This is an experimental interface and may change in the future. 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 Sub-device character device nodes, conventionally named /dev/v4l-subdev*, use major number 81.

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 . 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.

V4L2 sub-devices can notify applications of events as described in . 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.

Pad-level formats are only applicable to very complex device 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. 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 cropping and scaling 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 , where image scaling can be performed on both the video sensor and the host image processing hardware.

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; is then returned if the configuration is invalid. Pad-level image format configuration support can be tested by calling the &VIDIOC-SUBDEV-G-FMT; ioctl on pad 0. If the driver returns an &EINVAL; pad-level format configuration is not supported by the sub-device.

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. shows a sample configuration sequence for the pipeline described in (table columns list entity names and pad numbers). Sensor/0 Frontend/0 Frontend/1 Scaler/0 Scaler/1 Initial state 2048x1536 - - - - Configure frontend input 2048x1536 2048x1536 2046x1534 - - Configure scaler input 2048x1536 2048x1536 2046x1534 2046x1534 2046x1534 Configure scaler output 2048x1536 2048x1536 2046x1534 2046x1534 1280x960 Initial state. The sensor output is set to its native 3MP resolution. Resolutions on the host frontend and scaler input and output pads are undefined. The application configures the frontend input pad resolution to 2048x1536. The driver propagates the format to the frontend output pad. Note that the propagated output format can be different, as in this case, than the input format, as the hardware might need to crop pixels (for instance when converting a Bayer filter pattern to RGB or YUV). The application configures the scaler input pad resolution to 2046x1534 to match the frontend output resolution. The driver propagates the format to the scaler output pad. The application configures the scaler output pad resolution to 1280x960. When satisfied with the try results, applications can set the active formats by setting the which argument to V4L2_SUBDEV_FORMAT_TRY. 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.

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 a video sensor or 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 &v4l2-rect; by the coordinates of the top left corner and the rectangle size. Both the coordinates and sizes are expressed in pixels. The crop rectangle is retrieved and set using the &VIDIOC-SUBDEV-G-CROP; and &VIDIOC-SUBDEV-S-CROP; ioctls. Like for pad formats, drivers store try and active crop rectangles. The format negotiation mechanism applies to crop settings as well. On input pads, cropping is applied relatively 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 crop rectangle be entirely containted inside the input image size. Input crop rectangle are reset to their default value when the input image format is modified. Drivers should use the input image size as the crop rectangle default value, but hardware requirements may prevent this. Cropping behaviour on output pads is not defined.

&sub-subdev-formats;