camera-sensor.rst

  1.. SPDX-License-Identifier: GPL-2.0
  2
  3.. _media_using_camera_sensor_drivers:
  4
  5Using camera sensor drivers
  6===========================
  7
  8This section describes common practices for how the V4L2 sub-device interface is
  9used to control the camera sensor drivers.
 10
 11You may also find :ref:`media_writing_camera_sensor_drivers` useful.
 12
 13Sensor internal pipeline configuration
 14--------------------------------------
 15
 16Camera sensors have an internal processing pipeline including cropping and
 17binning functionality. The sensor drivers belong to two distinct classes, freely
 18configurable and register list-based drivers, depending on how the driver
 19configures this functionality.
 20
 21Freely configurable camera sensor drivers
 22~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 23
 24Freely configurable camera sensor drivers expose the device's internal
 25processing pipeline as one or more sub-devices with different cropping and
 26scaling configurations. The output size of the device is the result of a series
 27of cropping and scaling operations from the device's pixel array's size.
 28
 29An example of such a driver is the CCS driver.
 30
 31Register list-based drivers
 32~~~~~~~~~~~~~~~~~~~~~~~~~~~
 33
 34Register list-based drivers generally, instead of able to configure the device
 35they control based on user requests, are limited to a number of preset
 36configurations that combine a number of different parameters that on hardware
 37level are independent. How a driver picks such configuration is based on the
 38format set on a source pad at the end of the device's internal pipeline.
 39
 40Most sensor drivers are implemented this way.
 41
 42Frame interval configuration
 43----------------------------
 44
 45There are two different methods for obtaining possibilities for different frame
 46intervals as well as configuring the frame interval. Which one to implement
 47depends on the type of the device.
 48
 49Raw camera sensors
 50~~~~~~~~~~~~~~~~~~
 51
 52Instead of a high level parameter such as frame interval, the frame interval is
 53a result of the configuration of a number of camera sensor implementation
 54specific parameters. Luckily, these parameters tend to be the same for more or
 55less all modern raw camera sensors.
 56
 57The frame interval is calculated using the following equation::
 58
 59	frame interval = (analogue crop width + horizontal blanking) *
 60			 (analogue crop height + vertical blanking) / pixel rate
 61
 62The formula is bus independent and is applicable for raw timing parameters on
 63large variety of devices beyond camera sensors. Devices that have no analogue
 64crop, use the full source image size, i.e. pixel array size.
 65
 66Horizontal and vertical blanking are specified by ``V4L2_CID_HBLANK`` and
 67``V4L2_CID_VBLANK``, respectively. The unit of the ``V4L2_CID_HBLANK`` control
 68is pixels and the unit of the ``V4L2_CID_VBLANK`` is lines. The pixel rate in
 69the sensor's **pixel array** is specified by ``V4L2_CID_PIXEL_RATE`` in the same
 70sub-device. The unit of that control is pixels per second.
 71
 72Register list-based drivers need to implement read-only sub-device nodes for the
 73purpose. Devices that are not register list based need these to configure the
 74device's internal processing pipeline.
 75
 76The first entity in the linear pipeline is the pixel array. The pixel array may
 77be followed by other entities that are there to allow configuring binning,
 78skipping, scaling or digital crop, see :ref:`VIDIOC_SUBDEV_G_SELECTION
 79<VIDIOC_SUBDEV_G_SELECTION>`.
 80
 81USB cameras etc. devices
 82~~~~~~~~~~~~~~~~~~~~~~~~
 83
 84USB video class hardware, as well as many cameras offering a similar higher
 85level interface natively, generally use the concept of frame interval (or frame
 86rate) on device level in firmware or hardware. This means lower level controls
 87implemented by raw cameras may not be used on uAPI (or even kAPI) to control the
 88frame interval on these devices.
 89
 90Rotation, orientation and flipping
 91----------------------------------
 92
 93Some systems have the camera sensor mounted upside down compared to its natural
 94mounting rotation. In such cases, drivers shall expose the information to
 95userspace with the :ref:`V4L2_CID_CAMERA_SENSOR_ROTATION
 96<v4l2-camera-sensor-rotation>` control.
 97
 98Sensor drivers shall also report the sensor's mounting orientation with the
 99:ref:`V4L2_CID_CAMERA_SENSOR_ORIENTATION <v4l2-camera-sensor-orientation>`.
100
101Sensor drivers that have any vertical or horizontal flips embedded in the
102register programming sequences shall initialize the :ref:`V4L2_CID_HFLIP
103<v4l2-cid-hflip>` and :ref:`V4L2_CID_VFLIP <v4l2-cid-vflip>` controls with the
104values programmed by the register sequences. The default values of these
105controls shall be 0 (disabled). Especially these controls shall not be inverted,
106independently of the sensor's mounting rotation.