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
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
74Script
75~~~~~~
76
77There is a ``romlib_generate.py`` Python script that generates the necessary
78files for the "library at ROM" to work. It implements multiple functions:
79
801. ``romlib_generate.py gentbl [args]`` - Generates the jump table by parsing
81   the index file.
82
832. ``romlib_generator.py genvar [args]`` - Generates the jump table global
84   variable (**not** the jump table itself) with the absolute address in ROM.
85   This global variable is, basically, a pointer to the jump table.
86
873. ``romlib_generator.py genwrappers [args]`` - Generates a wrapper function for
88   each entry in the index file except for the ones that contain the keyword
89   ``patch``. The generated wrapper file is called ``<fn_name>.s``.
90
914. ``romlib_generator.py pre [args]`` - Preprocesses the index file which means
92   it resolves all the include commands in the file recursively. It can also
93   generate a dependency file of the included index files which can be directly
94   used in makefiles.
95
96Each ``romlib_generate.py`` function has its own manual which is accessible by
97runing ``romlib_generator.py [function] --help``.
98
99``romlib_generate.py`` requires Python 3 environment.
100
101
102Patching of functions in library at ROM
103~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
104
105The ``romlib_generator.py genwrappers`` does not generate wrappers for the
106entries in the index file that contain the keyword ``patch``. Thus, it allows
107calling the function from the actual library by breaking the link to the
108"library at ROM" version of this function.
109
110The calling sequence for a patched function is as follows:
111
112BL image --> function
113
114Memory impact
115~~~~~~~~~~~~~
116
117Using library at ROM will modify the memory layout of the BL images:
118
119- The ROM library needs a page aligned RAM section to hold the RW data. This
120  section is defined by the ROMLIB_RW_BASE and ROMLIB_RW_END macros.
121  On Arm platforms a section of 1 page (0x1000) is allocated at the top of SRAM.
122  This will have for effect to shift down all the BL images by 1 page.
123
124- Depending on the functions moved to the ROM library, the size of the BL images
125  will be reduced.
126  For example: moving MbedTLS function into the ROM library reduces BL1 and
127  BL2, but not BL31.
128
129- This change in BL images size can be taken into consideration to optimize the
130  memory layout when defining the BLx_BASE macros.
131
132Build library at ROM
133~~~~~~~~~~~~~~~~~~~~~
134
135The environment variable ``CROSS_COMPILE`` must be set appropriately. Refer to
136:ref:`Performing an Initial Build` for more information about setting this
137variable.
138
139In the below example the usage of ROMLIB together with mbed TLS is demonstrated
140to showcase the benefits of library at ROM - it's not mandatory.
141
142.. code:: shell
143
144    make PLAT=fvp                                                   \
145    MBEDTLS_DIR=</path/to/mbedtls/>                                 \
146    TRUSTED_BOARD_BOOT=1 GENERATE_COT=1                             \
147    ARM_ROTPK_LOCATION=devel_rsa                                    \
148    ROT_KEY=plat/arm/board/common/rotpk/arm_rotprivk_rsa.pem        \
149    BL33=</path/to/bl33.bin>                                        \
150    USE_ROMLIB=1                                                    \
151    all fip
152
153--------------
154
155*Copyright (c) 2019, Arm Limited. All rights reserved.*
156