Hi Sowjanya, On Tue, May 19, 2020 at 05:16:43PM -0700, Sowjanya Komatineni wrote: > On 5/19/20 3:53 PM, Laurent Pinchart wrote: > > 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 ? > > Our Tegra CSI receiver can detect LP transition so we don't need to > manually hold LP-11 state but we need to sync LP-11 state during sensor > stream with CSI receiver stream or during CSI pads calibration. > > So considering the fact that not all sensors support explicit LP-11 > state, we can handle in our driver to trigger pads calibration before > sensor stream and to check for calibration results after sensor stream > enable and then continue with capture. > > As CSI can detect 1st LP transition, we always keep CSI receiver ready > by starting CSI stream prior to sensor stream for capture. Sounds good to me :-) -- Regards, Laurent Pinchart
