1 .. Permission is granted to copy, distribute and/or modify this
2 .. document under the terms of the GNU Free Documentation License,
3 .. Version 1.1 or any later version published by the Free Software
4 .. Foundation, with no Invariant Sections, no Front-Cover Texts
5 .. and no Back-Cover Texts. A copy of the license is included at
6 .. Documentation/userspace-api/media/fdl-appendix.rst.
8 .. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
16 The complex nature of V4L2 devices, where hardware is often made of
17 several integrated circuits that need to interact with each other in a
18 controlled way, leads to complex V4L2 drivers. The drivers usually
19 reflect the hardware model in software, and model the different hardware
20 components as software blocks called sub-devices.
22 V4L2 sub-devices are usually kernel-only objects. If the V4L2 driver
23 implements the media device API, they will automatically inherit from
24 media entities. Applications will be able to enumerate the sub-devices
25 and discover the hardware topology using the media entities, pads and
26 links enumeration API.
28 In addition to make sub-devices discoverable, drivers can also choose to
29 make them directly configurable by applications. When both the
30 sub-device driver and the V4L2 device driver support this, sub-devices
31 will feature a character device node on which ioctls can be called to
33 - query, read and write sub-devices controls
35 - subscribe and unsubscribe to events and retrieve them
37 - negotiate image formats on individual pads
39 Sub-device character device nodes, conventionally named
40 ``/dev/v4l-subdev*``, use major number 81.
42 Drivers may opt to limit the sub-device character devices to only expose
43 operations that do not modify the device state. In such a case the sub-devices
44 are referred to as ``read-only`` in the rest of this documentation, and the
45 related restrictions are documented in individual ioctls.
51 Most V4L2 controls are implemented by sub-device hardware. Drivers
52 usually merge all controls and expose them through video device nodes.
53 Applications can control all sub-devices through a single interface.
55 Complex devices sometimes implement the same control in different pieces
56 of hardware. This situation is common in embedded platforms, where both
57 sensors and image processing hardware implement identical functions,
58 such as contrast adjustment, white balance or faulty pixels correction.
59 As the V4L2 controls API doesn't support several identical controls in a
60 single device, all but one of the identical controls are hidden.
62 Applications can access those hidden controls through the sub-device
63 node with the V4L2 control API described in :ref:`control`. The ioctls
64 behave identically as when issued on V4L2 device nodes, with the
65 exception that they deal only with controls implemented in the
68 Depending on the driver, those controls might also be exposed through
69 one (or several) V4L2 device nodes.
75 V4L2 sub-devices can notify applications of events as described in
76 :ref:`event`. The API behaves identically as when used on V4L2 device
77 nodes, with the exception that it only deals with events generated by
78 the sub-device. Depending on the driver, those events might also be
79 reported on one (or several) V4L2 device nodes.
82 .. _pad-level-formats:
89 Pad-level formats are only applicable to very complex devices that
90 need to expose low-level format configuration to user space. Generic
91 V4L2 applications do *not* need to use the API described in this
96 For the purpose of this section, the term *format* means the
97 combination of media bus data format, frame width and frame height.
99 Image formats are typically negotiated on video capture and output
100 devices using the format and
101 :ref:`selection <VIDIOC_SUBDEV_G_SELECTION>` ioctls. The driver is
102 responsible for configuring every block in the video pipeline according
103 to the requested format at the pipeline input and/or output.
105 For complex devices, such as often found in embedded systems, identical
106 image sizes at the output of a pipeline can be achieved using different
107 hardware configurations. One such example is shown on
108 :ref:`pipeline-scaling`, where image scaling can be performed on both
109 the video sensor and the host image processing hardware.
112 .. _pipeline-scaling:
114 .. kernel-figure:: pipeline.dot
118 Image Format Negotiation on Pipelines
120 High quality and high speed pipeline configuration
124 The sensor scaler is usually of less quality than the host scaler, but
125 scaling on the sensor is required to achieve higher frame rates.
126 Depending on the use case (quality vs. speed), the pipeline must be
127 configured differently. Applications need to configure the formats at
128 every point in the pipeline explicitly.
130 Drivers that implement the :ref:`media API <media-controller-intro>`
131 can expose pad-level image format configuration to applications. When
132 they do, applications can use the
133 :ref:`VIDIOC_SUBDEV_G_FMT <VIDIOC_SUBDEV_G_FMT>` and
134 :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` ioctls. to
135 negotiate formats on a per-pad basis.
137 Applications are responsible for configuring coherent parameters on the
138 whole pipeline and making sure that connected pads have compatible
139 formats. The pipeline is checked for formats mismatch at
140 :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` time, and an ``EPIPE`` error
141 code is then returned if the configuration is invalid.
143 Pad-level image format configuration support can be tested by calling
144 the :ref:`VIDIOC_SUBDEV_G_FMT` ioctl on pad
145 0. If the driver returns an ``EINVAL`` error code pad-level format
146 configuration is not supported by the sub-device.
152 Acceptable formats on pads can (and usually do) depend on a number of
153 external parameters, such as formats on other pads, active links, or
154 even controls. Finding a combination of formats on all pads in a video
155 pipeline, acceptable to both application and driver, can't rely on
156 formats enumeration only. A format negotiation mechanism is required.
158 Central to the format negotiation mechanism are the get/set format
159 operations. When called with the ``which`` argument set to
160 :ref:`V4L2_SUBDEV_FORMAT_TRY <VIDIOC_SUBDEV_G_FMT>`, the
161 :ref:`VIDIOC_SUBDEV_G_FMT <VIDIOC_SUBDEV_G_FMT>` and
162 :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` ioctls operate on
163 a set of formats parameters that are not connected to the hardware
164 configuration. Modifying those 'try' formats leaves the device state
165 untouched (this applies to both the software state stored in the driver
166 and the hardware state stored in the device itself).
168 While not kept as part of the device state, try formats are stored in
169 the sub-device file handles. A
170 :ref:`VIDIOC_SUBDEV_G_FMT <VIDIOC_SUBDEV_G_FMT>` call will return
171 the last try format set *on the same sub-device file handle*. Several
172 applications querying the same sub-device at the same time will thus not
173 interact with each other.
175 To find out whether a particular format is supported by the device,
177 :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` ioctl. Drivers
178 verify and, if needed, change the requested ``format`` based on device
179 requirements and return the possibly modified value. Applications can
180 then choose to try a different format or accept the returned value and
183 Formats returned by the driver during a negotiation iteration are
184 guaranteed to be supported by the device. In particular, drivers
185 guarantee that a returned format will not be further changed if passed
186 to an :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` call as-is
187 (as long as external parameters, such as formats on other pads or links'
188 configuration are not changed).
190 Drivers automatically propagate formats inside sub-devices. When a try
191 or active format is set on a pad, corresponding formats on other pads of
192 the same sub-device can be modified by the driver. Drivers are free to
193 modify formats as required by the device. However, they should comply
194 with the following rules when possible:
196 - Formats should be propagated from sink pads to source pads. Modifying
197 a format on a source pad should not modify the format on any sink
200 - Sub-devices that scale frames using variable scaling factors should
201 reset the scale factors to default values when sink pads formats are
202 modified. If the 1:1 scaling ratio is supported, this means that
203 source pads formats should be reset to the sink pads formats.
205 Formats are not propagated across links, as that would involve
206 propagating them from one sub-device file handle to another.
207 Applications must then take care to configure both ends of every link
208 explicitly with compatible formats. Identical formats on the two ends of
209 a link are guaranteed to be compatible. Drivers are free to accept
210 different formats matching device requirements as being compatible.
212 :ref:`sample-pipeline-config` shows a sample configuration sequence
213 for the pipeline described in :ref:`pipeline-scaling` (table columns
214 list entity names and pad numbers).
221 .. tabularcolumns:: |p{2.0cm}|p{2.3cm}|p{2.3cm}|p{2.3cm}|p{2.3cm}|p{2.3cm}|p{2.3cm}|
223 .. _sample-pipeline-config:
225 .. flat-table:: Sample Pipeline Configuration
228 :widths: 5 5 5 5 5 5 5
245 compose selection rectangle
258 * - Configure frontend sink format
271 * - Configure scaler sink format
288 * - Configure scaler sink compose selection
310 1. Initial state. The sensor source pad format is set to its native 3MP
311 size and V4L2_MBUS_FMT_SGRBG8_1X8 media bus code. Formats on the
312 host frontend and scaler sink and source pads have the default
313 values, as well as the compose rectangle on the scaler's sink pad.
315 2. The application configures the frontend sink pad format's size to
316 2048x1536 and its media bus code to V4L2_MBUS_FMT_SGRBG_1X8. The
317 driver propagates the format to the frontend source pad.
319 3. The application configures the scaler sink pad format's size to
320 2046x1534 and the media bus code to V4L2_MBUS_FMT_SGRBG_1X8 to
321 match the frontend source size and media bus code. The media bus code
322 on the sink pad is set to V4L2_MBUS_FMT_SGRBG_1X8. The driver
323 propagates the size to the compose selection rectangle on the
324 scaler's sink pad, and the format to the scaler source pad.
326 4. The application configures the size of the compose selection
327 rectangle of the scaler's sink pad 1280x960. The driver propagates
328 the size to the scaler's source pad format.
330 When satisfied with the try results, applications can set the active
331 formats by setting the ``which`` argument to
332 ``V4L2_SUBDEV_FORMAT_ACTIVE``. Active formats are changed exactly as try
333 formats by drivers. To avoid modifying the hardware state during format
334 negotiation, applications should negotiate try formats first and then
335 modify the active settings using the try formats returned during the
336 last negotiation iteration. This guarantees that the active format will
337 be applied as-is by the driver without being modified.
340 .. _v4l2-subdev-selections:
342 Selections: cropping, scaling and composition
343 ---------------------------------------------
345 Many sub-devices support cropping frames on their input or output pads
346 (or possible even on both). Cropping is used to select the area of
347 interest in an image, typically on an image sensor or a video decoder.
348 It can also be used as part of digital zoom implementations to select
349 the area of the image that will be scaled up.
351 Crop settings are defined by a crop rectangle and represented in a
352 struct :c:type:`v4l2_rect` by the coordinates of the top
353 left corner and the rectangle size. Both the coordinates and sizes are
356 As for pad formats, drivers store try and active rectangles for the
357 selection targets :ref:`v4l2-selections-common`.
359 On sink pads, cropping is applied relative to the current pad format.
360 The pad format represents the image size as received by the sub-device
361 from the previous block in the pipeline, and the crop rectangle
362 represents the sub-image that will be transmitted further inside the
363 sub-device for processing.
365 The scaling operation changes the size of the image by scaling it to new
366 dimensions. The scaling ratio isn't specified explicitly, but is implied
367 from the original and scaled image sizes. Both sizes are represented by
368 struct :c:type:`v4l2_rect`.
370 Scaling support is optional. When supported by a subdev, the crop
371 rectangle on the subdev's sink pad is scaled to the size configured
373 :ref:`VIDIOC_SUBDEV_S_SELECTION <VIDIOC_SUBDEV_G_SELECTION>` IOCTL
374 using ``V4L2_SEL_TGT_COMPOSE`` selection target on the same pad. If the
375 subdev supports scaling but not composing, the top and left values are
376 not used and must always be set to zero.
378 On source pads, cropping is similar to sink pads, with the exception
379 that the source size from which the cropping is performed, is the
380 COMPOSE rectangle on the sink pad. In both sink and source pads, the
381 crop rectangle must be entirely contained inside the source image size
382 for the crop operation.
384 The drivers should always use the closest possible rectangle the user
385 requests on all selection targets, unless specifically told otherwise.
386 ``V4L2_SEL_FLAG_GE`` and ``V4L2_SEL_FLAG_LE`` flags may be used to round
387 the image size either up or down. :ref:`v4l2-selection-flags`
390 Types of selection targets
391 --------------------------
397 Actual targets (without a postfix) reflect the actual hardware
398 configuration at any point of time. There is a BOUNDS target
399 corresponding to every actual target.
405 BOUNDS targets is the smallest rectangle that contains all valid actual
406 rectangles. It may not be possible to set the actual rectangle as large
407 as the BOUNDS rectangle, however. This may be because e.g. a sensor's
408 pixel array is not rectangular but cross-shaped or round. The maximum
409 size may also be smaller than the BOUNDS rectangle.
412 Order of configuration and format propagation
413 ---------------------------------------------
415 Inside subdevs, the order of image processing steps will always be from
416 the sink pad towards the source pad. This is also reflected in the order
417 in which the configuration must be performed by the user: the changes
418 made will be propagated to any subsequent stages. If this behaviour is
419 not desired, the user must set ``V4L2_SEL_FLAG_KEEP_CONFIG`` flag. This
420 flag causes no propagation of the changes are allowed in any
421 circumstances. This may also cause the accessed rectangle to be adjusted
422 by the driver, depending on the properties of the underlying hardware.
424 The coordinates to a step always refer to the actual size of the
425 previous step. The exception to this rule is the sink compose
426 rectangle, which refers to the sink compose bounds rectangle --- if it
427 is supported by the hardware.
429 1. Sink pad format. The user configures the sink pad format. This format
430 defines the parameters of the image the entity receives through the
431 pad for further processing.
433 2. Sink pad actual crop selection. The sink pad crop defines the crop
434 performed to the sink pad format.
436 3. Sink pad actual compose selection. The size of the sink pad compose
437 rectangle defines the scaling ratio compared to the size of the sink
438 pad crop rectangle. The location of the compose rectangle specifies
439 the location of the actual sink compose rectangle in the sink compose
442 4. Source pad actual crop selection. Crop on the source pad defines crop
443 performed to the image in the sink compose bounds rectangle.
445 5. Source pad format. The source pad format defines the output pixel
446 format of the subdev, as well as the other parameters with the
447 exception of the image width and height. Width and height are defined
448 by the size of the source pad actual crop selection.
450 Accessing any of the above rectangles not supported by the subdev will
451 return ``EINVAL``. Any rectangle referring to a previous unsupported
452 rectangle coordinates will instead refer to the previous supported
453 rectangle. For example, if sink crop is not supported, the compose
454 selection will refer to the sink pad format dimensions instead.
457 .. _subdev-image-processing-crop:
459 .. kernel-figure:: subdev-image-processing-crop.svg
460 :alt: subdev-image-processing-crop.svg
463 **Figure 4.5. Image processing in subdevs: simple crop example**
465 In the above example, the subdev supports cropping on its sink pad. To
466 configure it, the user sets the media bus format on the subdev's sink
467 pad. Now the actual crop rectangle can be set on the sink pad --- the
468 location and size of this rectangle reflect the location and size of a
469 rectangle to be cropped from the sink format. The size of the sink crop
470 rectangle will also be the size of the format of the subdev's source
474 .. _subdev-image-processing-scaling-multi-source:
476 .. kernel-figure:: subdev-image-processing-scaling-multi-source.svg
477 :alt: subdev-image-processing-scaling-multi-source.svg
480 **Figure 4.6. Image processing in subdevs: scaling with multiple sources**
482 In this example, the subdev is capable of first cropping, then scaling
483 and finally cropping for two source pads individually from the resulting
484 scaled image. The location of the scaled image in the cropped image is
485 ignored in sink compose target. Both of the locations of the source crop
486 rectangles refer to the sink scaling rectangle, independently cropping
487 an area at location specified by the source crop rectangle from it.
490 .. _subdev-image-processing-full:
492 .. kernel-figure:: subdev-image-processing-full.svg
493 :alt: subdev-image-processing-full.svg
496 **Figure 4.7. Image processing in subdevs: scaling and composition with multiple sinks and sources**
498 The subdev driver supports two sink pads and two source pads. The images
499 from both of the sink pads are individually cropped, then scaled and
500 further composed on the composition bounds rectangle. From that, two
501 independent streams are cropped and sent out of the subdev from the
projects / linux-2.6-microblaze.git / blob