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 v1: - Added power management documentation. The HTML docs are here: <URL:https://www.retiisi.eu/~sailus/v4l2/tmp/doc2/output/> .../driver-api/media/camera-sensor.rst | 129 ++++++++++++++++++ Documentation/driver-api/media/csi2.rst | 2 + Documentation/driver-api/media/index.rst | 1 + 3 files changed, 132 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..fa71f07731a0 --- /dev/null +++ b/Documentation/driver-api/media/camera-sensor.rst @@ -0,0 +1,129 @@ +.. 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. + +Power management +---------------- + +Always use runtime PM to manage the power states of your device. + +Existing camera sensor drivers may rely on the old +:c:type:`v4l2_subdev_core_ops`->s_power() callback for bridge or ISP drivers to +manage their power state. This is however **deprecated**. If you feel you need +to begin calling an s_power from an ISP or a bridge driver, instead please add +runtime PM support to the sensor driver you are using. Likewise, new drivers +should not use s_power. + +Please see examples in e.g. ``drivers/media/i2c/ov8856.c`` and +``drivers/media/i2c/smiapp/smiapp-core.c``. The two drivers work in both ACPI +and DT based systems. + +Control framework +~~~~~~~~~~~~~~~~~ + +``v4l2_ctrl_handler_setup()`` function may not be used in the device's runtime PM +``resume`` callback currently, as it has no way to figure out the power state of +the device. As callback is required to figure out the device's power state, it +can only know when the device is fully powered. This can be done using + +.. c:function:: + int pm_runtime_get_if_in_use(struct device *dev); + +The function returns a non-zero value if it succeeded getting the power count or +runtime PM was disabled, in either of which cases the driver may proceed to +access the device. 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 -- 2.20.1
