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>

Documentation/userspace-api/media/drivers/camera-sensor.rst

@@ -0,0 +1,104 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _media_using_camera_sensor_drivers:
+
+Using camera sensor drivers
+===========================
+
+This section describes common practices for how the V4L2 sub-device interface is
+used to control the camera sensor drivers.
+
+You may also find :ref:`media_writing_camera_sensor_drivers` useful.
+
+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.
+
+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.
+
+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, see :ref:`VIDIOC_SUBDEV_G_SELECTION
+<VIDIOC_SUBDEV_G_SELECTION>`.
+
+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>`.
+
+Sensor drivers that have any vertical or horizontal flips embedded in the
+register programming sequences shall initialize the :ref:`V4L2_CID_HFLIP
+<v4l2-cid-hflip>` and :ref:`V4L2_CID_VFLIP <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.

Documentation/userspace-api/media/drivers/index.rst

@@ -32,6 +32,7 @@ For more details see the file COPYING in the source distribution of Linux.
 	:numbered:
 
 	aspeed-video
+	camera-sensor
 	ccs
 	cx2341x-uapi
 	dw100

Documentation/userspace-api/media/v4l/control.rst

@@ -143,9 +143,13 @@ Control IDs
     recognise the difference between digital and analogue gain use
     controls ``V4L2_CID_DIGITAL_GAIN`` and ``V4L2_CID_ANALOGUE_GAIN``.
 
+.. _v4l2-cid-hflip:
+
 ``V4L2_CID_HFLIP`` ``(boolean)``
     Mirror the picture horizontally.
 
+.. _v4l2-cid-vflip:
+
 ``V4L2_CID_VFLIP`` ``(boolean)``
     Mirror the picture vertically.