135 lines
4.7 KiB
ReStructuredText
135 lines
4.7 KiB
ReStructuredText
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Library at ROM
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==============
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This document provides an overview of the "library at ROM" implementation in
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Trusted Firmware-A (TF-A).
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Introduction
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~~~~~~~~~~~~
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The "library at ROM" feature allows platforms to build a library of functions to
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be placed in ROM. This reduces SRAM usage by utilising the available space in
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ROM. The "library at ROM" contains a jump table with the list of functions that
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are placed in ROM. The capabilities of the "library at ROM" are:
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1. Functions can be from one or several libraries.
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2. Functions can be patched after they have been programmed into ROM.
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3. Platform-specific libraries can be placed in ROM.
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4. Functions can be accessed by one or more BL images.
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Index file
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~~~~~~~~~~
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.. image:: ../resources/diagrams/romlib_design.png
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:width: 600
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Library at ROM is described by an index file with the list of functions to be
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placed in ROM. The index file is platform specific and its format is:
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::
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lib function [patch]
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lib -- Name of the library the function belongs to
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function -- Name of the function to be placed in library at ROM
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[patch] -- Option to patch the function
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It is also possible to insert reserved spaces in the list by using the keyword
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"reserved" rather than the "lib" and "function" names as shown below:
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::
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reserved
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The reserved spaces can be used to add more functions in the future without
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affecting the order and location of functions already existing in the jump
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table. Also, for additional flexibility and modularity, the index file can
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include other index files.
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For an index file example, refer to ``lib/romlib/jmptbl.i``.
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Wrapper functions
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~~~~~~~~~~~~~~~~~
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.. image:: ../resources/diagrams/romlib_wrapper.png
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:width: 600
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When invoking a function of the "library at ROM", the calling sequence is as
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follows:
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BL image --> wrapper function --> jump table entry --> library at ROM
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The index file is used to create a jump table which is placed in ROM. Then, the
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wrappers refer to the jump table to call the "library at ROM" functions. The
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wrappers essentially contain a branch instruction to the jump table entry
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corresponding to the original function. Finally, the original function in the BL
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image(s) is replaced with the wrapper function.
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The "library at ROM" contains a necessary init function that initialises the
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global variables defined by the functions inside "library at ROM".
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Script
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~~~~~~
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There is a ``romlib_generate.py`` Python script that generates the necessary
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files for the "library at ROM" to work. It implements multiple functions:
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1. ``romlib_generate.py gentbl [args]`` - Generates the jump table by parsing
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the index file.
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2. ``romlib_generator.py genvar [args]`` - Generates the jump table global
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variable (**not** the jump table itself) with the absolute address in ROM.
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This global variable is, basically, a pointer to the jump table.
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3. ``romlib_generator.py genwrappers [args]`` - Generates a wrapper function for
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each entry in the index file except for the ones that contain the keyword
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``patch``. The generated wrapper file is called ``<fn_name>.s``.
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4. ``romlib_generator.py pre [args]`` - Preprocesses the index file which means
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it resolves all the include commands in the file recursively. It can also
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generate a dependency file of the included index files which can be directly
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used in makefiles.
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Each ``romlib_generate.py`` function has its own manual which is accessible by
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runing ``romlib_generator.py [function] --help``.
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``romlib_generate.py`` requires Python 3 environment.
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Patching of functions in library at ROM
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The ``romlib_generator.py genwrappers`` does not generate wrappers for the
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entries in the index file that contain the keyword ``patch``. Thus, it allows
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calling the function from the actual library by breaking the link to the
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"library at ROM" version of this function.
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The calling sequence for a patched function is as follows:
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BL image --> function
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Build library at ROM
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~~~~~~~~~~~~~~~~~~~~~
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The environment variable ``CROSS_COMPILE`` must be set as per the user guide.
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In the below example the usage of ROMLIB together with mbed TLS is demonstrated
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to showcase the benefits of library at ROM - it's not mandatory.
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.. code:: shell
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make PLAT=fvp \
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MBEDTLS_DIR=</path/to/mbedtls/> \
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TRUSTED_BOARD_BOOT=1 GENERATE_COT=1 \
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ARM_ROTPK_LOCATION=devel_rsa \
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ROT_KEY=plat/arm/board/common/rotpk/arm_rotprivk_rsa.pem \
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BL33=</path/to/bl33.bin> \
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USE_ROMLIB=1 \
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all fip
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--------------
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*Copyright (c) 2019, Arm Limited. All rights reserved.*
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