| .. SPDX-License-Identifier: GPL-2.0 |
| |
| 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`_. |
| |
| .. _`MIPI alliance`: http://www.mipi.org/ |
| |
| 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 |
| (:c:type:`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 |
| :c:type:`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 |
| |
| .. list-table:: variables in pixel rate calculation |
| :header-rows: 1 |
| |
| * - 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. |
| |
| 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. |
| |
| Receiver drivers |
| ---------------- |
| |
| Before the receiver driver may enable the CSI-2 transmitter by using |
| the :c:type:`v4l2_subdev_video_ops`->s_stream(), it must have powered |
| the transmitter up by using the |
| :c:type:`v4l2_subdev_core_ops`->s_power() callback. This may take |
| place either indirectly by using :c:func:`v4l2_pipeline_pm_get` or |
| directly. |
| |
| 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. |