1 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2
3 .. _subdev:
4
5 ********************
6 Sub-device Interface
7 ********************
8
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.
14
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.
20
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
25
26 - query, read and write sub-devices controls
27
28 - subscribe and unsubscribe to events and retrieve them
29
30 - negotiate image formats on individual pads
31
32 - inspect and modify internal data routing between pads of the same entity
33
34 Sub-device character device nodes, conventionally named
35 ``/dev/v4l-subdev*``, use major number 81.
36
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.
41
42
43 Controls
44 ========
45
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.
49
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.
56
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
61 sub-device.
62
63 Depending on the driver, those controls might also be exposed through
64 one (or several) V4L2 device nodes.
65
66
67 Events
68 ======
69
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.
75
76
77 .. _pad-level-formats:
78
79 Pad-level Formats
80 =================
81
82 .. warning::
83
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
87 section.
88
89 .. note::
90
91 For the purpose of this section, the term *format* means the
92 combination of media bus data format, frame width and frame height.
93
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.
99
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.
105
106
107 .. _pipeline-scaling:
108
109 .. kernel-figure:: pipeline.dot
110 :alt: pipeline.dot
111 :align: center
112
113 Image Format Negotiation on Pipelines
114
115 High quality and high speed pipeline configuration
116
117
118
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.
124
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.
131
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.
137
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.
142
143
144 Format Negotiation
145 ------------------
146
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.
152
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).
162
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.
169
170 To find out whether a particular format is supported by the device,
171 applications use the
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
176 continue.
177
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).
184
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:
190
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
193 pad.
194
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.
199
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.
206
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).
210
211
212 .. raw:: latex
213
214 \begingroup
215 \scriptsize
216 \setlength{\tabcolsep}{2pt}
217
218 .. tabularcolumns:: |p{2.0cm}|p{2.1cm}|p{2.1cm}|p{2.1cm}|p{2.1cm}|p{2.1cm}|p{2.1cm}|
219
220 .. _sample-pipeline-config:
221
222 .. flat-table:: Sample Pipeline Configuration
223 :header-rows: 1
224 :stub-columns: 0
225 :widths: 5 5 5 5 5 5 5
226
227 * -
228 - Sensor/0
229
230 format
231 - Frontend/0
232
233 format
234 - Frontend/1
235
236 format
237 - Scaler/0
238
239 format
240 - Scaler/0
241
242 compose selection rectangle
243 - Scaler/1
244
245 format
246 * - Initial state
247 - 2048x1536
248
249 SGRBG8_1X8
250 - (default)
251 - (default)
252 - (default)
253 - (default)
254 - (default)
255 * - Configure frontend sink format
256 - 2048x1536
257
258 SGRBG8_1X8
259 - *2048x1536*
260
261 *SGRBG8_1X8*
262 - *2046x1534*
263
264 *SGRBG8_1X8*
265 - (default)
266 - (default)
267 - (default)
268 * - Configure scaler sink format
269 - 2048x1536
270
271 SGRBG8_1X8
272 - 2048x1536
273
274 SGRBG8_1X8
275 - 2046x1534
276
277 SGRBG8_1X8
278 - *2046x1534*
279
280 *SGRBG8_1X8*
281 - *0,0/2046x1534*
282 - *2046x1534*
283
284 *SGRBG8_1X8*
285 * - Configure scaler sink compose selection
286 - 2048x1536
287
288 SGRBG8_1X8
289 - 2048x1536
290
291 SGRBG8_1X8
292 - 2046x1534
293
294 SGRBG8_1X8
295 - 2046x1534
296
297 SGRBG8_1X8
298 - *0,0/1280x960*
299 - *1280x960*
300
301 *SGRBG8_1X8*
302
303 .. raw:: latex
304
305 \endgroup
306
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.
311
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.
315
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.
322
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.
326
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.
335
336
337 .. _v4l2-subdev-selections:
338
339 Selections: cropping, scaling and composition
340 ---------------------------------------------
341
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.
347
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
351 expressed in pixels.
352
353 As for pad formats, drivers store try and active rectangles for the
354 selection targets :ref:`v4l2-selections-common`.
355
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.
361
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`.
366
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
369 using the
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.
374
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.
380
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`
385
386
387 Types of selection targets
388 --------------------------
389
390
391 Actual targets
392 ^^^^^^^^^^^^^^
393
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.
397
398
399 BOUNDS targets
400 ^^^^^^^^^^^^^^
401
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.
407
408
409 .. _format-propagation:
410
411 Order of configuration and format propagation
412 ---------------------------------------------
413
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.
422
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.
427
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.
431
432 2. Sink pad actual crop selection. The sink pad crop defines the crop
433 performed to the sink pad format.
434
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
439 bounds rectangle.
440
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.
443
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.
448
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.
454
455
456 .. _subdev-image-processing-crop:
457
458 .. kernel-figure:: subdev-image-processing-crop.svg
459 :alt: subdev-image-processing-crop.svg
460 :align: center
461
462 **Figure 4.5. Image processing in subdevs: simple crop example**
463
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
470 pad.
471
472
473 .. _subdev-image-processing-scaling-multi-source:
474
475 .. kernel-figure:: subdev-image-processing-scaling-multi-source.svg
476 :alt: subdev-image-processing-scaling-multi-source.svg
477 :align: center
478
479 **Figure 4.6. Image processing in subdevs: scaling with multiple sources**
480
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.
487
488
489 .. _subdev-image-processing-full:
490
491 .. kernel-figure:: subdev-image-processing-full.svg
492 :alt: subdev-image-processing-full.svg
493 :align: center
494
495 **Figure 4.7. Image processing in subdevs: scaling and composition with multiple sinks and sources**
496
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
501 source pads.
502
503
504 .. toctree::
505 :maxdepth: 1
506
507 subdev-formats
508
509 .. _subdev-routing:
510
511 Streams, multiplexed media pads and internal routing
512 ----------------------------------------------------
513
514 Simple V4L2 sub-devices do not support multiple, unrelated video streams,
515 and only a single stream can pass through a media link and a media pad.
516 Thus each pad contains a format and selection configuration for that
517 single stream. A subdev can do stream processing and split a stream into
518 two or compose two streams into one, but the inputs and outputs for the
519 subdev are still a single stream per pad.
520
521 Some hardware, e.g. MIPI CSI-2, support multiplexed streams, that is, multiple
522 data streams are transmitted on the same bus, which is represented by a media
523 link connecting a transmitter source pad with a sink pad on the receiver. For
524 example, a camera sensor can produce two distinct streams, a pixel stream and a
525 metadata stream, which are transmitted on the multiplexed data bus, represented
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
528 the its sink pad and allows to route them individually to one of its source
529 pads.
530
531 Subdevice drivers that support multiplexed streams are compatible with
532 non-multiplexed subdev drivers. However, if the driver at the sink end of a link
533 does not support streams, then only stream 0 of source end may be captured.
534 There may be additional limitations specific to the sink device.
535
536 Understanding streams
537 ^^^^^^^^^^^^^^^^^^^^^
538
539 A stream is a stream of content (e.g. pixel data or metadata) flowing through
540 the media pipeline from a source (e.g. a sensor) towards the final sink (e.g. a
541 receiver and demultiplexer in a SoC). Each media link carries all the enabled
542 streams from one end of the link to the other, and sub-devices have routing
543 tables which describe how the incoming streams from sink pads are routed to the
544 source pads.
545
546 A stream ID is a media pad-local identifier for a stream. Streams IDs of
547 the same stream must be equal on both ends of a link. In other words,
548 a particular stream ID must exist on both sides of a media
549 link, but another stream ID can be used for the same stream at the other side
550 of the sub-device.
551
552 A stream at a specific point in the media pipeline is identified by the
553 sub-device and a (pad, stream) pair. For sub-devices that do not support
554 multiplexed streams the 'stream' field is always 0.
555
556 Interaction between routes, streams, formats and selections
557 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
558
559 The addition of streams to the V4L2 sub-device interface moves the sub-device
560 formats and selections from pads to (pad, stream) pairs. Besides the
561 usual pad, also the stream ID needs to be provided for setting formats and
562 selections. The order of configuring formats and selections along a stream is
563 the same as without streams (see :ref:`format-propagation`).
564
565 Instead of the sub-device wide merging of streams from all sink pads
566 towards all source pads, data flows for each route are separate from each
567 other. Any number of routes from streams on sink pads towards streams on
568 source pads is allowed, to the extent supported by drivers. For every
569 stream on a source pad, however, only a single route is allowed.
570
571 Any configurations of a stream within a pad, such as format or selections,
572 are independent of similar configurations on other streams. This is
573 subject to change in the future.
574
575 Device types and routing setup
576 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
577
578 Different kinds of sub-devices have differing behaviour for route activation,
579 depending on the hardware. In all cases, however, only routes that have the
580 ``V4L2_SUBDEV_STREAM_FL_ACTIVE`` flag set are active.
581
582 Devices generating the streams may allow enabling and disabling some of the
583 routes or have a fixed routing configuration. If the routes can be disabled, not
584 declaring the routes (or declaring them without
585 ``V4L2_SUBDEV_STREAM_FL_ACTIVE`` flag set) in ``VIDIOC_SUBDEV_S_ROUTING`` will
586 disable the routes. ``VIDIOC_SUBDEV_S_ROUTING`` will still return such routes
587 back to the user in the routes array, with the ``V4L2_SUBDEV_STREAM_FL_ACTIVE``
588 flag unset.
589
590 Devices transporting the streams almost always have more configurability with
591 respect to routing. Typically any route between the sub-device's sink and source
592 pads is possible, and multiple routes (usually up to certain limited number) may
593 be active simultaneously. For such devices, no routes are created by the driver
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
596 configuration for format and selection rectangles.
597
598 Configuring streams
599 ^^^^^^^^^^^^^^^^^^^
600
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
603 is started.
604
605 There are three steps in configuring the streams:
606
607 1. Set up links. Connect the pads between sub-devices using the
608 :ref:`Media Controller API <media_controller>`
609
610 2. Streams. Streams are declared and their routing is configured by setting the
611 routing table for the sub-device using :ref:`VIDIOC_SUBDEV_S_ROUTING
612 <VIDIOC_SUBDEV_G_ROUTING>` ioctl. Note that setting the routing table will
613 reset formats and selections in the sub-device to default values.
614
615 3. Configure formats and selections. Formats and selections of each stream are
616 configured separately as documented for plain sub-devices in
617 :ref:`format-propagation`. The stream ID is set to the same stream ID
618 associated with either sink or source pads of routes configured using the
619 :ref:`VIDIOC_SUBDEV_S_ROUTING <VIDIOC_SUBDEV_G_ROUTING>` ioctl.
620
621 Multiplexed streams setup example
622 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
623
624 A simple example of a multiplexed stream setup might be as follows:
625
626 - Two identical sensors (Sensor A and Sensor B). Each sensor has a single source
627 pad (pad 0) which carries a pixel data stream.
628
629 - Multiplexer bridge (Bridge). The bridge has two sink pads, connected to the
630 sensors (pads 0, 1), and one source pad (pad 2), which outputs two streams.
631
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
634 engine. The receiver demultiplexes the incoming streams to the source pads.
635
636 - DMA Engines in the SoC (DMA Engine), one for each stream. Each DMA engine is
637 connected to a single source pad in the receiver.
638
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
641 modeled as V4L2 devices, exposed to userspace via /dev/videoX nodes.
642
643 To configure this pipeline, the userspace must take the following steps:
644
645 1. Set up media links between entities: connect the sensors to the bridge,
646 bridge to the receiver, and the receiver to the DMA engines. This step does
647 not differ from normal non-multiplexed media controller setup.
648
649 2. Configure routing
650
651 .. flat-table:: Bridge routing table
652 :header-rows: 1
653
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
666
667 .. flat-table:: Receiver routing table
668 :header-rows: 1
669
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
682
683 3. Configure formats and selections
684
685 After configuring routing, the next step is configuring the formats and
686 selections for the streams. This is similar to performing this step without
687 streams, with just one exception: the ``stream`` field needs to be assigned
688 to the value of the stream ID.
689
690 A common way to accomplish this is to start from the sensors and propagate
691 the configurations along the stream towards the receiver, using
692 :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` ioctls to configure each
693 stream endpoint in each sub-device.
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