This is an experimental interface and may change in the future. SDR is an abbreviation of Software Defined Radio, the radio device which uses application software for modulation or demodulation. This interface is intended for controlling and data streaming of such devices. SDR devices are accessed through character device special files named /dev/swradio0 to /dev/swradio255 with major number 81 and dynamically allocated minor numbers 0 to 255.

Devices supporting the SDR receiver interface set the V4L2_CAP_SDR_CAPTURE and V4L2_CAP_TUNER flag in the capabilities field of &v4l2-capability; returned by the &VIDIOC-QUERYCAP; ioctl. That flag means the device has an Analog to Digital Converter (ADC), which is a mandatory element for the SDR receiver. At least one of the read/write, streaming or asynchronous I/O methods must be supported.

SDR devices can support controls, and must support the tuner ioctls. Tuner ioctls are used for setting the ADC sampling rate (sampling frequency) and the possible RF tuner frequency. The V4L2_TUNER_ADC tuner type is used for ADC tuners, and the V4L2_TUNER_RF tuner type is used for RF tuners. The tuner index of the RF tuner (if any) must always follow the ADC tuner index. Normally the ADC tuner is #0 and the RF tuner is #1. The &VIDIOC-S-HW-FREQ-SEEK; ioctl is not supported.

The SDR capture device uses the format ioctls to select the capture format. Both the sampling resolution and the data streaming format are bound to that selectable format. In addition to the basic format ioctls, the &VIDIOC-ENUM-FMT; ioctl must be supported as well. To use the format ioctls applications set the type field of a &v4l2-format; to V4L2_BUF_TYPE_SDR_CAPTURE and use the &v4l2-sdr-format; sdr member of the fmt union as needed per the desired operation. Currently there is two fields, pixelformat and buffersize, of struct &v4l2-sdr-format; which are used. Content of the pixelformat is V4L2 FourCC code of the data format. The buffersize field is maximum buffer size in bytes required for data transfer, set by the driver in order to inform application. &cs-str; __u32 pixelformat The data format or type of compression, set by the application. This is a little endian four character code. V4L2 defines SDR formats in . __u32 buffersize Maximum size in bytes required for data. Value is set by the driver. __u8 reserved[24] This array is reserved for future extensions. Drivers and applications must set it to zero. An SDR device may support read/write and/or streaming (memory mapping or user pointer) I/O.