1Library at ROM 2============== 3 4This document provides an overview of the "library at ROM" implementation in 5Trusted Firmware-A (TF-A). 6 7Introduction 8~~~~~~~~~~~~ 9 10The "library at ROM" feature allows platforms to build a library of functions to 11be placed in ROM. This reduces SRAM usage by utilising the available space in 12ROM. The "library at ROM" contains a jump table with the list of functions that 13are placed in ROM. The capabilities of the "library at ROM" are: 14 151. Functions can be from one or several libraries. 16 172. Functions can be patched after they have been programmed into ROM. 18 193. Platform-specific libraries can be placed in ROM. 20 214. Functions can be accessed by one or more BL images. 22 23Index file 24~~~~~~~~~~ 25 26.. image:: ../resources/diagrams/romlib_design.png 27 :width: 600 28 29Library at ROM is described by an index file with the list of functions to be 30placed in ROM. The index file is platform specific and its format is: 31 32:: 33 34 lib function [patch] 35 36 lib -- Name of the library the function belongs to 37 function -- Name of the function to be placed in library at ROM 38 [patch] -- Option to patch the function 39 40It is also possible to insert reserved spaces in the list by using the keyword 41"reserved" rather than the "lib" and "function" names as shown below: 42 43:: 44 45 reserved reserved 46 47The reserved spaces can be used to add more functions in the future without 48affecting the order and location of functions already existing in the jump 49table. Also, for additional flexibility and modularity, the index file can 50include other index files. 51 52For an index file example, refer to ``lib/romlib/jmptbl.i``. 53 54Wrapper functions 55~~~~~~~~~~~~~~~~~ 56 57.. image:: ../resources/diagrams/romlib_wrapper.png 58 :width: 600 59 60When invoking a function of the "library at ROM", the calling sequence is as 61follows: 62 63BL image --> wrapper function --> jump table entry --> library at ROM 64 65The index file is used to create a jump table which is placed in ROM. Then, the 66wrappers refer to the jump table to call the "library at ROM" functions. The 67wrappers essentially contain a branch instruction to the jump table entry 68corresponding to the original function. Finally, the original function in the BL 69image(s) is replaced with the wrapper function. 70 71The "library at ROM" contains a necessary init function that initialises the 72global variables defined by the functions inside "library at ROM". 73 74Scripts 75~~~~~~~ 76 77There are several scripts that generate the necessary files for the "library at 78ROM" to work: 79 801. ``gentbl.sh`` - Generates the jump table by parsing the index file. 81 822. ``genvar.sh`` - Generates the jump table global variable (**not** the jump 83 table itself) with the absolute address in ROM. This global variable is, 84 basically, a pointer to the jump table. 85 863. ``genwrappers.sh`` - Generates a wrapper function for each entry in the index 87 file except for the ones that contain the keyword ``patch``. The generated 88 wrapper file is called ``<lib>_<fn_name>.S``. 89 90Patching of functions in library at ROM 91~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 92 93The ``genwrappers.sh`` script does not generate wrappers for the entries in the 94index file that contain the keyword ``patch``. Thus, it allows calling the 95function from the actual library by breaking the link to the "library at ROM" 96version of this function. 97 98The calling sequence for a patched function is as follows: 99 100BL image --> function 101 102Build library at ROM 103~~~~~~~~~~~~~~~~~~~~~ 104 105The environment variable ``CROSS_COMPILE`` must be set as per the user guide. 106In the below example the usage of ROMLIB together with mbed TLS is demonstrated 107to showcase the benefits of library at ROM - it's not mandatory. 108 109.. code:: shell 110 111 make PLAT=fvp \ 112 MBEDTLS_DIR=</path/to/mbedtls/> \ 113 TRUSTED_BOARD_BOOT=1 GENERATE_COT=1 \ 114 ARM_ROTPK_LOCATION=devel_rsa \ 115 ROT_KEY=plat/arm/board/common/rotpk/arm_rotprivk_rsa.pem \ 116 BL33=</path/to/bl33.bin> \ 117 USE_ROMLIB=1 \ 118 all fip 119 120Known issue 121----------- 122When building library at ROM, a clean build is always required. This is 123necessary when changes are made to the index files, e.g. adding new functions, 124patching existing ones etc. 125 126-------------- 127 128*Copyright (c) 2019, Arm Limited. All rights reserved.* 129
