1 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
9 The complex nature of V4L2 devices, where hardware is often made of
10 several integrated circuits that need to interact with each other in a
11 controlled way, leads to complex V4L2 drivers. The drivers usually
12 reflect the hardware model in software, and model the different hardware
13 components as software blocks called sub-devices.
15 V4L2 sub-devices are usually kernel-only objects. If the V4L2 driver
16 implements the media device API, they will automatically inherit from
17 media entities. Applications will be able to enumerate the sub-devices
18 and discover the hardware topology using the media entities, pads and
19 links enumeration API.
21 In addition to make sub-devices discoverable, drivers can also choose to
22 make them directly configurable by applications. When both the
23 sub-device driver and the V4L2 device driver support this, sub-devices
24 will feature a character device node on which ioctls can be called to
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
40 related restrictions are documented in individual ioctls.
46 Most V4L2 controls are implemented by sub-device hardware. Drivers
47 usually merge all controls and expose them through video device nodes.
48 Applications can control all sub-devices through a single interface.
50 Complex devices sometimes implement the same control in different pieces
51 of hardware. This situation is common in embedded platforms, where both
52 sensors and image processing hardware implement identical functions,
53 such as contrast adjustment, white balance or faulty pixels correction.
54 As the V4L2 controls API doesn't support several identical controls in a
55 single device, all but one of the identical controls are hidden.
57 Applications can access those hidden controls through the sub-device
58 node with the V4L2 control API described in :ref:`control`. The ioctls
59 behave identically as when issued on V4L2 device nodes, with the
60 exception that they deal only with controls implemented in the
63 Depending on the driver, those controls might also be exposed through
64 one (or several) V4L2 device nodes.
70 V4L2 sub-devices can notify applications of events as described in
71 :ref:`event`. The API behaves identically as when used on V4L2 device
72 nodes, with the exception that it only deals with events generated by
73 the sub-device. Depending on the driver, those events might also be
74 reported on one (or several) V4L2 device nodes.
77 .. _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
86 V4L2 applications do *not* need to use the API described in this
91 For the purpose of this section, the term *format* means the
92 combination of media bus data format, frame width and frame height.
94 Image formats are typically negotiated on video capture and output
95 devices using the format and
96 :ref:`selection <VIDIOC_SUBDEV_G_SELECTION>` ioctls. The driver is
97 responsible for configuring every block in the video pipeline according
98 to the requested format at the pipeline input and/or output.
100 For complex devices, such as often found in embedded systems, identical
101 image sizes at the output of a pipeline can be achieved using different
102 hardware configurations. One such example is shown on
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
115 High quality and high speed pipeline configuration
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.
121 Depending on the use case (quality vs. speed), the pipeline must be
122 configured differently. Applications need to configure the formats at
123 every point in the pipeline explicitly.
125 Drivers that implement the :ref:`media API <media-controller-intro>`
126 can expose pad-level image format configuration to applications. When
127 they do, applications can use the
128 :ref:`VIDIOC_SUBDEV_G_FMT <VIDIOC_SUBDEV_G_FMT>` and
129 :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` ioctls. to
130 negotiate formats on a per-pad basis.
132 Applications are responsible for configuring coherent parameters on the
133 whole pipeline and making sure that connected pads have compatible
134 formats. The pipeline is checked for formats mismatch at
135 :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` time, and an ``EPIPE`` error
136 code is then returned if the configuration is invalid.
138 Pad-level image format configuration support can be tested by calling
139 the :ref:`VIDIOC_SUBDEV_G_FMT` ioctl on pad
140 0. If the driver returns an ``EINVAL`` error code pad-level format
141 configuration is not supported by the sub-device.
147 Acceptable formats on pads can (and usually do) depend on a number of
148 external parameters, such as formats on other pads, active links, or
149 even controls. Finding a combination of formats on all pads in a video
150 pipeline, acceptable to both application and driver, can't rely on
151 formats enumeration only. A format negotiation mechanism is required.
153 Central to the format negotiation mechanism are the get/set format
154 operations. When called with the ``which`` argument set to
155 :ref:`V4L2_SUBDEV_FORMAT_TRY <VIDIOC_SUBDEV_G_FMT>`, the
156 :ref:`VIDIOC_SUBDEV_G_FMT <VIDIOC_SUBDEV_G_FMT>` and
157 :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` ioctls operate on
158 a set of formats parameters that are not connected to the hardware
159 configuration. Modifying those 'try' formats leaves the device state
160 untouched (this applies to both the software state stored in the driver
161 and the hardware state stored in the device itself).
163 While not kept as part of the device state, try formats are stored in
164 the sub-device file handles. A
165 :ref:`VIDIOC_SUBDEV_G_FMT <VIDIOC_SUBDEV_G_FMT>` call will return
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
168 interact with each other.
170 To find out whether a particular format is supported by the device,
172 :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` ioctl. Drivers
173 verify and, if needed, change the requested ``format`` based on device
174 requirements and return the possibly modified value. Applications can
175 then choose to try a different format or accept the returned value and
178 Formats returned by the driver during a negotiation iteration are
179 guaranteed to be supported by the device. In particular, drivers
180 guarantee that a returned format will not be further changed if passed
181 to an :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` call as-is
182 (as long as external parameters, such as formats on other pads or links'
183 configuration are not changed).
185 Drivers automatically propagate formats inside sub-devices. When a try
186 or active format is set on a pad, corresponding formats on other pads of
187 the same sub-device can be modified by the driver. Drivers are free to
188 modify formats as required by the device. However, they should comply
189 with the following rules when possible:
191 - Formats should be propagated from sink pads to source pads. Modifying
192 a format on a source pad should not modify the format on any sink
195 - Sub-devices that scale frames using variable scaling factors should
196 reset the scale factors to default values when sink pads formats are
197 modified. If the 1:1 scaling ratio is supported, this means that
198 source pads formats should be reset to the sink pads formats.
200 Formats are not propagated across links, as that would involve
201 propagating them from one sub-device file handle to another.
202 Applications must then take care to configure both ends of every link
203 explicitly with compatible formats. Identical formats on the two ends of
204 a link are guaranteed to be compatible. Drivers are free to accept
205 different formats matching device requirements as being compatible.
207 :ref:`sample-pipeline-config` shows a sample configuration sequence
208 for the pipeline described in :ref:`pipeline-scaling` (table columns
209 list entity names and pad numbers).
216 \setlength{\tabcolsep}{2pt}
218 .. tabularcolumns:: |p{2.0cm}|p{2.1cm}|p{2.1cm}|p{2.1cm}|p{2.1cm}|p{2.1cm}|p{2.1cm}|
220 .. _sample-pipeline-config:
222 .. flat-table:: Sample Pipeline Configuration
225 :widths: 5 5 5 5 5 5 5
242 compose selection rectangle
255 * - Configure frontend sink format
268 * - Configure scaler sink format
285 * - Configure scaler sink compose selection
307 1. Initial state. The sensor source pad format is set to its native 3MP
308 size and V4L2_MBUS_FMT_SGRBG8_1X8 media bus code. Formats on the
309 host frontend and scaler sink and source pads have the default
310 values, as well as the compose rectangle on the scaler's sink pad.
312 2. The application configures the frontend sink pad format's size to
313 2048x1536 and its media bus code to V4L2_MBUS_FMT_SGRBG_1X8. The
314 driver propagates the format to the frontend source pad.
316 3. The application configures the scaler sink pad format's size to
317 2046x1534 and the media bus code to V4L2_MBUS_FMT_SGRBG_1X8 to
318 match the frontend source size and media bus code. The media bus code
319 on the sink pad is set to V4L2_MBUS_FMT_SGRBG_1X8. The driver
320 propagates the size to the compose selection rectangle on the
321 scaler's sink pad, and the format to the scaler source pad.
323 4. The application configures the size of the compose selection
324 rectangle of the scaler's sink pad 1280x960. The driver propagates
325 the size to the scaler's source pad format.
327 When satisfied with the try results, applications can set the active
328 formats by setting the ``which`` argument to
329 ``V4L2_SUBDEV_FORMAT_ACTIVE``. Active formats are changed exactly as try
330 formats by drivers. To avoid modifying the hardware state during format
331 negotiation, applications should negotiate try formats first and then
332 modify the active settings using the try formats returned during the
333 last negotiation iteration. This guarantees that the active format will
334 be applied as-is by the driver without being modified.
337 .. _v4l2-subdev-selections:
339 Selections: cropping, scaling and composition
340 ---------------------------------------------
342 Many sub-devices support cropping frames on their input or output pads
343 (or possible even on both). Cropping is used to select the area of
344 interest in an image, typically on an image sensor or a video decoder.
345 It can also be used as part of digital zoom implementations to select
346 the area of the image that will be scaled up.
348 Crop settings are defined by a crop rectangle and represented in a
349 struct :c:type:`v4l2_rect` by the coordinates of the top
350 left corner and the rectangle size. Both the coordinates and sizes are
353 As for pad formats, drivers store try and active rectangles for the
354 selection targets :ref:`v4l2-selections-common`.
356 On sink pads, cropping is applied relative to the current pad format.
357 The pad format represents the image size as received by the sub-device
358 from the previous block in the pipeline, and the crop rectangle
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
363 dimensions. The scaling ratio isn't specified explicitly, but is implied
364 from the original and scaled image sizes. Both sizes are represented by
365 struct :c:type:`v4l2_rect`.
367 Scaling support is optional. When supported by a subdev, the crop
368 rectangle on the subdev's sink pad is scaled to the size configured
370 :ref:`VIDIOC_SUBDEV_S_SELECTION <VIDIOC_SUBDEV_G_SELECTION>` IOCTL
371 using ``V4L2_SEL_TGT_COMPOSE`` selection target on the same pad. If the
372 subdev supports scaling but not composing, the top and left values are
373 not used and must always be set to zero.
375 On source pads, cropping is similar to sink pads, with the exception
376 that the source size from which the cropping is performed, is the
377 COMPOSE rectangle on the sink pad. In both sink and source pads, the
378 crop rectangle must be entirely contained inside the source image size
379 for the crop operation.
381 The drivers should always use the closest possible rectangle the user
382 requests on all selection targets, unless specifically told otherwise.
383 ``V4L2_SEL_FLAG_GE`` and ``V4L2_SEL_FLAG_LE`` flags may be used to round
384 the image size either up or down. :ref:`v4l2-selection-flags`
387 Types of selection targets
388 --------------------------
394 Actual targets (without a postfix) reflect the actual hardware
395 configuration at any point of time. There is a BOUNDS target
396 corresponding to every actual target.
402 BOUNDS targets is the smallest rectangle that contains all valid actual
403 rectangles. It may not be possible to set the actual rectangle as large
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
406 size may also be smaller than the BOUNDS rectangle.
409 .. _format-propagation:
411 Order of configuration and format propagation
412 ---------------------------------------------
414 Inside subdevs, the order of image processing steps will always be from
415 the sink pad towards the source pad. This is also reflected in the order
416 in which the configuration must be performed by the user: the changes
417 made will be propagated to any subsequent stages. If this behaviour is
418 not desired, the user must set ``V4L2_SEL_FLAG_KEEP_CONFIG`` flag. This
419 flag causes no propagation of the changes are allowed in any
420 circumstances. This may also cause the accessed rectangle to be adjusted
421 by the driver, depending on the properties of the underlying hardware.
423 The coordinates to a step always refer to the actual size of the
424 previous step. The exception to this rule is the sink compose
425 rectangle, which refers to the sink compose bounds rectangle --- if it
426 is supported by the hardware.
428 1. Sink pad format. The user configures the sink pad format. This format
429 defines the parameters of the image the entity receives through the
430 pad for further processing.
432 2. Sink pad actual crop selection. The sink pad crop defines the crop
433 performed to the sink pad format.
435 3. Sink pad actual compose selection. The size of the sink pad compose
436 rectangle defines the scaling ratio compared to the size of the sink
437 pad crop rectangle. The location of the compose rectangle specifies
438 the location of the actual sink compose rectangle in the sink compose
441 4. Source pad actual crop selection. Crop on the source pad defines crop
442 performed to the image in the sink compose bounds rectangle.
444 5. Source pad format. The source pad format defines the output pixel
445 format of the subdev, as well as the other parameters with the
446 exception of the image width and height. Width and height are defined
447 by the size of the source pad actual crop selection.
449 Accessing any of the above rectangles not supported by the subdev will
450 return ``EINVAL``. Any rectangle referring to a previous unsupported
451 rectangle coordinates will instead refer to the previous supported
452 rectangle. For example, if sink crop is not supported, the compose
453 selection will refer to the sink pad format dimensions instead.
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**
464 In the above example, the subdev supports cropping on its sink pad. To
465 configure it, the user sets the media bus format on the subdev's sink
466 pad. Now the actual crop rectangle can be set on the sink pad --- the
467 location and size of this rectangle reflect the location and size of a
468 rectangle to be cropped from the sink format. The size of the sink crop
469 rectangle will also be the size of the format of the subdev's source
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**
481 In this example, the subdev is capable of first cropping, then scaling
482 and finally cropping for two source pads individually from the resulting
483 scaled image. The location of the scaled image in the cropped image is
484 ignored in sink compose target. Both of the locations of the source crop
485 rectangles refer to the sink scaling rectangle, independently cropping
486 an area at location specified by the source crop rectangle from it.
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**
497 The subdev driver supports two sink pads and two source pads. The images
498 from both of the sink pads are individually cropped, then scaled and
499 further composed on the composition bounds rectangle. From that, two
500 independent streams are cropped and sent out of the subdev from the
509 Streams, multiplexed media pads and internal routing
510 ----------------------------------------------------
512 Simple V4L2 sub-devices do not support multiple, unrelated video streams,
513 and only a single stream can pass through a media link and a media pad.
514 Thus each pad contains a format and selection configuration for that
515 single stream. A subdev can do stream processing and split a stream into
516 two or compose two streams into one, but the inputs and outputs for the
517 subdev are still a single stream per pad.
519 Some hardware, e.g. MIPI CSI-2, support multiplexed streams, that is, multiple
520 data streams are transmitted on the same bus, which is represented by a media
521 link connecting a transmitter source pad with a sink pad on the receiver. For
522 example, a camera sensor can produce two distinct streams, a pixel stream and a
523 metadata stream, which are transmitted on the multiplexed data bus, represented
524 by a media link which connects the single sensor's source pad with the receiver
525 sink pad. The stream-aware receiver will de-multiplex the streams received on
526 the its sink pad and allows to route them individually to one of its source
529 Subdevice drivers that support multiplexed streams are compatible with
530 non-multiplexed subdev drivers, but, of course, require a routing configuration
531 where the link between those two types of drivers contains only a single
534 Understanding streams
535 ^^^^^^^^^^^^^^^^^^^^^
537 A stream is a stream of content (e.g. pixel data or metadata) flowing through
538 the media pipeline from a source (e.g. a sensor) towards the final sink (e.g. a
539 receiver and demultiplexer in a SoC). Each media link carries all the enabled
540 streams from one end of the link to the other, and sub-devices have routing
541 tables which describe how the incoming streams from sink pads are routed to the
544 A stream ID is a media pad-local identifier for a stream. Streams IDs of
545 the same stream must be equal on both ends of a link. In other words,
546 a particular stream ID must exist on both sides of a media
547 link, but another stream ID can be used for the same stream at the other side
550 A stream at a specific point in the media pipeline is identified by the
551 sub-device and a (pad, stream) pair. For sub-devices that do not support
552 multiplexed streams the 'stream' field is always 0.
554 Interaction between routes, streams, formats and selections
555 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
557 The addition of streams to the V4L2 sub-device interface moves the sub-device
558 formats and selections from pads to (pad, stream) pairs. Besides the
559 usual pad, also the stream ID needs to be provided for setting formats and
560 selections. The order of configuring formats and selections along a stream is
561 the same as without streams (see :ref:`format-propagation`).
563 Instead of the sub-device wide merging of streams from all sink pads
564 towards all source pads, data flows for each route are separate from each
565 other. Any number of routes from streams on sink pads towards streams on
566 source pads is allowed, to the extent supported by drivers. For every
567 stream on a source pad, however, only a single route is allowed.
569 Any configurations of a stream within a pad, such as format or selections,
570 are independent of similar configurations on other streams. This is
571 subject to change in the future.
576 The configuration of the streams is done individually for each sub-device and
577 the validity of the streams between sub-devices is validated when the pipeline
580 There are three steps in configuring the streams:
582 1. Set up links. Connect the pads between sub-devices using the
583 :ref:`Media Controller API <media_controller>`
585 2. Streams. Streams are declared and their routing is configured by setting the
586 routing table for the sub-device using :ref:`VIDIOC_SUBDEV_S_ROUTING
587 <VIDIOC_SUBDEV_G_ROUTING>` ioctl. Note that setting the routing table will
588 reset formats and selections in the sub-device to default values.
590 3. Configure formats and selections. Formats and selections of each stream are
591 configured separately as documented for plain sub-devices in
592 :ref:`format-propagation`. The stream ID is set to the same stream ID
593 associated with either sink or source pads of routes configured using the
594 :ref:`VIDIOC_SUBDEV_S_ROUTING <VIDIOC_SUBDEV_G_ROUTING>` ioctl.
596 Multiplexed streams setup example
597 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
599 A simple example of a multiplexed stream setup might be as follows:
601 - Two identical sensors (Sensor A and Sensor B). Each sensor has a single source
602 pad (pad 0) which carries a pixel data stream.
604 - Multiplexer bridge (Bridge). The bridge has two sink pads, connected to the
605 sensors (pads 0, 1), and one source pad (pad 2), which outputs two streams.
607 - Receiver in the SoC (Receiver). The receiver has a single sink pad (pad 0),
608 connected to the bridge, and two source pads (pads 1-2), going to the DMA
609 engine. The receiver demultiplexes the incoming streams to the source pads.
611 - DMA Engines in the SoC (DMA Engine), one for each stream. Each DMA engine is
612 connected to a single source pad in the receiver.
614 The sensors, the bridge and the receiver are modeled as V4L2 sub-devices,
615 exposed to userspace via /dev/v4l-subdevX device nodes. The DMA engines are
616 modeled as V4L2 devices, exposed to userspace via /dev/videoX nodes.
618 To configure this pipeline, the userspace must take the following steps:
620 1. Set up media links between entities: connect the sensors to the bridge,
621 bridge to the receiver, and the receiver to the DMA engines. This step does
622 not differ from normal non-multiplexed media controller setup.
626 .. flat-table:: Bridge routing table
635 - V4L2_SUBDEV_ROUTE_FL_ACTIVE
636 - Pixel data stream from Sensor A
639 - V4L2_SUBDEV_ROUTE_FL_ACTIVE
640 - Pixel data stream from Sensor B
642 .. flat-table:: Receiver routing table
651 - V4L2_SUBDEV_ROUTE_FL_ACTIVE
652 - Pixel data stream from Sensor A
655 - V4L2_SUBDEV_ROUTE_FL_ACTIVE
656 - Pixel data stream from Sensor B
658 3. Configure formats and selections
660 After configuring routing, the next step is configuring the formats and
661 selections for the streams. This is similar to performing this step without
662 streams, with just one exception: the ``stream`` field needs to be assigned
663 to the value of the stream ID.
665 A common way to accomplish this is to start from the sensors and propagate
666 the configurations along the stream towards the receiver, using
667 :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` ioctls to configure each
668 stream endpoint in each sub-device.
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