side and provides a set of APIs to the upper
layers such as :term:`SPM`, :term:`RoT Service`.
The :term:`HAL` aims to cover the platform different aspects whereas common
architecturally defined aspects are done generically within the common
:term:`SPE`.
In some cases, although the operations are defined architecturally,
it may not be possible to generalize implementations because lots of information
is only known to platforms.
It is more efficient to define a :term:`HAL` API for those architectural
operations as well.

.. note::
  :term:`TBSA-M` provides the hardware requirements for security purposes.
  :term:`TF-M` :term:`HAL` tries to reference :term:`TBSA-M` recommendations in
  the interfaces from the software perspective only. Please reference
  :term:`TBSA-M` for your security hardware design.

Design Goals
============
:term:`TF-M` :term:`HAL` is designed to simplify the integration efforts on
different platforms.

:term:`TF-M` :term:`HAL` is designed to make it easy to use the hardware and
develop the :term:`SPM` and :term:`RoT Service` which need to access the
devices.

:term:`TF-M` :term:`HAL` is designed to make the structure clearer and let the
:term:`TF-M` mainly focus on :term:`PSA` implementation.

********
Overview
********
This section provides an overview of the abstraction layer structure.

.. figure:: media/hal_structure.png

Here lists a minimal set of necessary functionalities:

  - **Isolation API**: Provides the necessary isolation functionalities required
    by the :term:`PSA-FF-M` and :term:`TBSA-M`, and provides APIs to :term:`SPM`
    to check the permissions of memory access.
  - **Platform API**: Provides the platform initialization, platform-specific
    memory information, system reset, etc.
  - **Log dev API**: Provides the log system functions.
  - **Interrupt API**: Provides the interrupt functions.

.. note::
  - There is a non-secure :term:`HAL` that focuses on the mailbox operation API
    for Dual-core topology. For more information about it, please refer to
    :doc:`Mailbox Design in TF-M on Dual-core System
    </docs/design_documents/dual-cpu/mailbox_design_on_dual_core_system>`.
  - The minimal set of :term:`TF-M` :term:`HAL` is sufficient for Secure
    Partitions by using customized peripheral interfaces. To provide easier
    portability for the Secure Partitions, a Secure Partition :term:`HAL` is
    provided in this design too.
  - The debug mechanisms give the external entity the corresponding right to
    access the system assets. :term:`TF-M` ensures that the external entity is
    permitted access to those assets. Currently, :term:`TF-M` only needs the
    debug authentication. The whole debug mechanism and related :term:`HAL` will
    be enhanced in the future. Please refer to the :doc:`Debug authentication
    settings section </platform/readme>` for more detail.

*****************
Design Principles
*****************
As :term:`TF-M` runs on resource-constrained devices, the :term:`HAL` tries to
avoid multiple level abstractions which cost more resources.

Part of the :term:`HAL` interfaces does not focus on exact hardware operations
such as power on/off or PIN manipulation.
Instead, the :term:`HAL` abstracts higher-level interfaces to reserve the
implementation flexibility for the platform vendors.

The :term:`TF-M` :term:`HAL` should be easy to deprecate APIs and provide
compatibilities.
Any API incompatibility should be detected during building.

:term:`TF-M` relies on the :term:`HAL` APIs to be implemented correctly and
trusts the :term:`HAL` APIs.
:term:`TFM` can provide assertions to detect common programming errors but
essentially no further extensive checks will be provided.

************
Source Files
************
This section describes the source file of the :term:`TF-M` :term:`HAL`,
including the header and c files.

tfm_hal_defs.h
==============
This header file contains the definitions of common macros and types used by all
:term:`HAL` APIs. Please refer to `Status Codes`_ for detailed definitions.

tfm_hal_[module].[h/c]
======================
All other headers and c files are classified by the modules, such as isolation,
platform, interrupt, devices, etc.

.. note::
  There are common files in the platform folder include the implemented
  :term:`HAL` APIs. The platform vendors can use them directly but need to
  implement all the sub APIs.

************
Status Codes
************
These are common status and error codes for all :term:`HAL` APIs.

Types
=====
tfm_hal_status_t
----------------
This is a status code to be used as the return type of :term:`HAL` APIs.

.. code-block:: c

  enum tfm_hal_status_t {
      TFM_HAL_ERROR_MEM_FAULT = SCHAR_MIN,
      TFM_HAL_ERROR_MAX_VALUE,
      TFM_HAL_ERROR_BAD_STATE,
      TFM_HAL_ERROR_NOT_SUPPORTED,
      TFM_HAL_ERROR_INVALID_INPUT,
      TFM_HAL_ERROR_NOT_INIT,
      TFM_HAL_ERROR_GENERIC,
      TFM_HAL_SUCCESS = 0
  };

Error Codes
===========
Negative values indicate an error. Zero and positive values indicate success.

Here is the general list. The detailed usages for each error code are described
in the API introduction sections.

TFM_HAL_SUCCESS
---------------
Status code to indicate general success.

TFM_HAL_ERROR_GENERIC
---------------------
Status code to indicate an error that does not correspond to any defined failure
cause.

TFM_HAL_ERROR_NOT_INIT
----------------------
Status code to indicate that the module is not initialed.

TFM_HAL_ERROR_INVALID_INPUT
---------------------------
Status code to indicate that the input is invalid.

TFM_HAL_ERROR_NOT_SUPPORTED
---------------------------
Status code to indicate that the requested operation or a parameter is not
supported.

TFM_HAL_ERROR_BAD_STATE
-----------------------
Status code to indicate that the requested action cannot be performed in the
current state.

TFM_HAL_ERROR_MAX_VALUE
-----------------------
Status code to indicate that the current number has got the max value.

TFM_HAL_ERROR_MEM_FAULT
-----------------------
Status code to indicate that the memory check failed.

***************************
API Definition for TF-M SPM
***************************
This section describes the APIs defined for :term:`TF-M` :term:`SPM`.

Isolation API
=============
The :term:`PSA-FF-M` defines three isolation levels and a memory access rule to
provide diverse levels of securitiy. The isolation API provides the functions to
implement these requirements.

Memory Access Attributes
------------------------
The memory access attributes are encoded as bit fields, you can logic OR them to
have a combination of the atrributes, for example
``TFM_HAL_MEM_ATTR_UNPRIVILEGED | TFM_HAL_MEM_ATTR_READABLE`` is unprivileged
readable. The data type is `uint32_t`.

TFM_HAL_MEM_ATTR_EXECUTABLE
^^^^^^^^^^^^^^^^^^^^^^^^^^^
The memory is executable.

.. code-block:: c

  #define TFM_HAL_MEM_ATTR_EXECUTABLE (1UL << 0)

TFM_HAL_MEM_ATTR_READABLE
^^^^^^^^^^^^^^^^^^^^^^^^^
The memory is readable.

.. code-block:: c

  #define TFM_HAL_MEM_ATTR_READABLE (1UL << 1)

TFM_HAL_MEM_ATTR_WRITABLE
^^^^^^^^^^^^^^^^^^^^^^^^^
The memory is writable.

.. code-block:: c

  #define TFM_HAL_MEM_ATTR_WRITABLE (1UL << 2)

TFM_HAL_MEM_ATTR_UNPRIVILEGED
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The memory is unprivileged mode accessible.

.. code-block:: c

  #define TFM_HAL_MEM_ATTR_UNPRIVILEGED (1UL << 3)

TFM_HAL_MEM_ATTR_DEVICE
^^^^^^^^^^^^^^^^^^^^^^^
The memory is a MMIO device.

.. code-block:: c

  #define TFM_HAL_MEM_ATTR_DEVICE (1UL << 4)

TFM_HAL_MEM_ATTR_NS
^^^^^^^^^^^^^^^^^^^
The memory is accessible from :term:`NSPE`

.. code-block:: c

  #define TFM_HAL_MEM_ATTR_NS (1UL << 5)

APIs
----
tfm_hal_set_up_static_boundaries()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
**Prototype**

.. code-block:: c

  tfm_hal_status_t tfm_hal_set_up_static_boundaries(void)

**Description**

This API sets up the static isolation boundaries which are constant throughout
the runtime of the system.

The boundaries include:

- The SPE boundary between the :term:`SPE` and the :term:`NSPE`
- The PSA RoT isolation boundary between the PSA Root of Trust and the
  Application Root of Trust which is for isolation level 2 and 3 only.

Please refer to the :term:`PSA-FF-M` for the definitions of the isolation
boundaries.

**Return Values**

- ``TFM_HAL_SUCCESS`` - the isolation boundaries have been set up.
- ``TFM_HAL_ERROR_GENERIC`` - failed to set up the isolation boundaries.

tfm_hal_mpu_update_partition_boundary
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
**Prototype**

.. code-block:: c

  enum tfm_hal_status_t tfm_hal_mpu_update_partition_boundary(uintptr_t start,
                                                              uintptr_t end);

**Description**

This API updates the partition isolation boundary for isolation level 3.
Inside the partition isolation boundary is the private data of the running
Secure Partition.
This boundary is updated dynamically when :term:`SPM` switches Partitions in
isolation level 3.

The access permissions of the boundary is all privileged mode read-write.

Platforms decide which :term:`MPU` region the paritition boundary uses.

**Parameter**

- ``start`` - start address of the partition boundary.
- ``end`` - end address of the partition boundary.

**Return Values**

- ``TFM_HAL_SUCCESS`` - the isolation boundary has been set up.
- ``TFM_HAL_ERROR_GENERIC`` - failed to set upthe isolation boundary.

**Note**

This API is only for platforms using :term:`MPU` as isolation hardwares.
A generic API for all platforms will be introduced in future versions.

tfm_hal_memory_has_access()
^^^^^^^^^^^^^^^^^^^^^^^^^^^
**Prototype**

.. code-block:: c

  tfm_hal_status_t tfm_hal_memory_has_access(const uintptr_t base,
                                             size_t size,
                                             uint32_t attr)

**Description**

This API checks if the memory region defined by ``base`` and ``size`` has the
given access atrributes - ``attr``.

The Attributes include :term:`NSPE` access, privileged mode, and read-write
permissions.

**Parameter**

- ``base`` - The base address of the region.
- ``size`` - The size of the region.
- ``attr`` - The `Memory Access Attributes`_.

**Return Values**

- ``TFM_HAL_SUCCESS`` - The memory region has the access permissions.
- ``TFM_HAL_ERROR_MEM_FAULT`` - The memory region does not have the access
  permissions.
- ``TFM_HAL_ERROR_INVALID_INPUT`` - Invalid inputs.
- ``TFM_HAL_ERROR_GENERIC`` - An error occurred.

Log API
=======
The log API is used by the :term:`TF-M` :doc:`log system <tfm_log_system_design_document>`.
The log device could be uart, memory, usb, etc.

APIs
----
tfm_hal_output_partition_log()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
**Prototype**

.. code-block:: c

  int32_t tfm_hal_output_partition_log(const unsigned char *str, uint32_t len)

**Description**

This API is called by Secure Partition to output logs.

**Parameter**

- ``str`` - The string to output.
- ``len`` - Length of the string in bytes.

**Return Values**

- Positive values - Number of bytes output.
- ``TFM_HAL_ERROR_NOT_INIT`` - The log device has not been initialized.
- ``TFM_HAL_ERROR_INVALID_INPUT`` - Invalid inputs when ``str`` is ``NULL`` or
  ``len`` is zero.

**Note**

None.

tfm_hal_output_spm_log()
^^^^^^^^^^^^^^^^^^^^^^^^
**Prototype**

.. code-block:: c

  int32_t tfm_hal_output_spm_log(const unsigned char *str, uint32_t len)

**Description**

This API is called by :term:`SPM` to output logs.

**Parameter**

- ``str`` - The string to output.
- ``len`` - Length of the string in bytes.

**Return Values**

- Positive numbers - Number of bytes output.
- ``TFM_HAL_ERROR_NOT_INIT`` - The log device has not been initialized.
- ``TFM_HAL_ERROR_INVALID_INPUT`` - Invalid inputs when ``str`` is ``NULL``
  or ``len`` is zero.

**Note**

Please check :doc:`TF-M log system <tfm_log_system_design_document>` for more
information.

--------------

*Copyright (c) 2020, Arm Limited. All rights reserved.*