README.md
1# libpldm
2
3This is a library which deals with the encoding and decoding of PLDM messages.
4It should be possible to use this library by projects other than OpenBMC, and
5hence certain constraints apply to it:
6
7- keeping it light weight
8- implementation in C
9- minimal dynamic memory allocations
10- endian-safe
11- no OpenBMC specific dependencies
12
13Source files are named according to the PLDM Type, for eg base.[h/c], fru.[h/c],
14etc.
15
16Given a PLDM command "foo", the library will provide the following API: For the
17Requester function:
18
19```c
20encode_foo_req() - encode a foo request
21decode_foo_resp() - decode a response to foo
22```
23
24For the Responder function:
25
26```c
27decode_foo_req() - decode a foo request
28encode_foo_resp() - encode a response to foo
29```
30
31The library also provides API to pack and unpack PLDM headers.
32
33## To Build
34
35`libpldm` is configured and built using `meson`. Python's `pip` or
36[`pipx`][pipx] can be used to install a recent version on your machine:
37
38[pipx]: https://pipx.pypa.io/latest/
39
40```sh
41pipx install meson
42```
43
44Once `meson` is installed:
45
46```sh
47meson setup build && meson compile -C build
48```
49
50## To run unit tests
51
52```sh
53meson test -C build
54```
55
56## Working with `libpldm`
57
58Components of the library ABI[^1] (loosely, functions) are separated into three
59categories:
60
61[^1]: ["library API + compiler ABI = library ABI"][libstdc++-library-abi]
62
63[libstdc++-library-abi]:
64 https://gcc.gnu.org/onlinedocs/libstdc++/manual/abi.html
65
661. Stable
672. Testing
683. Deprecated
69
70Applications depending on `libpldm` should aim to only use functions from the
71stable category. However, this may not always be possible. What to do when
72required functions fall into the deprecated or testing categories is outlined
73below.
74
75### What does it mean to mark a function as stable?
76
77Marking a function as stable makes the following promise to users of the
78library:
79
80> We will not remove or change the symbol name, argument count, argument types,
81> return type, or interpretation of relevant values for the function before
82> first marking it as `LIBPLDM_ABI_DEPRECATED` and then subsequently creating a
83> tagged release
84
85Marking a function as stable does _not_ promise that it is free of
86implementation bugs. It is just a promise that the prototype won't change
87without notice.
88
89Given this, it is always okay to implement functions marked stable in terms of
90functions marked testing inside of libpldm. If we remove or change the prototype
91of a function marked testing the only impact is that we need to fix up any call
92sites of that function in the same patch.
93
94### The ABI lifecycle
95
96```mermaid
97---
98title: libpldm symbol lifecycle
99---
100stateDiagram-v2
101 direction LR
102 [*] --> Testing: Add
103 Testing --> Testing: Change
104 Testing --> [*]: Remove
105 Testing --> Stable: Stabilise
106 Stable --> Deprecated: Deprecate
107 Deprecated --> [*]: Remove
108```
109
110The ABI of the library produced by the build is controlled using the `abi` meson
111option. The following use cases determine how the `abi` option should be
112specified:
113
114| Use Case | Meson Configuration |
115| ----------- | --------------------------------- |
116| Production | `-Dabi=deprecated,stable` |
117| Maintenance | `-Dabi=stable` |
118| Development | `-Dabi=deprecated,stable,testing` |
119
120### Maintenance
121
122Applications and libraries that depend on `libpldm` can identify how to migrate
123off of deprecated APIs by constraining the library ABI to the stable category.
124This will force the compiler identify any call-sites that try to link against
125deprecated symbols.
126
127### Development
128
129Applications and libraries often require functionality that doesn't yet exist in
130`libpldm`. The work is thus in two parts:
131
1321. Add the required APIs to `libpldm`
1332. Use the new APIs from `libpldm` in the dependent application or library
134
135Adding APIs to a library is a difficult task. Generally, once an API is exposed
136in the library's ABI, any changes to the API risk breaking applications already
137making use of it. To make sure we have more than one shot at getting an API
138right, all new APIs must first be exposed in the testing category. Concretely:
139
140Patches adding new APIs MUST mark them as testing and MUST NOT mark them as
141stable.
142
143### Marking functions as testing, stable or deprecated
144
145Three macros are provided through `config.h` (automatically included for all
146translation units) to mark functions as testing, stable or deprecated:
147
1481. `LIBPLDM_ABI_TESTING`
1492. `LIBPLDM_ABI_STABLE`
1503. `LIBPLDM_ABI_DEPRECATED`
151
152These annotations go immediately before your function signature:
153
154```c
155LIBPLDM_ABI_TESTING
156pldm_requester_rc_t pldm_transport_send_msg(struct pldm_transport *transport,
157 pldm_tid_t tid,
158 const void *pldm_req_msg,
159 size_t req_msg_len)
160{
161 ...
162}
163```
164
165### Requirements for stabilising a function
166
167As mentioned above, all new functions must first be added in the testing
168category (using the `LIBPLDM_ABI_TESTING` annotation).
169
170To move a function from the testing category to the stable category, its
171required that patches demonstrating use of the function in a dependent
172application or library be linked in the commit message of the stabilisation
173change. We require this to demonstrate that the implementer has considered its
174use in context _before_ preventing us from making changes to the API.
175
176### Building a dependent application or library against a testing ABI
177
178Meson is broadly used in the OpenBMC ecosystem, the historical home of
179`libpldm`. Meson's subprojects are a relatively painless way of managing
180dependencies for the purpose of developing complex applications and libraries.
181Use of `libpldm` as a subproject is both supported and encouraged.
182
183`libpldm`'s ABI can be controlled from a parent project through meson's
184subproject configuration syntax:
185
186```shell
187meson setup ... -Dlibpldm:abi=deprecated,stable,testing ...
188```
189
190## OEM/vendor-specific functions
191
192libpldm accepts support for OEM or vendor-specific extensions. To add support
193for an OEM's extensions:
194
1951. Document the wire format for all OEM messages under `docs/oem/${OEM_NAME}/`
196
1972. Add public OEM API declarations and definitions under
198 `include/libpldm/oem/${OEM_NAME}/`, and install them to the same relative
199 location.
200
2013. Implement the public OEM APIs under `src/oem/${OEM_NAME}/`
202
2034. Implement the OEM API tests under `tests/oem/${OEM_NAME}/`
204
205The `${OEM_NAME}` folder must be created with the name of the OEM/vendor in
206lower case.
207
208Finally, the OEM name must be added to the list of choices for the `oem` meson
209option, and the `meson.build` files updated throughout the tree to guard
210integration of the OEM extensions.
211