media: Documentation: Split camera sensor documentation

Split camera sensor documentation into user and kernel portions. This
should make it easier for the user space developers to find the relevant
documentation.

Also add a list of exemplary drivers and add imx219 driver to it, besides
those that were already mentioned.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>

1 files changed, 38 insertions, 93 deletions

diff --git a/Documentation/driver-api/media/camera-sensor.rst b/Documentation/driver-api/media/camera-sensor.rst
index 2acc08142a1a..6456145f96ed 100644
--- a/Documentation/driver-api/media/camera-sensor.rst
+++ b/Documentation/driver-api/media/camera-sensor.rst
@@ -1,8 +1,14 @@
 .. SPDX-License-Identifier: GPL-2.0
 
+.. _media_writing_camera_sensor_drivers:
+
 Writing camera sensor drivers
 =============================
 
+This document covers the in-kernel APIs only. For the best practices on
+userspace API implementation in camera sensor drivers, please see
+:ref:`media_using_camera_sensor_drivers`.
+
 CSI-2 and parallel (BT.601 and BT.656) busses
 ---------------------------------------------
 
@@ -34,7 +40,8 @@ Devicetree
 
 The preferred way to achieve this is using ``assigned-clocks``,
 ``assigned-clock-parents`` and ``assigned-clock-rates`` properties. See the
-`clock device tree bindings <https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/clock/clock.yaml>`_
+`clock device tree bindings
+<https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/clock/clock.yaml>`_
 for more information. The driver then gets the frequency using
 ``clk_get_rate()``.
 
@@ -85,9 +92,7 @@ PM instead. If you feel you need to begin calling ``.s_power()`` from an ISP or
 a bridge driver, instead add runtime PM support to the sensor driver you are
 using and drop its ``.s_power()`` handler.
 
-See examples of runtime PM handling in e.g. ``drivers/media/i2c/ov8856.c`` and
-``drivers/media/i2c/ccs/ccs-core.c``. The two drivers work in both ACPI and DT
-based systems.
+Please also see :ref:`examples <media-camera-sensor-examples>`.
 
 Control framework
 ~~~~~~~~~~~~~~~~~
@@ -104,99 +109,39 @@ 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.
 
-Frame size
-----------
-
-There are two distinct ways to configure the frame size produced by camera
-sensors.
-
-Freely configurable camera sensor drivers
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-Freely configurable camera sensor drivers expose the device's internal
-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 CCS driver (see ``drivers/media/i2c/ccs``).
-
-Register list based drivers
-~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-Register list based drivers generally, instead of able to configure the device
-they control based on user requests, are limited to a number of preset
-configurations that combine a number of different parameters that on hardware
-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.
-
-Frame interval configuration
-----------------------------
-
-There are two different methods for obtaining possibilities for different frame
-intervals as well as configuring the frame interval. Which one to implement
-depends on the type of the device.
-
-Raw camera sensors
-~~~~~~~~~~~~~~~~~~
-
-Instead of a high level parameter such as frame interval, the frame interval is
-a result of the configuration of a number of camera sensor implementation
-specific parameters. Luckily, these parameters tend to be the same for more or
-less all modern raw camera sensors.
-
-The frame interval is calculated using the following equation::
-
-	frame interval = (analogue crop width + horizontal blanking) *
-			 (analogue crop height + vertical blanking) / pixel rate
-
-The formula is bus independent and is applicable for raw timing parameters on
-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 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
-device's internal processing pipeline.
-
-The first entity in the linear pipeline is the pixel array. The pixel array may
-be followed by other entities that are there to allow configuring binning,
-skipping, scaling or digital crop :ref:`v4l2-subdev-selections`.
-
-USB cameras etc. devices
-~~~~~~~~~~~~~~~~~~~~~~~~
-
-USB video class hardware, as well as many cameras offering a similar higher
-level interface natively, generally use the concept of frame interval (or frame
-rate) on device level in firmware or hardware. This means lower level controls
-implemented by raw cameras may not be used on uAPI (or even kAPI) to control the
-frame interval on these devices.
-
 Rotation, orientation and flipping
 ----------------------------------
 
-Some systems have the camera sensor mounted upside down compared to its natural
-mounting rotation. In such cases, drivers shall expose the information to
-userspace with the :ref:`V4L2_CID_CAMERA_SENSOR_ROTATION
-<v4l2-camera-sensor-rotation>` control.
-
-Sensor drivers shall also report the sensor's mounting orientation with the
-:ref:`V4L2_CID_CAMERA_SENSOR_ORIENTATION <v4l2-camera-sensor-orientation>`.
-
 Use ``v4l2_fwnode_device_parse()`` to obtain rotation and orientation
 information from system firmware and ``v4l2_ctrl_new_fwnode_properties()`` to
 register the appropriate controls.
 
-Sensor drivers that have any vertical or horizontal flips embedded in the
-register programming sequences shall initialize the V4L2_CID_HFLIP and
-V4L2_CID_VFLIP controls with the values programmed by the register sequences.
-The default values of these controls shall be 0 (disabled). Especially these
-controls shall not be inverted, independently of the sensor's mounting
-rotation.
+.. _media-camera-sensor-examples:
+
+Example drivers
+---------------
+
+Features implemented by sensor drivers vary, and depending on the set of
+supported features and other qualities, particular sensor drivers better serve
+the purpose of an example. The following drivers are known to be good examples:
+
+.. flat-table:: Example sensor drivers
+    :header-rows: 0
+    :widths:      1 1 1 2
+
+    * - Driver name
+      - File(s)
+      - Driver type
+      - Example topic
+    * - CCS
+      - ``drivers/media/i2c/ccs/``
+      - Freely configurable
+      - Power management (ACPI and DT), UAPI
+    * - imx219
+      - ``drivers/media/i2c/imx219.c``
+      - Register list based
+      - Power management (DT), UAPI, mode selection
+    * - imx319
+      - ``drivers/media/i2c/imx319.c``
+      - Register list based
+      - Power management (ACPI and DT)