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. 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 CSI-2 transmitter.

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

where

variables in pixel rate calculation

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.

6.1.1. Stopping the transmitter

A transmitter stops sending the stream of images as a result of calling the .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 cases.

6.2. Receiver drivers

Before the receiver driver may enable the CSI-2 transmitter by using the 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 v4l2_pipeline_pm_use() or directly.

6.3. Formats

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.