media: Documentation: media: Improve camera sensor documentation

Modernise the documentation to make it more precise and update the use of
pixel rate control and various other changes. In particular:

- Use non-proportional font for file names, properties as well as
  controls.

- The unit of the HBLANK control is pixels, not lines.

- The unit of PIXEL_RATE control is pixels per second, not Hz.

- Merge common requirements for CSI-2 and parallel busses.

- Include all DT properties needed for assigned clocks.

- Fix referencing the link rate control.

- SMIA driver's new name is CCS driver.

- The PIXEL_RATE control denotes pixel rate on the pixel array on camera
  sensors. Do not suggest it is used to tell the maximum pixel rate on the
  bus anymore.

- Improve ReST syntax (plain struct and function names).

- Remove the suggestion to use s_power() in receiver drivers.

- Make MIPI website URL use HTTPS, add Wikipedia links to BT.601 and
  BT.656.

Fixes: e4cf8c58af75 ("media: Documentation: media: Document how to write camera sensor drivers")
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Andrey Konovalov <andrey.konovalov@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

4 files changed, 135 insertions, 123 deletions

diff --git a/Documentation/driver-api/media/camera-sensor.rst b/Documentation/driver-api/media/camera-sensor.rst
index 7160336aa475..c7d4891bd24e 100644
--- a/Documentation/driver-api/media/camera-sensor.rst
+++ b/Documentation/driver-api/media/camera-sensor.rst
@@ -3,10 +3,10 @@
 Writing camera sensor drivers
 =============================
 
-CSI-2
------
+CSI-2 and parallel (BT.601 and BT.656) busses
+---------------------------------------------
 
-Please see what is written on :ref:`MIPI_CSI_2`.
+Please see :ref:`transmitter-receiver`.
 
 Handling clocks
 ---------------
@@ -26,15 +26,16 @@ user.
 ACPI
 ~~~~
 
-Read the "clock-frequency" _DSD property to denote the frequency. The driver can
-rely on this frequency being used.
+Read the ``clock-frequency`` _DSD property to denote the frequency. The driver
+can rely on this frequency being used.
 
 Devicetree
 ~~~~~~~~~~
 
-The currently preferred way to achieve this is using "assigned-clock-rates"
-property. See Documentation/devicetree/bindings/clock/clock-bindings.txt for
-more information. The driver then gets the frequency using clk_get_rate().
+The currently preferred way to achieve this is using ``assigned-clocks``,
+``assigned-clock-parents`` and ``assigned-clock-rates`` properties. See
+``Documentation/devicetree/bindings/clock/clock-bindings.txt`` for more
+information. The driver then gets the frequency using ``clk_get_rate()``.
 
 This approach has the drawback that there's no guarantee that the frequency
 hasn't been modified directly or indirectly by another driver, or supported by
@@ -55,7 +56,7 @@ processing pipeline as one or more sub-devices with different cropping and
 scaling configurations. The output size of the device is the result of a series
 of cropping and scaling operations from the device's pixel array's size.
 
-An example of such a driver is the smiapp driver (see drivers/media/i2c/smiapp).
+An example of such a driver is the CCS driver (see ``drivers/media/i2c/ccs``).
 
 Register list based drivers
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -67,7 +68,7 @@ level are independent. How a driver picks such configuration is based on the
 format set on a source pad at the end of the device's internal pipeline.
 
 Most sensor drivers are implemented this way, see e.g.
-drivers/media/i2c/imx319.c for an example.
+``drivers/media/i2c/imx319.c`` for an example.
 
 Frame interval configuration
 ----------------------------
@@ -94,9 +95,10 @@ large variety of devices beyond camera sensors. Devices that have no analogue
 crop, use the full source image size, i.e. pixel array size.
 
 Horizontal and vertical blanking are specified by ``V4L2_CID_HBLANK`` and
-``V4L2_CID_VBLANK``, respectively. The unit of these controls are lines. The
-pixel rate is specified by ``V4L2_CID_PIXEL_RATE`` in the same sub-device. The
-unit of that control is Hz.
+``V4L2_CID_VBLANK``, respectively. The unit of the ``V4L2_CID_HBLANK`` control
+is pixels and the unit of the ``V4L2_CID_VBLANK`` is lines. The pixel rate in
+the sensor's **pixel array** is specified by ``V4L2_CID_PIXEL_RATE`` in the same
+sub-device. The unit of that control is pixels per second.
 
 Register list based drivers need to implement read-only sub-device nodes for the
 purpose. Devices that are not register list based need these to configure the
@@ -125,14 +127,14 @@ general, the device must be powered on at least when its registers are being
 accessed and when it is streaming.
 
 Existing camera sensor drivers may rely on the old
-:c:type:`v4l2_subdev_core_ops`->s_power() callback for bridge or ISP drivers to
+struct v4l2_subdev_core_ops->s_power() callback for bridge or ISP drivers to
 manage their power state. This is however **deprecated**. If you feel you need
 to begin calling an s_power from an ISP or a bridge driver, instead please add
 runtime PM support to the sensor driver you are using. Likewise, new drivers
 should not use s_power.
 
 Please see examples in e.g. ``drivers/media/i2c/ov8856.c`` and
-``drivers/media/i2c/smiapp/smiapp-core.c``. The two drivers work in both ACPI
+``drivers/media/i2c/ccs/ccs-core.c``. The two drivers work in both ACPI
 and DT based systems.
 
 Control framework
@@ -149,16 +151,3 @@ used to obtain device's power state after the power state transition:
 The function returns a non-zero value if it succeeded getting the power count or
 runtime PM was disabled, in either of which cases the driver may proceed to
 access the device.
-
-Controls
---------
-
-For camera sensors that are connected to a bus where transmitter and receiver
-require common configuration set by drivers, such as CSI-2 or parallel (BT.601
-or BT.656) bus, the ``V4L2_CID_LINK_FREQ`` control is mandatory on transmitter
-drivers. Receiver drivers can use the ``V4L2_CID_LINK_FREQ`` to query the
-frequency used on the bus.
-
-The transmitter drivers should also implement ``V4L2_CID_PIXEL_RATE`` control in
-order to tell the maximum pixel rate to the receiver. This is required on raw
-camera sensors.
diff --git a/Documentation/driver-api/media/csi2.rst b/Documentation/driver-api/media/csi2.rst
deleted file mode 100644
index 11c52b0be8b8..000000000000
--- a/Documentation/driver-api/media/csi2.rst
+++ /dev/null
@@ -1,94 +0,0 @@
-.. SPDX-License-Identifier: GPL-2.0
-
-.. _MIPI_CSI_2:
-
-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/
-
-Media bus formats
------------------
-
-See :ref:`v4l2-mbus-pixelcode` for details on which media bus formats should
-be used for CSI-2 interfaces.
-
-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
-control 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 * 16 / k / 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.
-   * - k
-     - 16 for D-PHY and 7 for C-PHY
-
-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.
diff --git a/Documentation/driver-api/media/index.rst b/Documentation/driver-api/media/index.rst
index 813d7db59da7..08e206567408 100644
--- a/Documentation/driver-api/media/index.rst
+++ b/Documentation/driver-api/media/index.rst
@@ -37,7 +37,7 @@ Documentation/userspace-api/media/index.rst
     rc-core
     mc-core
     cec-core
-    csi2
+    tx-rx
     camera-sensor
 
     drivers/index
diff --git a/Documentation/driver-api/media/tx-rx.rst b/Documentation/driver-api/media/tx-rx.rst
new file mode 100644
index 000000000000..4c8584e7b6f2
--- /dev/null
+++ b/Documentation/driver-api/media/tx-rx.rst
@@ -0,0 +1,117 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _transmitter-receiver:
+
+Pixel data transmitter and receiver drivers
+===========================================
+
+V4L2 supports various devices that transmit and receiver pixel data. Examples of
+these devices include a camera sensor, a TV tuner and a parallel or a CSI-2
+receiver in an SoC.
+
+Bus types
+---------
+
+The following busses are the most common. This section discusses these two only.
+
+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`: https://www.mipi.org/
+
+Parallel
+^^^^^^^^
+
+`BT.601`_ and `BT.656`_ are the most common parallel busses.
+
+.. _`BT.601`: https://en.wikipedia.org/wiki/Rec._601
+.. _`BT.656`: https://en.wikipedia.org/wiki/ITU-R_BT.656
+
+Transmitter drivers
+-------------------
+
+Transmitter drivers generally need to provide the receiver drivers with the
+configuration of the transmitter. What is required depends on the type of the
+bus. These are common for both busses.
+
+Media bus pixel code
+^^^^^^^^^^^^^^^^^^^^
+
+See :ref:`v4l2-mbus-pixelcode`.
+
+Link frequency
+^^^^^^^^^^^^^^
+
+The :ref:`V4L2_CID_LINK_FREQ <v4l2-cid-link-freq>` control is used to tell the
+receiver the frequency of the bus (i.e. it is not the same as the symbol rate).
+
+``.s_stream()`` callback
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+The struct struct v4l2_subdev_video_ops->s_stream() callback is used by the
+receiver driver to control the transmitter driver's streaming state.
+
+
+CSI-2 transmitter drivers
+-------------------------
+
+Pixel rate
+^^^^^^^^^^
+
+The pixel rate on the bus is calculated as follows::
+
+	pixel_rate = link_freq * 2 * nr_of_lanes * 16 / k / 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
+     - Data is transferred on both rising and falling edge of the signal.
+   * - bits_per_sample
+     - Number of bits per sample.
+   * - k
+     - 16 for D-PHY and 7 for C-PHY
+
+.. note::
+
+	The pixel rate calculated this way is **not** the same thing as the
+	pixel rate on the camera sensor's pixel array which is indicated by the
+	:ref:`V4L2_CID_PIXEL_RATE <v4l2-cid-pixel-rate>` control.
+
+LP-11 and LP-111 modes
+^^^^^^^^^^^^^^^^^^^^^^
+
+The transmitter drivers must, if possible, configure the CSI-2 transmitter to
+*LP-11 or LP-111 mode* whenever the transmitter is powered on but not active,
+and maintain *LP-11 or LP-111 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.
+
+The receiver thus need to be configured to expect LP-11 or LP-111 mode from the
+transmitter before the transmitter driver's ``.s_stream()`` op is called.
+
+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.