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authorSakari Ailus <sakari.ailus@linux.intel.com>2016-10-19 17:28:45 +0300
committerMauro Carvalho Chehab <mchehab@s-opensource.com>2016-11-18 18:27:03 +0300
commitbb16d21cfa874a879b0ae40b8acc1c12f41e6850 (patch)
tree60ea23ebfb0d9154d3e33ce8a79730dc18634552 /Documentation/media/kapi
parentce3aeaf22c9f6d9dbbad00600b26ddd0263f8cbc (diff)
downloadlinux-bb16d21cfa874a879b0ae40b8acc1c12f41e6850.tar.xz
[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>
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+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.