LIRC stands for Linux Infrared Remote Control. The LIRC device interface is a bi-directional interface for transporting raw IR and decoded scancodes data between userspace and kernelspace. Fundamentally, it is just a chardev (/dev/lircX, for X = 0, 1, 2, …), with a number of standard struct file_operations defined on it. With respect to transporting raw IR and decoded scancodes to and fro, the essential fops are read, write and ioctl.
It is also possible to attach a BPF program to a LIRC device for decoding raw IR into scancodes.
Example dmesg output upon a driver registering w/LIRC:
$ dmesg |grep lirc_dev rc rc0: lirc_dev: driver mceusb registered at minor = 0, raw IR receiver, raw IR transmitter
What you should see for a chardev:
$ ls -l /dev/lirc* crw-rw---- 1 root root 248, 0 Jul 2 22:20 /dev/lirc0
Note that the package v4l-utils contains tools for working with LIRC devices:
ir-ctl: can receive raw IR and transmit IR, as well as query LIRC device features.
ir-keytable: can load keymaps; allows you to set IR kernel protocols; load BPF IR decoders and test IR decoding. Some BPF IR decoders are also provided.
6.2. LIRC modes¶
LIRC supports some modes of receiving and sending IR codes, as shown on the following table.
This mode is for both sending and receiving IR.
For transmitting (aka sending), create a
struct lirc_scancodewith the desired scancode set in the
rc_protoset to the IR protocol, and all other members set to 0. Write this struct to the lirc device.
For receiving, you read
struct lirc_scancodefrom the LIRC device. The
scancodefield is set to the received scancode and the IR protocol is set in
rc_proto. If the scancode maps to a valid key code, this is set in the
keycodefield, else it is set to
LIRC_SCANCODE_FLAG_TOGGLEset if the toggle bit is set in protocols that support it (e.g. rc-5 and rc-6), or
LIRC_SCANCODE_FLAG_REPEATfor when a repeat is received for protocols that support it (e.g. nec).
In the Sanyo and NEC protocol, if you hold a button on remote, rather than repeating the entire scancode, the remote sends a shorter message with no scancode, which just means button is held, a “repeat”. When this is received, the
LIRC_SCANCODE_FLAG_REPEATis set and the scancode and keycode is repeated.
With nec, there is no way to distinguish “button hold” from “repeatedly pressing the same button”. The rc-5 and rc-6 protocols have a toggle bit. When a button is released and pressed again, the toggle bit is inverted. If the toggle bit is set, the
timestampfield is filled with the time nanoseconds (in
CLOCK_MONOTONIC) when the scancode was decoded.
The driver returns a sequence of pulse and space codes to userspace, as a series of u32 values.
This mode is used only for IR receive.
The upper 8 bits determine the packet type, and the lower 24 bits the payload. Use
LIRC_VALUE()macro to get the payload, and the macro
LIRC_MODE2()will give you the type, which is one of:
Signifies the presence of IR in microseconds.
Signifies absence of IR in microseconds.
If measurement of the carrier frequency was enabled with ioctl LIRC_SET_MEASURE_CARRIER_MODE then this packet gives you the carrier frequency in Hertz.
If timeout reports are enabled with ioctl LIRC_SET_REC_TIMEOUT_REPORTS, when the timeout set with ioctl LIRC_GET_REC_TIMEOUT and LIRC_SET_REC_TIMEOUT expires due to no IR being detected, this packet will be sent, with the number of microseconds with no IR.
In pulse mode, a sequence of pulse/space integer values are written to the lirc device using LIRC write().
The values are alternating pulse and space lengths, in microseconds. The first and last entry must be a pulse, so there must be an odd number of entries.
This mode is used only for IR send.
6.3. BPF based IR decoder¶
The kernel has support for decoding the most common IR protocols, but there are many protocols which are not supported. To support these, it is possible to load an BPF program which does the decoding. This can only be done on LIRC devices which support reading raw IR.
First, using the bpf(2) syscall with the
program must be loaded of type
BPF_PROG_TYPE_LIRC_MODE2. Once attached
to the LIRC device, this program will be called for each pulse, space or
timeout event on the LIRC device. The context for the BPF program is a
pointer to a unsigned int, which is a LIRC_MODE_MODE2
value. When the program has decoded the scancode, it can be submitted using
the BPF functions
bpf_rc_repeat(). Mouse or pointer
movements can be reported using
Once you have the file descriptor for the
program, it can be attached to the LIRC device using the bpf(2) syscall.
The target must be the file descriptor for the LIRC device, and the
attach type must be
BPF_LIRC_MODE2. No more than 64 BPF programs can be
attached to a single LIRC device at a time.