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 Sub-device character device nodes, conventionally named
33 ``/dev/v4l-subdev*``, use major number 81.
35 Drivers may opt to limit the sub-device character devices to only expose
36 operations that do not modify the device state. In such a case the sub-devices
37 are referred to as ``read-only`` in the rest of this documentation, and the
38 related restrictions are documented in individual ioctls.
44 Most V4L2 controls are implemented by sub-device hardware. Drivers
45 usually merge all controls and expose them through video device nodes.
46 Applications can control all sub-devices through a single interface.
48 Complex devices sometimes implement the same control in different pieces
49 of hardware. This situation is common in embedded platforms, where both
50 sensors and image processing hardware implement identical functions,
51 such as contrast adjustment, white balance or faulty pixels correction.
52 As the V4L2 controls API doesn't support several identical controls in a
53 single device, all but one of the identical controls are hidden.
55 Applications can access those hidden controls through the sub-device
56 node with the V4L2 control API described in :ref:`control`. The ioctls
57 behave identically as when issued on V4L2 device nodes, with the
58 exception that they deal only with controls implemented in the
61 Depending on the driver, those controls might also be exposed through
62 one (or several) V4L2 device nodes.
68 V4L2 sub-devices can notify applications of events as described in
69 :ref:`event`. The API behaves identically as when used on V4L2 device
70 nodes, with the exception that it only deals with events generated by
71 the sub-device. Depending on the driver, those events might also be
72 reported on one (or several) V4L2 device nodes.
75 .. _pad-level-formats:
82 Pad-level formats are only applicable to very complex devices that
83 need to expose low-level format configuration to user space. Generic
84 V4L2 applications do *not* need to use the API described in this
89 For the purpose of this section, the term *format* means the
90 combination of media bus data format, frame width and frame height.
92 Image formats are typically negotiated on video capture and output
93 devices using the format and
94 :ref:`selection <VIDIOC_SUBDEV_G_SELECTION>` ioctls. The driver is
95 responsible for configuring every block in the video pipeline according
96 to the requested format at the pipeline input and/or output.
98 For complex devices, such as often found in embedded systems, identical
99 image sizes at the output of a pipeline can be achieved using different
100 hardware configurations. One such example is shown on
101 :ref:`pipeline-scaling`, where image scaling can be performed on both
102 the video sensor and the host image processing hardware.
105 .. _pipeline-scaling:
107 .. kernel-figure:: pipeline.dot
111 Image Format Negotiation on Pipelines
113 High quality and high speed pipeline configuration
117 The sensor scaler is usually of less quality than the host scaler, but
118 scaling on the sensor is required to achieve higher frame rates.
119 Depending on the use case (quality vs. speed), the pipeline must be
120 configured differently. Applications need to configure the formats at
121 every point in the pipeline explicitly.
123 Drivers that implement the :ref:`media API <media-controller-intro>`
124 can expose pad-level image format configuration to applications. When
125 they do, applications can use the
126 :ref:`VIDIOC_SUBDEV_G_FMT <VIDIOC_SUBDEV_G_FMT>` and
127 :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` ioctls. to
128 negotiate formats on a per-pad basis.
130 Applications are responsible for configuring coherent parameters on the
131 whole pipeline and making sure that connected pads have compatible
132 formats. The pipeline is checked for formats mismatch at
133 :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` time, and an ``EPIPE`` error
134 code is then returned if the configuration is invalid.
136 Pad-level image format configuration support can be tested by calling
137 the :ref:`VIDIOC_SUBDEV_G_FMT` ioctl on pad
138 0. If the driver returns an ``EINVAL`` error code pad-level format
139 configuration is not supported by the sub-device.
145 Acceptable formats on pads can (and usually do) depend on a number of
146 external parameters, such as formats on other pads, active links, or
147 even controls. Finding a combination of formats on all pads in a video
148 pipeline, acceptable to both application and driver, can't rely on
149 formats enumeration only. A format negotiation mechanism is required.
151 Central to the format negotiation mechanism are the get/set format
152 operations. When called with the ``which`` argument set to
153 :ref:`V4L2_SUBDEV_FORMAT_TRY <VIDIOC_SUBDEV_G_FMT>`, the
154 :ref:`VIDIOC_SUBDEV_G_FMT <VIDIOC_SUBDEV_G_FMT>` and
155 :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` ioctls operate on
156 a set of formats parameters that are not connected to the hardware
157 configuration. Modifying those 'try' formats leaves the device state
158 untouched (this applies to both the software state stored in the driver
159 and the hardware state stored in the device itself).
161 While not kept as part of the device state, try formats are stored in
162 the sub-device file handles. A
163 :ref:`VIDIOC_SUBDEV_G_FMT <VIDIOC_SUBDEV_G_FMT>` call will return
164 the last try format set *on the same sub-device file handle*. Several
165 applications querying the same sub-device at the same time will thus not
166 interact with each other.
168 To find out whether a particular format is supported by the device,
170 :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` ioctl. Drivers
171 verify and, if needed, change the requested ``format`` based on device
172 requirements and return the possibly modified value. Applications can
173 then choose to try a different format or accept the returned value and
176 Formats returned by the driver during a negotiation iteration are
177 guaranteed to be supported by the device. In particular, drivers
178 guarantee that a returned format will not be further changed if passed
179 to an :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` call as-is
180 (as long as external parameters, such as formats on other pads or links'
181 configuration are not changed).
183 Drivers automatically propagate formats inside sub-devices. When a try
184 or active format is set on a pad, corresponding formats on other pads of
185 the same sub-device can be modified by the driver. Drivers are free to
186 modify formats as required by the device. However, they should comply
187 with the following rules when possible:
189 - Formats should be propagated from sink pads to source pads. Modifying
190 a format on a source pad should not modify the format on any sink
193 - Sub-devices that scale frames using variable scaling factors should
194 reset the scale factors to default values when sink pads formats are
195 modified. If the 1:1 scaling ratio is supported, this means that
196 source pads formats should be reset to the sink pads formats.
198 Formats are not propagated across links, as that would involve
199 propagating them from one sub-device file handle to another.
200 Applications must then take care to configure both ends of every link
201 explicitly with compatible formats. Identical formats on the two ends of
202 a link are guaranteed to be compatible. Drivers are free to accept
203 different formats matching device requirements as being compatible.
205 :ref:`sample-pipeline-config` shows a sample configuration sequence
206 for the pipeline described in :ref:`pipeline-scaling` (table columns
207 list entity names and pad numbers).
214 \setlength{\tabcolsep}{2pt}
216 .. tabularcolumns:: |p{2.0cm}|p{2.1cm}|p{2.1cm}|p{2.1cm}|p{2.1cm}|p{2.1cm}|p{2.1cm}|
218 .. _sample-pipeline-config:
220 .. flat-table:: Sample Pipeline Configuration
223 :widths: 5 5 5 5 5 5 5
240 compose selection rectangle
253 * - Configure frontend sink format
266 * - Configure scaler sink format
283 * - Configure scaler sink compose selection
305 1. Initial state. The sensor source pad format is set to its native 3MP
306 size and V4L2_MBUS_FMT_SGRBG8_1X8 media bus code. Formats on the
307 host frontend and scaler sink and source pads have the default
308 values, as well as the compose rectangle on the scaler's sink pad.
310 2. The application configures the frontend sink pad format's size to
311 2048x1536 and its media bus code to V4L2_MBUS_FMT_SGRBG_1X8. The
312 driver propagates the format to the frontend source pad.
314 3. The application configures the scaler sink pad format's size to
315 2046x1534 and the media bus code to V4L2_MBUS_FMT_SGRBG_1X8 to
316 match the frontend source size and media bus code. The media bus code
317 on the sink pad is set to V4L2_MBUS_FMT_SGRBG_1X8. The driver
318 propagates the size to the compose selection rectangle on the
319 scaler's sink pad, and the format to the scaler source pad.
321 4. The application configures the size of the compose selection
322 rectangle of the scaler's sink pad 1280x960. The driver propagates
323 the size to the scaler's source pad format.
325 When satisfied with the try results, applications can set the active
326 formats by setting the ``which`` argument to
327 ``V4L2_SUBDEV_FORMAT_ACTIVE``. Active formats are changed exactly as try
328 formats by drivers. To avoid modifying the hardware state during format
329 negotiation, applications should negotiate try formats first and then
330 modify the active settings using the try formats returned during the
331 last negotiation iteration. This guarantees that the active format will
332 be applied as-is by the driver without being modified.
335 .. _v4l2-subdev-selections:
337 Selections: cropping, scaling and composition
338 ---------------------------------------------
340 Many sub-devices support cropping frames on their input or output pads
341 (or possible even on both). Cropping is used to select the area of
342 interest in an image, typically on an image sensor or a video decoder.
343 It can also be used as part of digital zoom implementations to select
344 the area of the image that will be scaled up.
346 Crop settings are defined by a crop rectangle and represented in a
347 struct :c:type:`v4l2_rect` by the coordinates of the top
348 left corner and the rectangle size. Both the coordinates and sizes are
351 As for pad formats, drivers store try and active rectangles for the
352 selection targets :ref:`v4l2-selections-common`.
354 On sink pads, cropping is applied relative to the current pad format.
355 The pad format represents the image size as received by the sub-device
356 from the previous block in the pipeline, and the crop rectangle
357 represents the sub-image that will be transmitted further inside the
358 sub-device for processing.
360 The scaling operation changes the size of the image by scaling it to new
361 dimensions. The scaling ratio isn't specified explicitly, but is implied
362 from the original and scaled image sizes. Both sizes are represented by
363 struct :c:type:`v4l2_rect`.
365 Scaling support is optional. When supported by a subdev, the crop
366 rectangle on the subdev's sink pad is scaled to the size configured
368 :ref:`VIDIOC_SUBDEV_S_SELECTION <VIDIOC_SUBDEV_G_SELECTION>` IOCTL
369 using ``V4L2_SEL_TGT_COMPOSE`` selection target on the same pad. If the
370 subdev supports scaling but not composing, the top and left values are
371 not used and must always be set to zero.
373 On source pads, cropping is similar to sink pads, with the exception
374 that the source size from which the cropping is performed, is the
375 COMPOSE rectangle on the sink pad. In both sink and source pads, the
376 crop rectangle must be entirely contained inside the source image size
377 for the crop operation.
379 The drivers should always use the closest possible rectangle the user
380 requests on all selection targets, unless specifically told otherwise.
381 ``V4L2_SEL_FLAG_GE`` and ``V4L2_SEL_FLAG_LE`` flags may be used to round
382 the image size either up or down. :ref:`v4l2-selection-flags`
385 Types of selection targets
386 --------------------------
392 Actual targets (without a postfix) reflect the actual hardware
393 configuration at any point of time. There is a BOUNDS target
394 corresponding to every actual target.
400 BOUNDS targets is the smallest rectangle that contains all valid actual
401 rectangles. It may not be possible to set the actual rectangle as large
402 as the BOUNDS rectangle, however. This may be because e.g. a sensor's
403 pixel array is not rectangular but cross-shaped or round. The maximum
404 size may also be smaller than the BOUNDS rectangle.
407 Order of configuration and format propagation
408 ---------------------------------------------
410 Inside subdevs, the order of image processing steps will always be from
411 the sink pad towards the source pad. This is also reflected in the order
412 in which the configuration must be performed by the user: the changes
413 made will be propagated to any subsequent stages. If this behaviour is
414 not desired, the user must set ``V4L2_SEL_FLAG_KEEP_CONFIG`` flag. This
415 flag causes no propagation of the changes are allowed in any
416 circumstances. This may also cause the accessed rectangle to be adjusted
417 by the driver, depending on the properties of the underlying hardware.
419 The coordinates to a step always refer to the actual size of the
420 previous step. The exception to this rule is the sink compose
421 rectangle, which refers to the sink compose bounds rectangle --- if it
422 is supported by the hardware.
424 1. Sink pad format. The user configures the sink pad format. This format
425 defines the parameters of the image the entity receives through the
426 pad for further processing.
428 2. Sink pad actual crop selection. The sink pad crop defines the crop
429 performed to the sink pad format.
431 3. Sink pad actual compose selection. The size of the sink pad compose
432 rectangle defines the scaling ratio compared to the size of the sink
433 pad crop rectangle. The location of the compose rectangle specifies
434 the location of the actual sink compose rectangle in the sink compose
437 4. Source pad actual crop selection. Crop on the source pad defines crop
438 performed to the image in the sink compose bounds rectangle.
440 5. Source pad format. The source pad format defines the output pixel
441 format of the subdev, as well as the other parameters with the
442 exception of the image width and height. Width and height are defined
443 by the size of the source pad actual crop selection.
445 Accessing any of the above rectangles not supported by the subdev will
446 return ``EINVAL``. Any rectangle referring to a previous unsupported
447 rectangle coordinates will instead refer to the previous supported
448 rectangle. For example, if sink crop is not supported, the compose
449 selection will refer to the sink pad format dimensions instead.
452 .. _subdev-image-processing-crop:
454 .. kernel-figure:: subdev-image-processing-crop.svg
455 :alt: subdev-image-processing-crop.svg
458 **Figure 4.5. Image processing in subdevs: simple crop example**
460 In the above example, the subdev supports cropping on its sink pad. To
461 configure it, the user sets the media bus format on the subdev's sink
462 pad. Now the actual crop rectangle can be set on the sink pad --- the
463 location and size of this rectangle reflect the location and size of a
464 rectangle to be cropped from the sink format. The size of the sink crop
465 rectangle will also be the size of the format of the subdev's source
469 .. _subdev-image-processing-scaling-multi-source:
471 .. kernel-figure:: subdev-image-processing-scaling-multi-source.svg
472 :alt: subdev-image-processing-scaling-multi-source.svg
475 **Figure 4.6. Image processing in subdevs: scaling with multiple sources**
477 In this example, the subdev is capable of first cropping, then scaling
478 and finally cropping for two source pads individually from the resulting
479 scaled image. The location of the scaled image in the cropped image is
480 ignored in sink compose target. Both of the locations of the source crop
481 rectangles refer to the sink scaling rectangle, independently cropping
482 an area at location specified by the source crop rectangle from it.
485 .. _subdev-image-processing-full:
487 .. kernel-figure:: subdev-image-processing-full.svg
488 :alt: subdev-image-processing-full.svg
491 **Figure 4.7. Image processing in subdevs: scaling and composition with multiple sinks and sources**
493 The subdev driver supports two sink pads and two source pads. The images
494 from both of the sink pads are individually cropped, then scaled and
495 further composed on the composition bounds rectangle. From that, two
496 independent streams are cropped and sent out of the subdev from the
projects / linux-2.6-microblaze.git / blob