When multiple applications share a device it may be desirable to assign them different priorities. Contrary to the traditional "rm -rf /" school of thought a video recording application could for example block other applications from changing video controls or switching the current TV channel. Another objective is to permit low priority applications working in background, which can be preempted by user controlled applications and automatically regain control of the device at a later time.
Since these features cannot be implemented entirely in user
space V4L2 defines the VIDIOC_G_PRIORITY and VIDIOC_S_PRIORITY
ioctls to request and query the access priority associate with a file
descriptor. Opening a device assigns a medium priority, compatible
with earlier versions of V4L2 and drivers not supporting these ioctls.
Applications requiring a different priority will usually call
VIDIOC_S_PRIORITY after verifying the device with
the VIDIOC_QUERYCAP ioctl.
Ioctls changing driver properties, such as VIDIOC_S_INPUT,
return an EBUSY error code after another application obtained higher priority.
An event mechanism to notify applications about asynchronous property
changes has been proposed but not added yet.