6. MIPI CSI-2¶
CSI-2 is a data bus intended for transferring images from cameras to the host SoC. It is defined by the MIPI alliance.
6.1. Media bus formats¶
See Media Bus Pixel Codes for details on which media bus formats should be used for CSI-2 interfaces.
6.2. Transmitter drivers¶
CSI-2 transmitter, such as a sensor or a TV tuner, drivers need to
provide the CSI-2 receiver with information on the CSI-2 bus
configuration. These include the V4L2_CID_LINK_FREQ and
V4L2_CID_PIXEL_RATE controls and
v4l2_subdev_video_ops->s_stream() callback). These
interface elements must be present on the sub-device represents the
The V4L2_CID_LINK_FREQ control is used to tell the receiver driver the
frequency (and not the symbol rate) of the link. The
V4L2_CID_PIXEL_RATE is may be used by the receiver to obtain the pixel
rate the transmitter uses. The
v4l2_subdev_video_ops->s_stream() callback provides an
ability to start and stop the stream.
The value of the V4L2_CID_PIXEL_RATE is calculated as follows:
pixel_rate = link_freq * 2 * nr_of_lanes / bits_per_sample
|variable or constant||description|
|link_freq||The value of the V4L2_CID_LINK_FREQ integer64 menu item.|
|nr_of_lanes||Number of data lanes used on the CSI-2 link. This can be obtained from the OF endpoint configuration.|
|2||Two bits are transferred per clock cycle per lane.|
|bits_per_sample||Number of bits per sample.|
The transmitter drivers must, if possible, configure the CSI-2 transmitter to LP-11 mode whenever the transmitter is powered on but not active, and maintain LP-11 mode until stream on. Only at stream on should the transmitter activate the clock on the clock lane and transition to HS mode.
Some transmitters do this automatically but some have to be explicitly programmed to do so, and some are unable to do so altogether due to hardware constraints.
6.2.1. Stopping the transmitter¶
A transmitter stops sending the stream of images as a result of
.s_stream() callback. Some transmitters may stop the
stream at a frame boundary whereas others stop immediately,
effectively leaving the current frame unfinished. The receiver driver
should not make assumptions either way, but function properly in both
6.3. Receiver drivers¶
Before the receiver driver may enable the CSI-2 transmitter by using
v4l2_subdev_video_ops->s_stream(), it must have powered
the transmitter up by using the
v4l2_subdev_core_ops->s_power() callback. This may take
place either indirectly by using
The media bus pixel codes document parallel formats. Should the pixel data be transported over a serial bus, the media bus pixel code that describes a parallel format that transfers a sample on a single clock cycle is used.