Document the interface between the CSI-2 transmitter and receiver drivers. Signed-off-by: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx> --- since v2: - Add bits_per_sample variable to the formula. It should be correct now. Documentation/media/kapi/csi2.rst | 61 ++++++++++++++++++++++++++++++++++++++ Documentation/media/media_kapi.rst | 1 + 2 files changed, 62 insertions(+) create mode 100644 Documentation/media/kapi/csi2.rst diff --git a/Documentation/media/kapi/csi2.rst b/Documentation/media/kapi/csi2.rst new file mode 100644 index 0000000..2004db0 --- /dev/null +++ b/Documentation/media/kapi/csi2.rst @@ -0,0 +1,61 @@ +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. diff --git a/Documentation/media/media_kapi.rst b/Documentation/media/media_kapi.rst index f282ca2..bc06389 100644 --- a/Documentation/media/media_kapi.rst +++ b/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 -- 2.7.4 -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
