4.13. Sub-device Interface

The complex nature of V4L2 devices, where hardware is often made of several integrated circuits that need to interact with each other in a controlled way, leads to complex V4L2 drivers. The drivers usually reflect the hardware model in software, and model the different hardware components as software blocks called sub-devices.

V4L2 sub-devices are usually kernel-only objects. If the V4L2 driver implements the media device API, they will automatically inherit from media entities. Applications will be able to enumerate the sub-devices and discover the hardware topology using the media entities, pads and links enumeration API.

In addition to make sub-devices discoverable, drivers can also choose to make them directly configurable by applications. When both the sub-device driver and the V4L2 device driver support this, sub-devices will feature a character device node on which ioctls can be called to

• query, read and write sub-devices controls

• subscribe and unsubscribe to events and retrieve them

• negotiate image formats on individual pads

• inspect and modify internal data routing between pads of the same entity

Sub-device character device nodes, conventionally named /dev/v4l-subdev*, use major number 81.

Drivers may opt to limit the sub-device character devices to only expose operations that do not modify the device state. In such a case the sub-devices are referred to as read-only in the rest of this documentation, and the related restrictions are documented in individual ioctls.

4.13.1. Controls

Most V4L2 controls are implemented by sub-device hardware. Drivers usually merge all controls and expose them through video device nodes. Applications can control all sub-devices through a single interface.

Complex devices sometimes implement the same control in different pieces of hardware. This situation is common in embedded platforms, where both sensors and image processing hardware implement identical functions, such as contrast adjustment, white balance or faulty pixels correction. As the V4L2 controls API doesn’t support several identical controls in a single device, all but one of the identical controls are hidden.

Applications can access those hidden controls through the sub-device node with the V4L2 control API described in User Controls. The ioctls behave identically as when issued on V4L2 device nodes, with the exception that they deal only with controls implemented in the sub-device.

Depending on the driver, those controls might also be exposed through one (or several) V4L2 device nodes.

4.13.2. Events

V4L2 sub-devices can notify applications of events as described in Event Interface. The API behaves identically as when used on V4L2 device nodes, with the exception that it only deals with events generated by the sub-device. Depending on the driver, those events might also be reported on one (or several) V4L2 device nodes.

4.13.3. Pad-level Formats

Warning

Pad-level formats are only applicable to very complex devices that need to expose low-level format configuration to user space. Generic V4L2 applications do not need to use the API described in this section.

Note

For the purpose of this section, the term format means the combination of media bus data format, frame width and frame height.

Image formats are typically negotiated on video capture and output devices using the format and selection ioctls. The driver is responsible for configuring every block in the video pipeline according to the requested format at the pipeline input and/or output.

For complex devices, such as often found in embedded systems, identical image sizes at the output of a pipeline can be achieved using different hardware configurations. One such example is shown on Image Format Negotiation on Pipelines, where image scaling can be performed on both the video sensor and the host image processing hardware.

Image Format Negotiation on Pipelines

High quality and high speed pipeline configuration

The sensor scaler is usually of less quality than the host scaler, but scaling on the sensor is required to achieve higher frame rates. Depending on the use case (quality vs. speed), the pipeline must be configured differently. Applications need to configure the formats at every point in the pipeline explicitly.

Drivers that implement the media API can expose pad-level image format configuration to applications. When they do, applications can use the VIDIOC_SUBDEV_G_FMT and VIDIOC_SUBDEV_S_FMT ioctls. to negotiate formats on a per-pad basis.

Applications are responsible for configuring coherent parameters on the whole pipeline and making sure that connected pads have compatible formats. The pipeline is checked for formats mismatch at VIDIOC_STREAMON time, and an EPIPE error code is then returned if the configuration is invalid.

Pad-level image format configuration support can be tested by calling the ioctl VIDIOC_SUBDEV_G_FMT, VIDIOC_SUBDEV_S_FMT ioctl on pad 0. If the driver returns an EINVAL error code pad-level format configuration is not supported by the sub-device.

4.13.3.1. Format Negotiation

Acceptable formats on pads can (and usually do) depend on a number of external parameters, such as formats on other pads, active links, or even controls. Finding a combination of formats on all pads in a video pipeline, acceptable to both application and driver, can’t rely on formats enumeration only. A format negotiation mechanism is required.

Central to the format negotiation mechanism are the get/set format operations. When called with the which argument set to V4L2_SUBDEV_FORMAT_TRY, the VIDIOC_SUBDEV_G_FMT and VIDIOC_SUBDEV_S_FMT ioctls operate on a set of formats parameters that are not connected to the hardware configuration. Modifying those ‘try’ formats leaves the device state untouched (this applies to both the software state stored in the driver and the hardware state stored in the device itself).

While not kept as part of the device state, try formats are stored in the sub-device file handles. A VIDIOC_SUBDEV_G_FMT call will return the last try format set on the same sub-device file handle. Several applications querying the same sub-device at the same time will thus not interact with each other.

To find out whether a particular format is supported by the device, applications use the VIDIOC_SUBDEV_S_FMT ioctl. Drivers verify and, if needed, change the requested format based on device requirements and return the possibly modified value. Applications can then choose to try a different format or accept the returned value and continue.

Formats returned by the driver during a negotiation iteration are guaranteed to be supported by the device. In particular, drivers guarantee that a returned format will not be further changed if passed to an VIDIOC_SUBDEV_S_FMT call as-is (as long as external parameters, such as formats on other pads or links’ configuration are not changed).

Drivers automatically propagate formats inside sub-devices. When a try or active format is set on a pad, corresponding formats on other pads of the same sub-device can be modified by the driver. Drivers are free to modify formats as required by the device. However, they should comply with the following rules when possible:

• Formats should be propagated from sink pads to source pads. Modifying a format on a source pad should not modify the format on any sink pad.

• Sub-devices that scale frames using variable scaling factors should reset the scale factors to default values when sink pads formats are modified. If the 1:1 scaling ratio is supported, this means that source pads formats should be reset to the sink pads formats.

Formats are not propagated across links, as that would involve propagating them from one sub-device file handle to another. Applications must then take care to configure both ends of every link explicitly with compatible formats. Identical formats on the two ends of a link are guaranteed to be compatible. Drivers are free to accept different formats matching device requirements as being compatible.

Sample Pipeline Configuration shows a sample configuration sequence for the pipeline described in Image Format Negotiation on Pipelines (table columns list entity names and pad numbers).

Sample Pipeline Configuration

	Sensor/0 format	Frontend/0 format	Frontend/1 format	Scaler/0 format	Scaler/0 compose selection rectangle	Scaler/1 format
Initial state	2048x1536 SGRBG8_1X8	(default)	(default)	(default)	(default)	(default)
Configure frontend sink format	2048x1536 SGRBG8_1X8	2048x1536 SGRBG8_1X8	2046x1534 SGRBG8_1X8	(default)	(default)	(default)
Configure scaler sink format	2048x1536 SGRBG8_1X8	2048x1536 SGRBG8_1X8	2046x1534 SGRBG8_1X8	2046x1534 SGRBG8_1X8	0,0/2046x1534	2046x1534 SGRBG8_1X8
Configure scaler sink compose selection	2048x1536 SGRBG8_1X8	2048x1536 SGRBG8_1X8	2046x1534 SGRBG8_1X8	2046x1534 SGRBG8_1X8	0,0/1280x960	1280x960 SGRBG8_1X8

1. Initial state. The sensor source pad format is set to its native 3MP size and V4L2_MBUS_FMT_SGRBG8_1X8 media bus code. Formats on the host frontend and scaler sink and source pads have the default values, as well as the compose rectangle on the scaler’s sink pad.

2. The application configures the frontend sink pad format’s size to 2048x1536 and its media bus code to V4L2_MBUS_FMT_SGRBG_1X8. The driver propagates the format to the frontend source pad.

3. The application configures the scaler sink pad format’s size to 2046x1534 and the media bus code to V4L2_MBUS_FMT_SGRBG_1X8 to match the frontend source size and media bus code. The media bus code on the sink pad is set to V4L2_MBUS_FMT_SGRBG_1X8. The driver propagates the size to the compose selection rectangle on the scaler’s sink pad, and the format to the scaler source pad.

4. The application configures the size of the compose selection rectangle of the scaler’s sink pad 1280x960. The driver propagates the size to the scaler’s source pad format.

When satisfied with the try results, applications can set the active formats by setting the which argument to V4L2_SUBDEV_FORMAT_ACTIVE. Active formats are changed exactly as try formats by drivers. To avoid modifying the hardware state during format negotiation, applications should negotiate try formats first and then modify the active settings using the try formats returned during the last negotiation iteration. This guarantees that the active format will be applied as-is by the driver without being modified.

4.13.3.2. Selections: cropping, scaling and composition

Many sub-devices support cropping frames on their input or output pads (or possible even on both). Cropping is used to select the area of interest in an image, typically on an image sensor or a video decoder. It can also be used as part of digital zoom implementations to select the area of the image that will be scaled up.

Crop settings are defined by a crop rectangle and represented in a struct v4l2_rect by the coordinates of the top left corner and the rectangle size. Both the coordinates and sizes are expressed in pixels.

As for pad formats, drivers store try and active rectangles for the selection targets Common selection definitions.

On sink pads, cropping is applied relative to the current pad format. The pad format represents the image size as received by the sub-device from the previous block in the pipeline, and the crop rectangle represents the sub-image that will be transmitted further inside the sub-device for processing.

The scaling operation changes the size of the image by scaling it to new dimensions. The scaling ratio isn’t specified explicitly, but is implied from the original and scaled image sizes. Both sizes are represented by struct v4l2_rect.

Scaling support is optional. When supported by a subdev, the crop rectangle on the subdev’s sink pad is scaled to the size configured using the VIDIOC_SUBDEV_S_SELECTION IOCTL using V4L2_SEL_TGT_COMPOSE selection target on the same pad. If the subdev supports scaling but not composing, the top and left values are not used and must always be set to zero.

On source pads, cropping is similar to sink pads, with the exception that the source size from which the cropping is performed, is the COMPOSE rectangle on the sink pad. In both sink and source pads, the crop rectangle must be entirely contained inside the source image size for the crop operation.

The drivers should always use the closest possible rectangle the user requests on all selection targets, unless specifically told otherwise. V4L2_SEL_FLAG_GE and V4L2_SEL_FLAG_LE flags may be used to round the image size either up or down. Selection flags

4.13.3.3. Types of selection targets

4.13.3.3.1. Actual targets

Actual targets (without a postfix) reflect the actual hardware configuration at any point of time. There is a BOUNDS target corresponding to every actual target.

4.13.3.3.2. BOUNDS targets

BOUNDS targets is the smallest rectangle that contains all valid actual rectangles. It may not be possible to set the actual rectangle as large as the BOUNDS rectangle, however. This may be because e.g. a sensor’s pixel array is not rectangular but cross-shaped or round. The maximum size may also be smaller than the BOUNDS rectangle.

4.13.3.4. Order of configuration and format propagation

Inside subdevs, the order of image processing steps will always be from the sink pad towards the source pad. This is also reflected in the order in which the configuration must be performed by the user: the changes made will be propagated to any subsequent stages. If this behaviour is not desired, the user must set V4L2_SEL_FLAG_KEEP_CONFIG flag. This flag causes no propagation of the changes are allowed in any circumstances. This may also cause the accessed rectangle to be adjusted by the driver, depending on the properties of the underlying hardware.

The coordinates to a step always refer to the actual size of the previous step. The exception to this rule is the sink compose rectangle, which refers to the sink compose bounds rectangle --- if it is supported by the hardware.

1. Sink pad format. The user configures the sink pad format. This format defines the parameters of the image the entity receives through the pad for further processing.

2. Sink pad actual crop selection. The sink pad crop defines the crop performed to the sink pad format.

3. Sink pad actual compose selection. The size of the sink pad compose rectangle defines the scaling ratio compared to the size of the sink pad crop rectangle. The location of the compose rectangle specifies the location of the actual sink compose rectangle in the sink compose bounds rectangle.

4. Source pad actual crop selection. Crop on the source pad defines crop performed to the image in the sink compose bounds rectangle.

5. Source pad format. The source pad format defines the output pixel format of the subdev, as well as the other parameters with the exception of the image width and height. Width and height are defined by the size of the source pad actual crop selection.

Accessing any of the above rectangles not supported by the subdev will return EINVAL. Any rectangle referring to a previous unsupported rectangle coordinates will instead refer to the previous supported rectangle. For example, if sink crop is not supported, the compose selection will refer to the sink pad format dimensions instead.

Image processing in subdevs: simple crop example

In the above example, the subdev supports cropping on its sink pad. To configure it, the user sets the media bus format on the subdev’s sink pad. Now the actual crop rectangle can be set on the sink pad --- the location and size of this rectangle reflect the location and size of a rectangle to be cropped from the sink format. The size of the sink crop rectangle will also be the size of the format of the subdev’s source pad.

Image processing in subdevs: scaling with multiple sources

In this example, the subdev is capable of first cropping, then scaling and finally cropping for two source pads individually from the resulting scaled image. The location of the scaled image in the cropped image is ignored in sink compose target. Both of the locations of the source crop rectangles refer to the sink scaling rectangle, independently cropping an area at location specified by the source crop rectangle from it.

Image processing in subdevs: scaling and composition with multiple sinks and sources

The subdev driver supports two sink pads and two source pads. The images from both of the sink pads are individually cropped, then scaled and further composed on the composition bounds rectangle. From that, two independent streams are cropped and sent out of the subdev from the source pads.

• 4.13.3.4.1. Media Bus Formats

4.13.4. Streams, multiplexed media pads and internal routing

Simple V4L2 sub-devices do not support multiple, unrelated video streams, and only a single stream can pass through a media link and a media pad. Thus each pad contains a format and selection configuration for that single stream. A subdev can do stream processing and split a stream into two or compose two streams into one, but the inputs and outputs for the subdev are still a single stream per pad.

Some hardware, e.g. MIPI CSI-2, support multiplexed streams, that is, multiple data streams are transmitted on the same bus, which is represented by a media link connecting a transmitter source pad with a sink pad on the receiver. For example, a camera sensor can produce two distinct streams, a pixel stream and a metadata stream, which are transmitted on the multiplexed data bus, represented by a media link which connects the single sensor’s source pad with the receiver sink pad. The stream-aware receiver will de-multiplex the streams received on the its sink pad and allows to route them individually to one of its source pads.

Subdevice drivers that support multiplexed streams are compatible with non-multiplexed subdev drivers. However, if the driver at the sink end of a link does not support streams, then only stream 0 of source end may be captured. There may be additional limitations specific to the sink device.

4.13.4.1. Understanding streams

A stream is a stream of content (e.g. pixel data or metadata) flowing through the media pipeline from a source (e.g. a sensor) towards the final sink (e.g. a receiver and demultiplexer in a SoC). Each media link carries all the enabled streams from one end of the link to the other, and sub-devices have routing tables which describe how the incoming streams from sink pads are routed to the source pads.

A stream ID is a media pad-local identifier for a stream. Streams IDs of the same stream must be equal on both ends of a link. In other words, a particular stream ID must exist on both sides of a media link, but another stream ID can be used for the same stream at the other side of the sub-device.

A stream at a specific point in the media pipeline is identified by the sub-device and a (pad, stream) pair. For sub-devices that do not support multiplexed streams the ‘stream’ field is always 0.

4.13.4.2. Internal sink pads and routing

Cases where a single sub-device source pad is traversed by multiple streams, one or more of which originate from within the sub-device itself, are special as there is no external sink pad for such routes. In those cases, the sources of the internally generated streams are represented by internal sink pads, which are sink pads that have the MEDIA_PAD_FL_INTERNAL pad flag set.

Internal pads have all the properties of an external pad, including formats and selections. The format in this case is the source format of the stream. An internal pad always has a single stream only (0).

Routes from an internal sink pad to an external source pad are typically not modifiable but they can be activated and deactivated using the V4L2_SUBDEV_ROUTE_FL_ACTIVE flag, depending on driver capabilities. The V4L2_SUBDEV_ROUTE_FL_IMMUTABLE flag indicates that the V4L2_SUBDEV_ROUTE_FLAG_ACTIVE of the route may not be unset.

4.13.4.3. Interaction between routes, streams, formats and selections

The addition of streams to the V4L2 sub-device interface moves the sub-device formats and selections from pads to (pad, stream) pairs. Besides the usual pad, also the stream ID needs to be provided for setting formats and selections. The order of configuring formats and selections along a stream is the same as without streams (see Order of configuration and format propagation).

Instead of the sub-device wide merging of streams from all sink pads towards all source pads, data flows for each route are separate from each other. Any number of routes from streams on sink pads towards streams on source pads is allowed, to the extent supported by drivers. For every stream on a source pad, however, only a single route is allowed.

Any configurations of a stream within a pad, such as format or selections, are independent of similar configurations on other streams. This is subject to change in the future.

4.13.4.4. Device types and routing setup

Different kinds of sub-devices have differing behaviour for route activation, depending on the hardware. In all cases, however, only routes that have the V4L2_SUBDEV_STREAM_FL_ACTIVE flag set are active.

Devices generating the streams may allow enabling and disabling some of the routes or have a fixed routing configuration. If the routes can be disabled, not declaring the routes (or declaring them without V4L2_SUBDEV_STREAM_FL_ACTIVE flag set) in VIDIOC_SUBDEV_S_ROUTING will disable the routes. VIDIOC_SUBDEV_S_ROUTING will still return such routes back to the user in the routes array, with the V4L2_SUBDEV_STREAM_FL_ACTIVE flag unset.

Devices transporting the streams almost always have more configurability with respect to routing. Typically any route between the sub-device’s sink and source pads is possible, and multiple routes (usually up to certain limited number) may be active simultaneously. For such devices, no routes are created by the driver and user-created routes are fully replaced when VIDIOC_SUBDEV_S_ROUTING is called on the sub-device. Such newly created routes have the device’s default configuration for format and selection rectangles.

4.13.4.5. Configuring streams

The configuration of the streams is done individually for each sub-device and the validity of the streams between sub-devices is validated when the pipeline is started.

There are three steps in configuring the streams:

1. Set up links. Connect the pads between sub-devices using the Media Controller API

2. Streams. Streams are declared and their routing is configured by setting the routing table for the sub-device using VIDIOC_SUBDEV_S_ROUTING ioctl. Note that setting the routing table will reset formats and selections in the sub-device to default values.

3. Configure formats and selections. Formats and selections of each stream are configured separately as documented for plain sub-devices in Order of configuration and format propagation. The stream ID is set to the same stream ID associated with either sink or source pads of routes configured using the VIDIOC_SUBDEV_S_ROUTING ioctl.

4.13.4.6. Multiplexed streams setup example

A simple example of a multiplexed stream setup might be as follows:

• Two identical sensors (Sensor A and Sensor B). Each sensor has a single source pad (pad 0) which carries a pixel data stream.

• Multiplexer bridge (Bridge). The bridge has two sink pads, connected to the sensors (pads 0, 1), and one source pad (pad 2), which outputs two streams.

• Receiver in the SoC (Receiver). The receiver has a single sink pad (pad 0), connected to the bridge, and two source pads (pads 1-2), going to the DMA engine. The receiver demultiplexes the incoming streams to the source pads.

• DMA Engines in the SoC (DMA Engine), one for each stream. Each DMA engine is connected to a single source pad in the receiver.

The sensors, the bridge and the receiver are modeled as V4L2 sub-devices, exposed to userspace via /dev/v4l-subdevX device nodes. The DMA engines are modeled as V4L2 devices, exposed to userspace via /dev/videoX nodes.

To configure this pipeline, the userspace must take the following steps:

1. Set up media links between entities: connect the sensors to the bridge, bridge to the receiver, and the receiver to the DMA engines. This step does not differ from normal non-multiplexed media controller setup.

2. Configure routing

Bridge routing table

Sink Pad/Stream	Source Pad/Stream	Routing Flags	Comments
0/0	2/0	V4L2_SUBDEV_ROUTE_FL_ACTIVE	Pixel data stream from Sensor A
1/0	2/1	V4L2_SUBDEV_ROUTE_FL_ACTIVE	Pixel data stream from Sensor B

Receiver routing table

Sink Pad/Stream	Source Pad/Stream	Routing Flags	Comments
0/0	1/0	V4L2_SUBDEV_ROUTE_FL_ACTIVE	Pixel data stream from Sensor A
0/1	2/0	V4L2_SUBDEV_ROUTE_FL_ACTIVE	Pixel data stream from Sensor B

3. Configure formats and selections

   After configuring routing, the next step is configuring the formats and selections for the streams. This is similar to performing this step without streams, with just one exception: the stream field needs to be assigned to the value of the stream ID.

   A common way to accomplish this is to start from the sensors and propagate the configurations along the stream towards the receiver, using VIDIOC_SUBDEV_S_FMT ioctls to configure each stream endpoint in each sub-device.

4.13.4.7. Internal pads setup example

A simple example of a multiplexed stream setup might be as follows:

• An IMX219 camera sensor source sub-device, with one source pad (0), one internal sink pad (1) as the source of the image data and an internal sink pad (2) as the source of the embedded data. There are two routes, one from the internal sink pad 1 to the source pad 0 (image data) and another from the internal sink pad 2 to the source pad 0 (embedded data). Both streams are always active, i.e. there is no need to separately enable the embedded data stream. The sensor uses the CSI-2 interface.

• A CSI-2 receiver in the SoC. The receiver has a single sink pad (pad 0), connected to the sensor, and two source pads (pads 1 and 2), to the two DMA engines. The receiver demultiplexes the incoming streams to the source pads.

• DMA engines in the SoC, one for each stream. Each DMA engine is connected to a single source pad of the receiver.

The sensor and the receiver are modelled as V4L2 sub-devices, exposed to userspace via /dev/v4l-subdevX device nodes. The DMA engines are modelled as V4L2 devices, exposed to userspace via /dev/videoX nodes.

To configure this pipeline, the userspace must take the following steps:

1. Set up media links between entities: enable the links from the sensor to the receiver and from the receiver to the DMA engines. This step does not differ from normal non-multiplexed media controller setup.

2. Configure routing

Camera sensor. There are no configurable routes.

Sink Pad/Stream	Source Pad/Stream	Routing Flags	Comments
1/0	0/0	V4L2_SUBDEV_ROUTE_FL_ACTIVE	Pixel data stream from the internal image sink pad
2/0	0/1	V4L2_SUBDEV_ROUTE_FL_ACTIVE	Metadata stream from the internal embedded data sink pad

The options available in the sensor’s routing configuration are dictated by hardware capabilities: typically camera sensors always produce an image data stream while it may be possible to enable and disable the embedded data stream.

Receiver routing table. Typically both routes need to be explicitly set.

Sink Pad/Stream	Source Pad/Stream	Routing Flags	Comments
0/0	1/0	V4L2_SUBDEV_ROUTE_FL_ACTIVE	Pixel data stream from camera sensor
0/1	2/0	V4L2_SUBDEV_ROUTE_FL_ACTIVE	Metadata stream from camera sensor

3. Configure formats and selections

   This example assumes that the formats are propagated from sink pad to the source pad as-is. The tables contain fields of both struct v4l2_subdev_format and struct v4l2_mbus_framefmt.

Formats set on the sub-devices. Bold values are set, others are static or propagated. The order is aligned with configured routes.

Sub-device	Pad/Stream	Width	Height	Code
IMX219	1/0	3296	2480	MEDIA_BUS_FMT_SRGGB10
	0/0	3296	2480	MEDIA_BUS_FMT_SRGGB10
	2/0	3296	2	MEDIA_BUS_FMT_IMX219_EMBEDDED
	0/1	3296	2	MEDIA_BUS_FMT_META_10
CSI-2 receiver	0/0	3296	2480	MEDIA_BUS_FMT_SRGGB10
	1/0	3296	2480	MEDIA_BUS_FMT_SRGGB10
	0/1	3296	2	MEDIA_BUS_FMT_META_10
	2/0	3296	2	MEDIA_BUS_FMT_META_10

The embedded data format does not need to be configured on the sensor’s pads as the format is dictated by the pixel data format in this case.

4.13.5. Sub-device configuration models

A sub-device configuration model specifies in detail what the user space can expect from a sub-device in terms of V4L2 sub-device interface support, including but not limited to IOCTLs, selection rectangles and controls.

A sub-device may implement more than one configuration model at the same time. The implemented configuration models can be obtained from the sub-device’s V4L2_CID_CONFIG_MODEL control.

4.13.5.1. Common raw camera sensor model

The common raw camera sensor model defines the configurability of a superset that covers the vast majority of raw camera sensors. Not all of the configuration and enumeration interfaces are offered by all drivers.

A sub-device complies with the common raw sensor model if the V4L2_CONFIG_MODEL_COMMON_RAW bit is set in the V4L2_CID_CONFIG_MODEL control of the sub-device.

The common raw camera sensor model is aligned with Using camera sensor drivers. Please refer to that regarding aspects not specified here.

Each camera sensor implementing the common raw sensor model exposes a single V4L2 sub-device. The sub-device contains a single source pad (0) and two or more internal pads: an image data internal pad (1) and optionally an embedded data pad (2). Additionally, further internal pads may be supported for other features, in which case they are documented separately for the given device.

This is show in Common raw sensor sub-device.

Common raw sensor sub-device

4.13.5.1.1. Routes

A sub-device conforming to common raw camera sensor model implements the following routes.

Routes

Sink pad/stream	Source pad/stream	Static (X/M(aybe)/-)	Mandatory (X/-)	Synopsis
1/0	0/0	X	X	Image data
2/0	0/1	M		Embedded data

Some devices may support enabling and disabling the embedded data stream. Others may not support it at all, in which case the embedded data route does not exist.

4.13.5.1.2. Sensor pixel array size, cropping and binning

The sensor’s pixel array is divided into one or more areas. The areas around the edge of the pixel array, usually one one or more sides, may contain optical black pixels, dummy pixels and other non-image pixels.

A rectangle within the pixel area contains the visible pixels. Capturing the non-visible pixels may be supported by the sensor.

The sensor can perform three operations that affect the output image size. First comes analogue crop. This configuration limits parts of the pixel array which the sensor will read, affecting sensor timing as well. The granularity of the analogue crop configuration varies greatly across sensors: some sensors support a few different analogue crop configurations whereas others may support anything divisible by a given number of pixels.

The default analogue crop rectangle corresponds to the visible pixel area if supported by the hardware.

In the next step, binning is performed on the image data read from camera sensor’s pixel array. This will effectively result in an image smaller than the original by given proportions horizontally and vertically. Typical values are 1/2 and 1/3 but others may well be supported by the hardware as well.

The combination of the analogue crop and binning operations may result in an image size that may be larger than desirable. For this purpose, a digital crop operation may be performed on the binned image. The resulting image size is further outputted by the sensor.

Selection targets on pads

Pad/Stream	Selection target/format	Mandatory (X/-)	Synopsis
1/0	Format	X	Image data format. The width and height fields of this format are the same than those for the V4L2_SEL_TGT_CROP_BOUNDS rectangle. The media bus code of this format reflects the native pixel depth of the sensor.
1/0	V4L2_SEL_TGT_NATIVE_SIZE	X	The full size of the pixel array, including all pixels in the pixel array, even if they cannot be captured. This rectangle is relative to the format on the same (pad, stream).
1/0	V4L2_SEL_TGT_CROP_BOUNDS	X	The crop rectangle bounds. No pixels outside this area can be captured. This rectangle is relative to the V4L2_SEL_TGT_NATIVE_SIZE rectangle on the same (pad, stream).
1/0	V4L2_SEL_TGT_CROP_DEFAULT	X	The visible pixel area. This rectangle is relative to the V4L2_SEL_TGT_NATIVE_SIZE rectangle on the same (pad, stream).
1/0	V4L2_SEL_TGT_CROP	-	Analogue crop. Analogue crop typically has a coarse granularity. This rectangle is relative to the V4L2_SEL_TGT_NATIVE_SIZE rectangle on the same (pad, stream).
1/0	V4L2_SEL_TGT_COMPOSE	-	Binning. This rectangle is relative to the V4L2_SEL_TGT_CROP rectangle on the same (pad, stream).
2/0	Format	X	Embedded data format.
0/0	V4L2_SEL_TGT_CROP	-	Digital crop. This rectangle is relative to the V4L2_SEL_TGT_COMPOSE rectangle on (pad, stream) pair 1/0.
0/0	Format	X	Image data source format. The width and height fields of the format are the same than for the V4L2_SEL_TGT crop rectangle on (pad, stream) pair 0/0 where as the media bus code reflects the pixel data output of the sensor.
0/1	Format	X	Embedded data source format.

4.13.5.1.3. Embedded data

The embedded data stream is produced by the sensor when the corresponding route is enabled. The embedded data route may also be immutable or not exist at all, in case the sensor (or the driver) does not support it.

Generally the sensor embedded data width is determined by the width of the image data whereas the number of lines are constant for the embedded data. The user space may obtain the size of the embedded data once the image data size on the source pad has been configured.

Also see Embedded data.