On 12/05/2020 12:59, Sakari Ailus wrote: > While we have had some example drivers, there has been up to date no > formal documentation on how camera sensor drivers should be written; what > are the practices, why, and where they apply. > > Signed-off-by: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx> > --- > The HTML documentation can be found here: > > <URL:https://www.retiisi.eu/~sailus/v4l2/tmp/doc/output/driver-api/media/camera-sensor.html> > > .../driver-api/media/camera-sensor.rst | 98 +++++++++++++++++++ > Documentation/driver-api/media/csi2.rst | 2 + > Documentation/driver-api/media/index.rst | 1 + > 3 files changed, 101 insertions(+) > create mode 100644 Documentation/driver-api/media/camera-sensor.rst > > diff --git a/Documentation/driver-api/media/camera-sensor.rst b/Documentation/driver-api/media/camera-sensor.rst > new file mode 100644 > index 000000000000..345e3ae30340 > --- /dev/null > +++ b/Documentation/driver-api/media/camera-sensor.rst > @@ -0,0 +1,98 @@ > +.. SPDX-License-Identifier: GPL-2.0 > + > +Writing camera sensor drivers > +============================= > + > +CSI-2 > +----- > + > +Please see what is written on :ref:`MIPI_CSI_2`. > + > +Handling clocks > +--------------- > + > +Camera sensors have an internal clock tree including a PLL and a number of > +divisors. The clock tree is generally configured by the driver based on a few > +input parameters that are specific to the hardware:: the external clock frequency > +and the link frequency. The two parameters generally are obtained from system > +firmware. No other frequencies should be used in any circumstances. > + > +The reason why the clock frequencies are so important is that the clock signals > +come out of the SoC, and in many cases a specific frequency is designed to be > +used in the system. Using another frequency may cause harmful effects > +elsewhere. Therefore only the pre-determined frequencies are configurable by the > +user. > + > +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 smiapp driver (see drivers/media/i2c/smiapp). > + > +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 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. > + > +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 higher level > +control interface, generally use the concept of frame interval (or frame rate) > +on the level of device hardware interface. This means lower level controls > +exposed by raw cameras may not be used as an interface to control the frame > +interval on these devices. > diff --git a/Documentation/driver-api/media/csi2.rst b/Documentation/driver-api/media/csi2.rst > index e111ff7bfd3d..da8b356389f0 100644 > --- a/Documentation/driver-api/media/csi2.rst > +++ b/Documentation/driver-api/media/csi2.rst > @@ -1,5 +1,7 @@ > .. SPDX-License-Identifier: GPL-2.0 > > +.. _MIPI_CSI_2: > + > MIPI CSI-2 > ========== > > diff --git a/Documentation/driver-api/media/index.rst b/Documentation/driver-api/media/index.rst > index 328350924853..c140692454b1 100644 > --- a/Documentation/driver-api/media/index.rst > +++ b/Documentation/driver-api/media/index.rst > @@ -34,6 +34,7 @@ Please see: > mc-core > cec-core > csi2 > + camera-sensor > > drivers/index > > Can you add a section on power management? I've CC-ed Sowjanya as well, since she had some questions about that (off-line), and I don't know the answer on the right way to handle power management for sensors. Regards, Hans
