On 8/3/20 6:25 PM, Sakari Ailus wrote: > Hi Helen, > > On Mon, Aug 03, 2020 at 12:25:54PM -0300, Helen Koike wrote: >> Hi Sakari, >> >> Thanks for this. > > Thanks for the comments! > >> >> On 7/30/20 1:20 PM, 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> >>> --- >>> since v2: >>> >>> - More verbose explanation on sensor driver's responsibilities. >>> >>> - Reword the explanation on power state vs. v4l2_ctrl_handler_setup. >>> >>> .../driver-api/media/camera-sensor.rst | 134 ++++++++++++++++++ >>> Documentation/driver-api/media/csi2.rst | 2 + >>> Documentation/driver-api/media/index.rst | 1 + >>> 3 files changed, 137 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..2121586e8ede >>> --- /dev/null >>> +++ b/Documentation/driver-api/media/camera-sensor.rst >>> @@ -0,0 +1,134 @@ >>> +.. 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 >> >> Double colon > > This is ReST. This should translate into a single colon. > >> >>> +and the link frequency. The two parameters generally are obtained from system >>> +firmware. No other frequencies should be used in any circumstances. >> >> If I understand correctly, the firmware exposes a list of options that >> userspace can chose from, right? > > Correct. > >> >>> + >>> +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. >> >> If I understand correctly, the difference between the first and second way, >> is that the first allows userspace to change the format on the subdevice's pads, >> and the second one needs to be calculated from the format set in the video devices, >> is this correct? > > Yes, but video devices aren't necessarily involved. In a MC-centric drivers > the interface is available through sub-device nodes in both above cases. So the definition of a "Freely configurable camera sensor driver" is when we have MC-centric driver exposing format configuration through pads, and a "Register list based driver" is when the format configuration is propagated in the pipeline from somewhere else right? And it can be a MC-centric device where it doesn't expose format configuration through pads, or non Mc-centric, where the subdevice is not exposed, is this correct? I was wondering why it is called "Register list". > >> >>> + >>> +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 >> >> A pixel array entity is the one that represents the "source" of the image, right? > > Correct. > > Please wrap lines before 80. > >> >>> +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. >> >> Sorry, it's not clear to me if you are referring to driver->hardware configuration, or >> userspace->driver configuration (but maybe it's just me and my lack of knowledge). > > Both actually. Would this be clearer: > > 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. > Yes :) Thanks, Helen
