Hi Sowjanya, On Mon, May 18, 2020 at 08:19:55AM -0700, Sowjanya Komatineni wrote: > On 5/18/20 3:35 AM, Sakari Ailus wrote: > > On Mon, May 18, 2020 at 11:50:34AM +0200, Hans Verkuil wrote: > >> 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. > > > > Sure. There's nothing special in here per se, but given the history and > > interaction with the control framework it's worth documenting that > > separately. Many drivers are also being used on both ACPI and DT that makes > > the drivers somewhat more convoluted. > > Hi Sakari, > > Are there any generic implementation guidelines for sensor drivers > regarding keeping pads in LP-11 when they are powered on and not being used? > > Also is it mandatory for sensor drivers to implement s_power callback > where during on time it powers on and keeps pads in LP-11 state? > > I see some sensor drivers have RPM enabled and keep sensor power on only > when they are being used during configuring and during streaming other > wise sensor power will be off and also not all drivers have s_power > callback implemented and some drivers with s_power implemented does only > power on but does not keep pads in LP-11. > > Reason for asking is for Tegra CSI receiver, we need to perform pads > calibration after every power on of Tegra CSI MIPI Pads. > > Calibration will be done only when link is is LP-11 state. > > Would like to have proper implementation for Tegra CSI MIPI pads > calibration based on this. I came across a similar issue recently with the i.MX6 CSI-2 receiver, when used with an MT9M114 sensor. The MT9M114 drives the CSI-2 lines to LP-00 when in standby mode. When taken out of standby, it transitions to LP-11, and then automatically transitions to high-speed mode after a short delay. There is no way (at least according to the documentation) to switch to the LP-11 state and keep it manually before going to high-speed mode. How would your CSI-2 receiver work with such a sensor ? -- Regards, Laurent Pinchart
