media/v4l/dev-subdev.rst

1 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
6 Sub-device Interface
13 components as software blocks called sub-devices.
15 V4L2 sub-devices are usually kernel-only objects. If the V4L2 driver
17 media entities. Applications will be able to enumerate the sub-devices
21 In addition to make sub-devices discoverable, drivers can also choose to
23 sub-device driver and the V4L2 device driver support this, sub-devices
26 -  query, read and write sub-devices controls
28 -  subscribe and unsubscribe to events and retrieve them
30 -  negotiate image formats on individual pads
32 -  inspect and modify internal data routing between pads of the same entity
34 Sub-device character device nodes, conventionally named
35 ``/dev/v4l-subdev*``, use major number 81.
37 Drivers may opt to limit the sub-device character devices to only expose
38 operations that do not modify the device state. In such a case the sub-devices
39 are referred to as ``read-only`` in the rest of this documentation, and the
46 Most V4L2 controls are implemented by sub-device hardware. Drivers
48 Applications can control all sub-devices through a single interface.
52 sensors and image processing hardware implement identical functions,
57 Applications can access those hidden controls through the sub-device
61 sub-device.
70 V4L2 sub-devices can notify applications of events as described in
73 the sub-device. Depending on the driver, those events might also be
77 .. _pad-level-formats:
79 Pad-level Formats
84     Pad-level formats are only applicable to very complex devices that
85     need to expose low-level format configuration to user space. Generic
94 Image formats are typically negotiated on video capture and output
101 image sizes at the output of a pipeline can be achieved using different
103 :ref:`pipeline-scaling`, where image scaling can be performed on both
104 the video sensor and the host image processing hardware.
107 .. _pipeline-scaling:
109 .. kernel-figure:: pipeline.dot
113     Image Format Negotiation on Pipelines
119 The sensor scaler is usually of less quality than the host scaler, but
120 scaling on the sensor is required to achieve higher frame rates.
125 Drivers that implement the :ref:`media API <media-controller-intro>`
126 can expose pad-level image format configuration to applications. When
130 negotiate formats on a per-pad basis.
138 Pad-level image format configuration support can be tested by calling
140 0. If the driver returns an ``EINVAL`` error code pad-level format
141 configuration is not supported by the sub-device.
145 ------------------
164 the sub-device file handles. A
166 the last try format set *on the same sub-device file handle*. Several
167 applications querying the same sub-device at the same time will thus not
181 to an :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` call as-is
185 Drivers automatically propagate formats inside sub-devices. When a try
187 the same sub-device can be modified by the driver. Drivers are free to
191 -  Formats should be propagated from sink pads to source pads. Modifying
195 -  Sub-devices that scale frames using variable scaling factors should
201 propagating them from one sub-device file handle to another.
207 :ref:`sample-pipeline-config` shows a sample configuration sequence
208 for the pipeline described in :ref:`pipeline-scaling` (table columns
220 .. _sample-pipeline-config:
222 .. flat-table:: Sample Pipeline Configuration
223     :header-rows:  1
224     :stub-columns: 0
227     * -
228       - Sensor/0
231       - Frontend/0
234       - Frontend/1
237       - Scaler/0
240       - Scaler/0
243       - Scaler/1
246     * - Initial state
247       - 2048x1536
250       - (default)
251       - (default)
252       - (default)
253       - (default)
254       - (default)
255     * - Configure frontend sink format
256       - 2048x1536
259       - *2048x1536*
262       - *2046x1534*
265       - (default)
266       - (default)
267       - (default)
268     * - Configure scaler sink format
269       - 2048x1536
272       - 2048x1536
275       - 2046x1534
278       - *2046x1534*
281       - *0,0/2046x1534*
282       - *2046x1534*
285     * - Configure scaler sink compose selection
286       - 2048x1536
289       - 2048x1536
292       - 2046x1534
295       - 2046x1534
298       - *0,0/1280x960*
299       - *1280x960*
307 1. Initial state. The sensor source pad format is set to its native 3MP
334 be applied as-is by the driver without being modified.
337 .. _v4l2-subdev-selections:
340 ---------------------------------------------
342 Many sub-devices support cropping frames on their input or output pads
344 interest in an image, typically on an image sensor or a video decoder.
346 the area of the image that will be scaled up.
354 selection targets :ref:`v4l2-selections-common`.
357 The pad format represents the image size as received by the sub-device
359 represents the sub-image that will be transmitted further inside the
360 sub-device for processing.
362 The scaling operation changes the size of the image by scaling it to new
364 from the original and scaled image sizes. Both sizes are represented by
378 crop rectangle must be entirely contained inside the source image size
384 the image size either up or down. :ref:`v4l2-selection-flags`
388 --------------------------
404 as the BOUNDS rectangle, however. This may be because e.g. a sensor's
405 pixel array is not rectangular but cross-shaped or round. The maximum
409 .. _format-propagation:
412 ---------------------------------------------
414 Inside subdevs, the order of image processing steps will always be from
425 rectangle, which refers to the sink compose bounds rectangle --- if it
429    defines the parameters of the image the entity receives through the
442    performed to the image in the sink compose bounds rectangle.
446    exception of the image width and height. Width and height are defined
456 .. _subdev-image-processing-crop:
458 .. kernel-figure:: subdev-image-processing-crop.svg
459     :alt:   subdev-image-processing-crop.svg
462     **Figure 4.5. Image processing in subdevs: simple crop example**
466 pad. Now the actual crop rectangle can be set on the sink pad --- the
473 .. _subdev-image-processing-scaling-multi-source:
475 .. kernel-figure:: subdev-image-processing-scaling-multi-source.svg
476     :alt:   subdev-image-processing-scaling-multi-source.svg
479     **Figure 4.6. Image processing in subdevs: scaling with multiple sources**
483 scaled image. The location of the scaled image in the cropped image is
489 .. _subdev-image-processing-full:
491 .. kernel-figure:: subdev-image-processing-full.svg
492     :alt:    subdev-image-processing-full.svg
495 …**Figure 4.7. Image processing in subdevs: scaling and composition with multiple sinks and sources…
507     subdev-formats
509 .. _subdev-routing:
512 ----------------------------------------------------
514 Simple V4L2 sub-devices do not support multiple, unrelated video streams,
521 Some hardware, e.g. MIPI CSI-2, support multiplexed streams, that is, multiple
524 example, a camera sensor can produce two distinct streams, a pixel stream and a
526 by a media link which connects the single sensor's source pad with the receiver
527 sink pad. The stream-aware receiver will de-multiplex the streams received on
532 non-multiplexed subdev drivers. However, if the driver at the sink end of a link
540 the media pipeline from a source (e.g. a sensor) towards the final sink (e.g. a
542 streams from one end of the link to the other, and sub-devices have routing
546 A stream ID is a media pad-local identifier for a stream. Streams IDs of
550 of the sub-device.
553 sub-device and a (pad, stream) pair. For sub-devices that do not support
559 The addition of streams to the V4L2 sub-device interface moves the sub-device
563 the same as without streams (see :ref:`format-propagation`).
565 Instead of the sub-device wide merging of streams from all sink pads
578 Different kinds of sub-devices have differing behaviour for route activation,
591 respect to routing. Typically any route between the sub-device's sink and source
594 and user-created routes are fully replaced when ``VIDIOC_SUBDEV_S_ROUTING`` is
595 called on the sub-device. Such newly created routes have the device's default
601 The configuration of the streams is done individually for each sub-device and
602 the validity of the streams between sub-devices is validated when the pipeline
607 1. Set up links. Connect the pads between sub-devices using the
611    routing table for the sub-device using :ref:`VIDIOC_SUBDEV_S_ROUTING
613    reset formats and selections in the sub-device to default values.
616    configured separately as documented for plain sub-devices in
617    :ref:`format-propagation`. The stream ID is set to the same stream ID
626 - Two identical sensors (Sensor A and Sensor B). Each sensor has a single source
629 - Multiplexer bridge (Bridge). The bridge has two sink pads, connected to the
632 - Receiver in the SoC (Receiver). The receiver has a single sink pad (pad 0),
633   connected to the bridge, and two source pads (pads 1-2), going to the DMA
636 - DMA Engines in the SoC (DMA Engine), one for each stream. Each DMA engine is
639 The sensors, the bridge and the receiver are modeled as V4L2 sub-devices,
640 exposed to userspace via /dev/v4l-subdevX device nodes. The DMA engines are
647    not differ from normal non-multiplexed media controller setup.
651 .. flat-table:: Bridge routing table
652     :header-rows:  1
654     * - Sink Pad/Stream
655       - Source Pad/Stream
656       - Routing Flags
657       - Comments
658     * - 0/0
659       - 2/0
660       - V4L2_SUBDEV_ROUTE_FL_ACTIVE
661       - Pixel data stream from Sensor A
662     * - 1/0
663       - 2/1
664       - V4L2_SUBDEV_ROUTE_FL_ACTIVE
665       - Pixel data stream from Sensor B
667 .. flat-table:: Receiver routing table
668     :header-rows:  1
670     * - Sink Pad/Stream
671       - Source Pad/Stream
672       - Routing Flags
673       - Comments
674     * - 0/0
675       - 1/0
676       - V4L2_SUBDEV_ROUTE_FL_ACTIVE
677       - Pixel data stream from Sensor A
678     * - 0/1
679       - 2/0
680       - V4L2_SUBDEV_ROUTE_FL_ACTIVE
681       - Pixel data stream from Sensor B
693    stream endpoint in each sub-device.