The LIRC device interface is a bi-directional interface for transporting raw IR 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 data to and fro, the essential fops are read, write and ioctl.
Example dmesg output upon a driver registering w/LIRC:
$ dmesg |grep lirc_dev
lirc_dev: IR Remote Control driver registered, major 248
rc rc0: lirc_dev: driver ir-lirc-codec (mceusb) registered at minor = 0
What you should see for a chardev:
$ ls -l /dev/lirc*
crw-rw---- 1 root root 248, 0 Jul 2 22:20 /dev/lirc0
The lircd userspace daemon reads raw IR data from the LIRC chardev. The exact format of the data depends on what modes a driver supports, and what mode has been selected. lircd obtains supported modes and sets the active mode via the ioctl interface, detailed at the section called “LIRC ioctl fop”. The generally preferred mode is LIRC_MODE_MODE2, in which packets containing an int value describing an IR signal are read from the chardev.
See also http://www.lirc.org/html/technical.html for more info.
The data written to the chardev is a pulse/space sequence of integer values. Pulses and spaces are only marked implicitly by their position. The data must start and end with a pulse, therefore, the data must always include an uneven number of samples. The write function must block until the data has been transmitted by the hardware. If more data is provided than the hardware can send, the driver returns EINVAL.
The LIRC device's ioctl definition is bound by the ioctl function definition of struct file_operations, leaving us with an unsigned int for the ioctl command and an unsigned long for the arg. For the purposes of ioctl portability across 32-bit and 64-bit, these values are capped to their 32-bit sizes.
The following ioctls can be used to change specific hardware settings. In general each driver should have a default set of settings. The driver implementation is expected to re-apply the default settings when the device is closed by user-space, so that every application opening the device can rely on working with the default settings initially.
Obviously, get the underlying hardware device's features. If a driver does not announce support of certain features, calling of the corresponding ioctls is undefined.
Get supported transmit mode. Only LIRC_MODE_PULSE is supported by lircd.
Get supported receive modes. Only LIRC_MODE_MODE2 and LIRC_MODE_LIRCCODE are supported by lircd.
Get carrier frequency (in Hz) currently used for transmit.
Get carrier frequency (in Hz) currently used for IR reception.
Get/set the duty cycle (from 0 to 100) of the carrier signal. Currently, no special meaning is defined for 0 or 100, but this could be used to switch off carrier generation in the future, so these values should be reserved.
Some receiver have maximum resolution which is defined by internal sample rate or data format limitations. E.g. it's common that signals can only be reported in 50 microsecond steps. This integer value is used by lircd to automatically adjust the aeps tolerance value in the lircd config file.
Some devices have internal timers that can be used to detect when there's no IR activity for a long time. This can help lircd in detecting that a IR signal is finished and can speed up the decoding process. Returns an integer value with the minimum/maximum timeout that can be set. Some devices have a fixed timeout, in that case both ioctls will return the same value even though the timeout cannot be changed.
Some devices are able to filter out spikes in the incoming signal using given filter rules. These ioctls return the hardware capabilities that describe the bounds of the possible filters. Filter settings depend on the IR protocols that are expected. lircd derives the settings from all protocols definitions found in its config file.
Retrieves the code length in bits (only for LIRC_MODE_LIRCCODE). Reads on the device must be done in blocks matching the bit count. The bit could should be rounded up so that it matches full bytes.
Set send/receive mode. Largely obsolete for send, as only LIRC_MODE_PULSE is supported.
Set send/receive carrier (in Hz).
This enables the given set of transmitters. The first transmitter is encoded by the least significant bit, etc. When an invalid bit mask is given, i.e. a bit is set, even though the device does not have so many transitters, then this ioctl returns the number of available transitters and does nothing otherwise.
Sets the integer value for IR inactivity timeout (cf. LIRC_GET_MIN_TIMEOUT and LIRC_GET_MAX_TIMEOUT). A value of 0 (if supported by the hardware) disables all hardware timeouts and data should be reported as soon as possible. If the exact value cannot be set, then the next possible value _greater_ than the given value should be set.
Enable (1) or disable (0) timeout reports in LIRC_MODE_MODE2. By default, timeout reports should be turned off.
Pulses/spaces shorter than this are filtered out by hardware. If filters cannot be set independently for pulse/space, the corresponding ioctls must return an error and LIRC_SET_REC_FILTER shall be used instead.
Enable (1)/disable (0) measure mode. If enabled, from the next key press on, the driver will send LIRC_MODE2_FREQUENCY packets. By default this should be turned off.
To set a range use LIRC_SET_REC_DUTY_CYCLE_RANGE/LIRC_SET_REC_CARRIER_RANGE with the lower bound first and later LIRC_SET_REC_DUTY_CYCLE/LIRC_SET_REC_CARRIER with the upper bound.
This ioctl is called by lircd whenever a successful decoding of an incoming IR signal could be done. This can be used by supporting hardware to give visual feedback to the user e.g. by flashing a LED.
Setting of several driver parameters can be optimized by encapsulating the according ioctl calls with LIRC_SETUP_START/LIRC_SETUP_END. When a driver receives a LIRC_SETUP_START ioctl it can choose to not commit further setting changes to the hardware until a LIRC_SETUP_END is received. But this is open to the driver implementation and every driver must also handle parameter changes which are not encapsulated by LIRC_SETUP_START and LIRC_SETUP_END. Drivers can also choose to ignore these ioctls.
Some receivers are equipped with special wide band receiver which is intended to be used to learn output of existing remote. Calling that ioctl with (1) will enable it, and with (0) disable it. This might be useful of receivers that have otherwise narrow band receiver that prevents them to be used with some remotes. Wide band receiver might also be more precise On the other hand its disadvantage it usually reduced range of reception. Note: wide band receiver might be implictly enabled if you enable carrier reports. In that case it will be disabled as soon as you disable carrier reports. Trying to disable wide band receiver while carrier reports are active will do nothing.
On success 0 is returned, on error -1 and the errno variable is set appropriately. The generic error codes are described at the Generic Error Codes chapter.