Devices supporting the radio interface set the V4L2_CAP_RADIO and V4L2_CAP_TUNER or V4L2_CAP_MODULATOR flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl. Other combinations of capability flags are reserved for future extensions.

Radio devices can support controls, and must support the tuner or modulator ioctls.

They do not support the video input or output, audio input or output, video standard, cropping and scaling, compression and streaming parameter, or overlay ioctls. All other ioctls and I/O methods are reserved for future extensions.

Radio devices may have a couple audio controls (as discussed in the section called “User Controls”) such as a volume control, possibly custom controls. Further all radio devices have one tuner or modulator (these are discussed in the section called “Tuners and Modulators”) with index number zero to select the radio frequency and to determine if a monaural or FM stereo program is received/emitted. Drivers switch automatically between AM and FM depending on the selected frequency. The VIDIOC_G_TUNER or VIDIOC_G_MODULATOR ioctl reports the supported frequency range.