Drivers Media Drive Cameras

7.1. CSI-2¶

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Please see what is written on MIPI CSI-2.

7.2. Handling clocks¶

Camera sensors have an internal clock tree including a PLL and a number ofdivisors. The clock tree is generally configured by the driver based on a fewinput parameters that are specific to the hardware:: the external clock frequencyand the link frequency. The two parameters generally are obtained from systemfirmware. No other frequencies should be used in any circumstances.

The reason why the clock frequencies are so important is that the clock signalscome out of the SoC, and in many cases a specific frequency is designed to beused in the system. Using another frequency may cause harmful effectselsewhere. Therefore only the pre-determined frequencies are configurable by theuser.

7.3. Frame size¶

There are two distinct ways to configure the frame size produced by camerasensors.

7.3.1. Freely configurable camera sensor drivers¶

Freely configurable camera sensor drivers expose the device’s internalprocessing pipeline as one or more sub-devices with different cropping andscaling configurations. The output size of the device is the result of a seriesof 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).

7.3.2. Register list based drivers¶

Register list based drivers generally, instead of able to configure the devicethey control based on user requests, are limited to a number of presetconfigurations that combine a number of different parameters that on hardwarelevel are independent. How a driver picks such configuration is based on theformat 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.

7.4. Frame interval configuration¶

There are two different methods for obtaining possibilities for different frameintervals as well as configuring the frame interval. Which one to implementdepends on the type of the device.

Drivers Media Drive Cameras Wireless

7.4.1. Raw camera sensors¶

Instead of a high level parameter such as frame interval, the frame interval isa result of the configuration of a number of camera sensor implementationspecific parameters. Luckily, these parameters tend to be the same for more orless all modern raw camera sensors.

The frame interval is calculated using the following equation:

The formula is bus independent and is applicable for raw timing parameters onlarge variety of devices beyond camera sensors. Devices that have no analoguecrop, use the full source image size, i.e. pixel array size.

Horizontal and vertical blanking are specified by V4L2_CID_HBLANK andV4L2_CID_VBLANK, respectively. The unit of these controls are lines. Thepixel rate is specified by V4L2_CID_PIXEL_RATE in the same sub-device. Theunit of that control is Hz.

Register list based drivers need to implement read-only sub-device nodes for thepurpose. Devices that are not register list based need these to configure thedevice’s internal processing pipeline.

The first entity in the linear pipeline is the pixel array. The pixel array maybe followed by other entities that are there to allow configuring binning,skipping, scaling or digital crop Selections: cropping, scaling and composition.

Cameras

7.4.2. USB cameras etc. devices¶

USB video class hardware, as well as many cameras offering a similar higherlevel interface natively, generally use the concept of frame interval (or framerate) on device level in firmware or hardware. This means lower level controlsimplemented by raw cameras may not be used on uAPI (or even kAPI) to control theframe interval on these devices.

7.5. Power management¶

Always use runtime PM to manage the power states of your device. Camera sensordrivers are in no way special in this respect: they are responsible forcontrolling the power state of the device they otherwise control as well. Ingeneral, the device must be powered on at least when its registers are beingaccessed and when it is streaming.

Cameras

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Existing camera sensor drivers may rely on the oldv4l2_subdev_core_ops->s_power() callback for bridge or ISP drivers tomanage their power state. This is however deprecated. If you feel you needto begin calling an s_power from an ISP or a bridge driver, instead please addruntime PM support to the sensor driver you are using. Likewise, new driversshould not use s_power.

Please see examples in e.g. drivers/media/i2c/ov8856.c anddrivers/media/i2c/smiapp/smiapp-core.c. The two drivers work in both ACPIand DT based systems.

7.5.1. Control framework¶

v4l2_ctrl_handler_setup() function may not be used in the device’s runtimePM runtime_resume callback, as it has no way to figure out the power stateof the device. This is because the power state of the device is only changedafter the power state transition has taken place. The s_ctrl callback can beused to obtain device’s power state after the power state transition:

int pm_runtime_get_if_in_use(struct device *dev);

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The function returns a non-zero value if it succeeded getting the power count orruntime PM was disabled, in either of which cases the driver may proceed toaccess the device.

7.6. Controls¶

For camera sensors that are connected to a bus where transmitter and receiverrequire common configuration set by drivers, such as CSI-2 or parallel (BT.601or BT.656) bus, the V4L2_CID_LINK_FREQ control is mandatory on transmitterdrivers. Receiver drivers can use the V4L2_CID_LINK_FREQ to query thefrequency used on the bus.

The transmitter drivers should also implement V4L2_CID_PIXEL_RATE control inorder to tell the maximum pixel rate to the receiver. This is required on rawcamera sensors.