[media] doc-rst: v4l: Add documentation on CSI-2 bus configuration

Document the interface between the CSI-2 transmitter and receiver drivers.
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/kapi/csi2.rst

+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 configure the CSI-2 transmitter to *LP-11
+mode* whenever the transmitter is powered on but not active. Some
+transmitters do this automatically but some have to be explicitly
+programmed to do so.
+
+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_use` or
+directly.

Documentation/media/media_kapi.rst

@@ -33,3 +33,4 @@ For more details see the file COPYING in the source distribution of Linux.
     kapi/rc-core
     kapi/mc-core
     kapi/cec-core
+    kapi/csi2