romlib-design.rst

Library at ROM
==============

This document provides an overview of the "library at ROM" implementation in
Trusted Firmware-A (TF-A).

Introduction
~~~~~~~~~~~~

The "library at ROM" feature allows platforms to build a library of functions to
be placed in ROM. This reduces SRAM usage by utilising the available space in
ROM. The "library at ROM" contains a jump table with the list of functions that
are placed in ROM. The capabilities of the "library at ROM" are:

1. Functions can be from one or several libraries.

2. Functions can be patched after they have been programmed into ROM.

3. Platform-specific libraries can be placed in ROM.

4. Functions can be accessed by one or more BL images.

Index file
~~~~~~~~~~

.. image:: diagrams/romlib_design.png
    :width: 600

Library at ROM is described by an index file with the list of functions to be
placed in ROM. The index file is platform specific and its format is:

::

    lib function    [patch]

    lib      -- Name of the library the function belongs to
    function -- Name of the function to be placed in library at ROM
    [patch]  -- Option to patch the function

It is also possible to insert reserved spaces in the list by using the keyword
"reserved" rather than the "lib" and "function" names as shown below:

::

    reserved    reserved

The reserved spaces can be used to add more functions in the future without
affecting the order and location of functions already existing in the jump
table. Also, for additional flexibility and modularity, the index file can
include other index files.

For an index file example, refer to ``lib/romlib/jmptbl.i``.

Wrapper functions
~~~~~~~~~~~~~~~~~

.. image:: diagrams/romlib_wrapper.png
    :width: 600

When invoking a function of the "library at ROM", the calling sequence is as
follows:

BL image --> wrapper function --> jump table entry --> library at ROM

The index file is used to create a jump table which is placed in ROM. Then, the
wrappers refer to the jump table to call the "library at ROM" functions. The
wrappers essentially contain a branch instruction to the jump table entry
corresponding to the original function. Finally, the original function in the BL
image(s) is replaced with the wrapper function.

The "library at ROM" contains a necessary init function that initialises the
global variables defined by the functions inside "library at ROM".

Scripts
~~~~~~~

There are several scripts that generate the necessary files for the "library at
ROM" to work:

1. ``gentbl.sh`` - Generates the jump table by parsing the index file.

2. ``genvar.sh`` - Generates the jump table global variable (**not** the jump
   table itself) with the absolute address in ROM. This global variable is,
   basically, a pointer to the jump table.

3. ``genwrappers.sh`` - Generates a wrapper function for each entry in the index
   file except for the ones that contain the keyword ``patch``. The generated
   wrapper file is called ``<lib>_<fn_name>.S``.

Patching of functions in library at ROM
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The ``genwrappers.sh`` script does not generate wrappers for the entries in the
index file that contain the keyword ``patch``. Thus, it allows calling the
function from the actual library by breaking the link to the  "library at ROM"
version of this function.

The calling sequence for a patched function is as follows:

BL image --> function

Build library at ROM
~~~~~~~~~~~~~~~~~~~~~

The environment variable ``CROSS_COMPILE`` must be set as per the user guide.
In the below example the usage of ROMLIB together with mbed TLS is demonstrated
to showcase the benefits of library at ROM - it's not mandatory.

.. code:: shell

    make PLAT=fvp                                                   \
    MBEDTLS_DIR=</path/to/mbedtls/>                                 \
    TRUSTED_BOARD_BOOT=1 GENERATE_COT=1                             \
    ARM_ROTPK_LOCATION=devel_rsa                                    \
    ROT_KEY=plat/arm/board/common/rotpk/arm_rotprivk_rsa.pem        \
    BL33=</path/to/bl33.bin>                                        \
    USE_ROMLIB=1                                                    \
    all fip

Known issue
-----------
When building library at ROM, a clean build is always required. This is
necessary when changes are made to the index files, e.g. adding new functions,
patching existing ones etc.

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

*Copyright (c) 2019, Arm Limited. All rights reserved.*