2. Using camera sensor drivers

This section describes common practices for how the V4L2 sub-device interface is used to control the camera sensor drivers.

You may also find Writing camera sensor drivers useful.

2.1. Frame size

There are two distinct ways to configure the frame size produced by camera sensors.

2.1.1. Freely configurable camera sensor drivers

Freely configurable camera sensor drivers expose the device's internal processing pipeline as one or more sub-devices with different cropping and scaling configurations. The output size of the device is the result of a series of cropping and scaling operations from the device's pixel array's size.

An example of such a driver is the CCS driver.

2.1.2. Register list based drivers

Register list based drivers generally, instead of able to configure the device they control based on user requests, are limited to a number of preset configurations that combine a number of different parameters that on hardware level are independent. How a driver picks such configuration is based on the format set on a source pad at the end of the device's internal pipeline.

Most sensor drivers are implemented this way.

2.2. Frame interval configuration

There are two different methods for obtaining possibilities for different frame intervals as well as configuring the frame interval. Which one to implement depends on the type of the device.

2.2.1. Raw camera sensors

Instead of a high level parameter such as frame interval, the frame interval is a result of the configuration of a number of camera sensor implementation specific parameters. Luckily, these parameters tend to be the same for more or less all modern raw camera sensors.

The frame interval is calculated using the following equation:

frame interval = (analogue crop width + horizontal blanking) *
                 (analogue crop height + vertical blanking) / pixel rate

The formula is bus independent and is applicable for raw timing parameters on large variety of devices beyond camera sensors. Devices that have no analogue crop, use the full source image size, i.e. pixel array size.

Horizontal and vertical blanking are specified by V4L2_CID_HBLANK and V4L2_CID_VBLANK, respectively. The unit of the V4L2_CID_HBLANK control is pixels and the unit of the V4L2_CID_VBLANK is lines. The pixel rate in the sensor's pixel array is specified by V4L2_CID_PIXEL_RATE in the same sub-device. The unit of that control is pixels per second.

Register list based drivers need to implement read-only sub-device nodes for the purpose. Devices that are not register list based need these to configure the device's internal processing pipeline.

The first entity in the linear pipeline is the pixel array. The pixel array may be followed by other entities that are there to allow configuring binning, skipping, scaling or digital crop, see VIDIOC_SUBDEV_G_SELECTION.

2.2.2. USB cameras etc. devices

USB video class hardware, as well as many cameras offering a similar higher level interface natively, generally use the concept of frame interval (or frame rate) on device level in firmware or hardware. This means lower level controls implemented by raw cameras may not be used on uAPI (or even kAPI) to control the frame interval on these devices.

2.3. Rotation, orientation and flipping

Some systems have the camera sensor mounted upside down compared to its natural mounting rotation. In such cases, drivers shall expose the information to userspace with the V4L2_CID_CAMERA_SENSOR_ROTATION control.

Sensor drivers shall also report the sensor's mounting orientation with the V4L2_CID_CAMERA_SENSOR_ORIENTATION.

Sensor drivers that have any vertical or horizontal flips embedded in the register programming sequences shall initialize the V4L2_CID_HFLIP and V4L2_CID_VFLIP controls with the values programmed by the register sequences. The default values of these controls shall be 0 (disabled). Especially these controls shall not be inverted, independently of the sensor's mounting rotation.