LIRC libraries: Driver API manual excerpt

LIRC libraries Linux Infrared Remote Control

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Driver API manual excerpt

Driver API manual excerpt

# Documentation of the LIRC driver API version 2.

This document describes the API used by the LIRC userspace drivers in version 2.

It is targeted at active C programmers, and not written with the intention to be understandable by non-programmers.

The guide aims at being fully compatible with the upcoming release LIRC 0.9.2. (CLARIFY)

# Introduction

There are three kinds of drivers in LIRC:

1. Kernel drivers, also called kernel modules.
2. User space, statically linked drivers.
3. User space, dynamically linked drivers.

Kernel drivers will not be covered in the present article. User space drivers are nothing else but normal compiled C functions, running as the invoking user (which may or may not be root). Therefore, the decomposition between "program" and driver is a priori not always necessary, but instead serves modularization and structuring.

In the beginning, due to its focus on very simple hardware, LIRC was very centered around kernal modules like directly connected LEDs or IR sensors. Modern hardware, due to embedded micro processors etc., have less requirements on precise timing, and user space drivers are normally the preferred solution. One such driver consists of a C file, containing a number of function, which are linked into the executing program (e. g., Lircd). Traditionally (at least for LIRC) linking takes place during the build process (statically linking). The API for these drivers we will call "Driver API version 1".

Dynamically loaded drivers was introduced LIRC just recently, replacing the statically linked drivers. It turned out to be necessary to slighty augment the driver API version 1. The new version will be referred to as "Driver API version 2".

In this guide, the word plugin refers to the so-file on disk. Each such file contains one or more driver(s) visible in the user interface.

# Two flavors of drivers

An IR signal consists of a sequence of on-times (pulses) and off-times (gaps). (We disregard modulation for the time being.) A full-fledged IR driver, on reading, can deliver the timing of these period ("durations"). On sending, it can be fed with a list of durations, and it sends these on- and off-periods as (modulated) IR signals. Such a piece of hardware is suitable for receiving and/or transmitting "arbitrary" IR sequences.

Much IR hardware was not designed as general-purpose IR hardware, but to allow e.g. a laptop computer or a TV card to be be controlled by the supplied remote. This hardware in general decodes the IR signal in its own hardware, and delivers, for recognized signals, an integer code denoting the code received (like "play"). For signals not following the protocol, no output is generated. This property is indicated in the drivers as the feature `LIRC_CAN_REC_LIRCCODE`.

As usable and flexible IR hardware for LIRC, these devices are second choice. The "learns" (configuration files) from such a device is of very limited value, and not portable to other IR devices, since timing information is missing. In the configuration file, this can be determined by the lines `one 0 0` and `one 0 0`, which is clearly not usable without the context, namely the hardware used for capturing.

This guide is intended for programmers writing and maintain "real" drivers.

At the time of writing, there are about 44 plugins and 54 drivers. Of the drivers, 13 are "general receivers"; the others are LIRCCODE drivers. (The actual numbers vary depending on configuration i. e. libraries installed on the build system.) Since hardware related to LIRCCODE drivers (typically a TV-card) normally doesn't support sending there is no LIRCCODE driver which can send data.

The tool lirc-lsplugins can be used to generate overviews of the drivers and their capabilities.

# The C interface

A (user space) driver is a C file where all the functions are declared `static`, i.e. not (directly) visible from outside of the file. The only thing visible from outside is a particular data structure, containing some data elements, and some pointers to functions, that in these way effectively are made public API. In this way a certain encapsulation is achieved. The data structure will be described next.

## The `struct driver`

The `struct driver` is the data structure representing the driver for LIRC. It is defined in `driver.h` (in directory `lib`).

    <pre>

struct driver { char device; int fd; __u32 features; __u32 send_mode; __u32 rec_mode; __u32 code_length; int (*init_func) (void); int (*deinit_func) (void); int (*send_func) (struct ir_remote remote, struct ir_ncode* code); char* (rec_func) (struct ir_remote remotes); int (decode_func) (struct ir_remote remote, struct decode_ctx_t* ctx); int (drvctl_func) (unsigned int cmd, void arg); lirc_t(*readdata) (lirc_t timeout); char *name; unsigned int resolution;

/* The following fields are API version 2 extensions */ const int api_version; const char* driver_version; const char* info; int (const close_func)(void); int (*const open_func)(const char device); };

These fields will next be described. Note that a driver sometimes "misuses" a field; e.g. the UDP driver expects a port number (as string) in the `device` field.

Also note that some function pointer may be NULL, indicating that the driver does not implement the said functionality (e.g. sending of IR signals).

The structures `ir_remote, ir_ncode` and, `ir_code` are declared in the file `ir_remote_types.h`. The data type `lirc_t` (what a meaningful name!) is an integer value used in many places for storing timing data. It is defined by `#define int` in `lirc.h`.

device

  Typically the name of the device as text string in Linux. This corresponds to the `–device` argument to lircd Not all drivers respect the device field as input; some have hardcoded device name(s), some use autodetecting code.

fd

  A file descriptor associated with the driver.

features
  Code for the features of the present device, see Appendix. It consist of the bitwise or of a number of possible features.
send_mode
  Use `LIRC_MODE_PULSE`.
rec_mode
  Use `LIRC_MODE_MODE2`.
code_length
  Only of relevance for LIRC_MODE_LIRCCODE drivers, for which it is the size in bits of the LIRCCODE.
init_func
  Function pointer, see below.
deinit_func
  Function pointer, see below.
send_func
  Function pointer, see below. NULL if the driver cannot transmit.
rec_func
  Function pointer, see below. NULL if the driver cannot receive.
decode_func
  Function pointer, for "real" drivers (not using LIRC_MODE_LIRCCODE), just use `receive_decode`, a function defined in `receive.c`.
drvctl_func
  Function pointer, see below. May be NULL if there are no ioctls.
readdata
  Function pointer, see below. Is NULL if the driver reads uses LIRC_MODE_LIRCCODE.
name
  Name of the driver in a human readable form, as listed e.g. by `lircd –driver help` or `irrecord –driver help`. Although not enforced, it is recommended to use names following the syntax for C identifiers.
resolution
  The resolution in microseconds of the recorded durations when reading signals. Used as default value for the `aeps` parameter in remotes.
api_version
  Use 2.
driver_version
  Free form string used to identify the version of the driver, if desired.
info
  Free form string, overall driver information.
open_func
  Function pointer, see below.
close_func
  Function pointer, see below.

## Driver lifecycle and `lircd`

The driver is loaded, used and unloaded by the executing program according to the following:

1. The program Lircd code determines what driver to use (normally the `–driver` runtime option) and invokes `hw_choose_driver()` (in `drv_admin.c`) which loads the driver and exposes the driver interface in the global variable `drv` (which is a `struct driver`).
2. `Lircd` invokes the `open_func()` which establishes the device to use, usually based on the runtime option `–device`.
3. When a client connects, either for sending or receiving, the function `init_func()` is invoked, initializing the driver. In some drivers, the capabilities of the drivers, i.e. the field `features`, are available only after initializing. The driver also exports the `fd` field, a file descriptor (a positive int) for the underlying file.
4. From this point, calls to `read_func()` and `send_func()` are possible.
5. When the last client is terminated, executing program calls `deinit_func()`. Note that the Lircd program may need the device again e.g., if another sending request is received, so a hard closing of a device may not be optimal here.
6. As a last operation, executing program should call the `close_func()`. This relinquishes all resources allocated by the driver.
7. Calls to `drvctl_func()` are possible at any time. They are the responsibility of the programmer. In particular, ioctl operations may fail under some circumstances.
8. The associated functions are not guaranteed to be executed in the order described. In general, the driver should not break if e.g., the `init_func()` is called on an already open device, or `deinit_func()` called on a device not open.

## Driver structure

Typical structure of a driver file will be shown next.

    /* Include system includes files as needed */

     /* The overall include, mandatory, often sufficient.  */
          /* For drivers using serial hardware. */

    /* Global (but "static") variable definition. */

    /* static function definitions. */
    static int generic_init() {
        /* Open device/hardware */

        send_buffer_init();
        return 1;
    }

    static int generic_deinit(void) {
        /* close device */
        return 1;
    }

    static int generic_send(struct ir_remote* remote, struct ir_ncode* code) {
        result = send_buffer_put(remote, code);
        if (!result)
            return 0;

        /* Payload signal is now available in global variable send_buf. */
        /* Process sendbuf (see transmit.h). */
        return success;
    }

    static char *rec_func(struct ir_remote* remotes) {
        if (!rec_buffer_clear) {
            /* handle errors */
            return NULL;
        }
        return decode_all(remotes);
    }

    static char *decode_func(struct ir_remote* remote, struct decode_ctx_t* ctx) {
        return receive_decode(remote, ctx);
    }

    static int generic_drvctl(unsigned int cmd, void *arg) {
        /* Do something device specific, e.g.
           return ioctl(drv.fd, cmd, arg); */
        return cmd == 42 ? 1 : 0;
    }

    static lirc_t *readdata(lirc_t timeout) {
        /* compute and return the next duration in microseconds */
    }

    struct driver my_driver = {
       /* see above. */
    }

    struct driver* hardwares[] = { &my_hardware, NULL };

### Syntax and semantics of the functions

Note that, even if implementing the functions in C++, there is no need to declare them as `external "C"`.

#### `init_func`

`int myinit_func(void)`

Function called for initializing the driver and the hardware. It should return nonzero in the case of success. This function simple opens the device. In addition, the functions `rec_buffer_init(void)` and/or `send_buffer_init(void)` needs to be called.

#### `deinit_func`

`int mydeinit_func(void)`

Function called for suspending the driver. Zero return value indicates failure, all other return values success.

#### `send_func`

`int mysend_func(struct ir_remote *remote, struct ir_ncode *code)`

Function called for sending an IR code, residing in the second argument. For this, the function `send_buffer_put(struct ir_remote *remote, struct ir_ncode *code)` is called, with the same arguments. This makes the global variable `send_bug` containing the durations in micro seconds for the IR signal. The field `data` is a pointer to `wptr` number of numbers, having the data type `lirc_t`. Returns non-zero if operation successful.

Called from `ir_remote.c`.

#### `rec_func`

`char* myrec_func(struct ir_remote* remotes)`

The canonical implementation for non-LIRCCODE drivers is

            if (!rec_buffer_clear) {
                /* handle errors */
                return NULL;
            }
            return decode_all(remotes);

This is a non-blocking function which returns NULL if there is no data available currently, but might very well succeed later. The device might get closed as part of error handling, so calling code should be prepared to reopen device if required.

#### `decode_func`

`char mydecode_func(struct ir_remote * remote, struct decode_ctx_t ctx)`

For non-LIRC_MODE_LIRCCODE, just call `receive_decode`, or write `receive_decode` directly in the `struct driver`.

#### `drvctl_func`

`mydrvctl_func(unsigned int cmd, void *arg)`

Depending on the particular driver and hardware, additional functionality can be implemented here, with semantics as determined by the driver. There are some cmd definitions in driver.h which could be used by any driver. Driver-specific cmd constants should be > DRVCTL_MAX.

drvctl() returns 0 if OK, else a positive error code some of which defined in driver.h

driver.h defines the DRVCTL_SET_OPTION cmd. This is a generic way to set a named key to value which should cover many (most?) needs for the drvctl() function.

#### `readdata`

`lirc_t myreaddata(lirc_t timeout)`

This function returns one integer of read information from the device. For this, it may wait for a time determined by the argument, in micro seconds. A wait time of 0 (or < 0) blocks indefinitely. The return value has the semantic of a duration (pulse or gap) in microseconds. For this reason, 0 is an "impossible" value, and would indicate an error. In LIRC, only the lower 24 bits are used for the length of the duration (making the largest duration that LIRC can represent 2^24 - 1 = 16777215 microseconds). See the example code.

The function is called from the daemon Lircd as well as from irrecord, and mode2.

#### `close_func`

`int close_func(void)`

Hard close of the device. zero return value indicates success, other values indicates an error. Some standard error codes are defined in driver.h.

#### `open_func`

`int open_func(void)`

Open the device. This is the basic, possibly expensive steps taken to make the device usable. Returns 0 on success, else an error code, some of which defined in driver.h.

When running using the effective-user option, this function is called running as root - other functions are called running as the effective-user optionn.

### Comments

- Since no elements are exported, except for through the `hardwares` array, an associated header file (`.h`-file) is neither necessary nor desired. (The only exception would be special constants for usage with the drvctl function.)
- In the interest of portability and encapsulation, the only LIRC files that should be included are `lirc_driver.h` (which includes `driver.h, lirc_log.h, receive.h`, and `transmit.h`) and if required also `serial.h`. Inclusions of other LIRC files (including `config.h`) should be avoided.
- The LIRC name of the driver is taken from the `name` field of the `hardware` struct. The file name is thus irrelevant. For files containing only one driver, it is recommended to keep the name of the file equal to the name of the driver (with added file extension).

## Generating console output

It is possible to generate console output in any way; writing on `stdout` or `stderr`, using e.g. stdio or C++ streams. However, to conform with the working of the rest of LIRC, it is recommended to use only the various `void log_(const char *format_str, ...)` and `void log_perror_ (const char *format, ...)` which are declared in the header `lirc_log.h`. The normal set of`log_error(), log_warn(), log_notice() ...` and `log_perror_err(), log_perror_warn(), log_perror_debug()` etc. are available

## Timing issues

### Sending

The daemon `Lircd` takes care of the timing between the IR sequences, calling the `send_func` as it sees fit. Also, the final gap is the responsibility of Lircd. For example, if a signal is to be sent repeatedly every x milliseconds, `Lircd` will take care of the timing, at least as long as `send_func` does not consume too much time.

### Receiving

Except for the timeout argument to `readdata`, there does not appear to be any timing issues the driver author needs to address.

## Compiling

### Compiling in-tree

To compile in-tree and assuming autogen.sh and ./configure has been run just copy the plugin source code to the plugins directory and run

        $ cd plugins;
        $ ./make-pluginlist.sh > pluginlist.am
        $ make

### Compiling out-of-tree

Plugins can also easily be built out-of tree. Only some include files from LIRC are needed. Just a trivial compilation is needed. A simple generic Makefile is provided in the Appendix, another example is available in the sources as Makefile. The autotools are not needed, in particular not libtool.

## The anatomy of a complete driver package.

A complete driver package consists of three files: the driver code, the configuration support and the documentation.

    lirc
     |
     |-----doc
     |      |
     |      |
     |     plugindocs ------
     |                     |
     |                  foo.html
     |-----configs
     |        |
     |     foo.conf
     |
     |----plugins
              |
           foo.so

<p<blockquote>‍

The driver code has been described all over in this document. There is also plenty of examples in the plugins/ directory. The compiled .so file could just be dropped into a directory in the -U/–plugins search path which basically could be anywhere.

The configuration support is a single file in the configs/ directory. The file configs/README describes the format. This is used by tools like lirc-setup, but also to create the list of all drivers. New files are automatically picked up.

The documentation is a single html file in the doc/plugindocs directory. The plugindocs directory contains a makefile which could be used to update the main html documents when the plugindocs/ contents have changed. See example in drivers/default/Makefile

The lirc-driver pkgconfig file defines three variables useful when installing files: plugindir, configdir and plugindocs. These are the configured locations for each file. E. g., to get the plugindir location

            $ pkg-config --variable=plugindir lirc-driver
            /usr/lib64/lirc/plugins

## Using C++

Plugins can also be written in C++. The used include files are required to be "C++-safe" i. e., the functions to be called from C++ have to be declared `extern "C"`, to make sure that the generated code follows C's calling conventions. Since the plugin code is called only indirectly through the hardware struct, no `extern "C"` declarations are required in the plugin code.

### Adding additional libraries

Extra libraries, as well as include files, can be simply added to the Makefile (or Makefile.am for the case of in-tree builds). Note that for libraries located outside of the "standard" directories, it may be required to use an `-L` and an `-rpath` argument to the linker arguments. See the Makefile in Appendix 2.

# Appendix. List of features (`lirc.h`).

There are four "modes of operation" of LIRC: LIRC_MODE_RAW, LIRC_MODE_PULSE, LIRC_MODE_MODE2, LIRC_MODE_LIRCCODE. In the current code, no semantic difference between the first three can be inferred. For the meaning of the last, see above ("Two flavors of drivers").

There are a number of "features" that a driver can have or not have. These are documented in the lirc(4)manpage which has been upstreamed from the LIRC project. The last LIRC version is available in the sources.

# Appendix 2. Generic Makefile

    LIRC_SRC=/home/bengt/lirc/master

    PLUGINDIR := /home/bengt/lirc/root/lib/lirc/plugins

    MACHINE := -m64
    INCLUDE := -I$(LIRC_SRC)/lib -I$(LIRC_SRC)
    OPTIMIZE := -O2
    DEBUG := -g
    SHARED :=-shared -fPIC
    WARNINGS=-Wall
    CC := gcc
    CPP := g++

    %.so: %.c
                    -o $@ $<

    %.so: %.cpp
                   -o $@ $<

    default:
            @echo "There is no default target in this makefile."
            @echo "Type \"make plugin.so\" to compile the plugin named plugin,"
            @echo "and \"make install\" to install it"

    install:
            cp *.so

    clean:
            rm -f *.so