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Based on kernel version 4.2. Page generated on 2015-09-09 12:12 EST.

1	  <title>Sub-device Interface</title>
3	  <note>
4	    <title>Experimental</title>
5	    <para>This is an <link linkend="experimental">experimental</link>
6	    interface and may change in the future.</para>
7	  </note>
9	  <para>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 reflect
12	  the hardware model in software, and model the different hardware components
13	  as software blocks called sub-devices.</para>
15	  <para>V4L2 sub-devices are usually kernel-only objects. If the V4L2 driver
16	  implements the media device API, they will automatically inherit from media
17	  entities. Applications will be able to enumerate the sub-devices and discover
18	  the hardware topology using the media entities, pads and links enumeration
19	  API.</para>
21	  <para>In addition to make sub-devices discoverable, drivers can also choose
22	  to make them directly configurable by applications. When both the sub-device
23	  driver and the V4L2 device driver support this, sub-devices will feature a
24	  character device node on which ioctls can be called to
25	  <itemizedlist>
26	    <listitem><para>query, read and write sub-devices controls</para></listitem>
27	    <listitem><para>subscribe and unsubscribe to events and retrieve them</para></listitem>
28	    <listitem><para>negotiate image formats on individual pads</para></listitem>
29	  </itemizedlist>
30	  </para>
32	  <para>Sub-device character device nodes, conventionally named
33	  <filename>/dev/v4l-subdev*</filename>, use major number 81.</para>
35	  <section>
36	    <title>Controls</title>
37	    <para>Most V4L2 controls are implemented by sub-device hardware. Drivers
38	    usually merge all controls and expose them through video device nodes.
39	    Applications can control all sub-devices through a single interface.</para>
41	    <para>Complex devices sometimes implement the same control in different
42	    pieces of hardware. This situation is common in embedded platforms, where
43	    both sensors and image processing hardware implement identical functions,
44	    such as contrast adjustment, white balance or faulty pixels correction. As
45	    the V4L2 controls API doesn't support several identical controls in a single
46	    device, all but one of the identical controls are hidden.</para>
48	    <para>Applications can access those hidden controls through the sub-device
49	    node with the V4L2 control API described in <xref linkend="control" />. The
50	    ioctls behave identically as when issued on V4L2 device nodes, with the
51	    exception that they deal only with controls implemented in the sub-device.
52	    </para>
54	    <para>Depending on the driver, those controls might also be exposed through
55	    one (or several) V4L2 device nodes.</para>
56	  </section>
58	  <section>
59	    <title>Events</title>
60	    <para>V4L2 sub-devices can notify applications of events as described in
61	    <xref linkend="event" />. The API behaves identically as when used on V4L2
62	    device nodes, with the exception that it only deals with events generated by
63	    the sub-device. Depending on the driver, those events might also be reported
64	    on one (or several) V4L2 device nodes.</para>
65	  </section>
67	  <section id="pad-level-formats">
68	    <title>Pad-level Formats</title>
70	    <warning><para>Pad-level formats are only applicable to very complex device that
71	    need to expose low-level format configuration to user space. Generic V4L2
72	    applications do <emphasis>not</emphasis> need to use the API described in
73	    this section.</para></warning>
75	    <note><para>For the purpose of this section, the term
76	    <wordasword>format</wordasword> means the combination of media bus data
77	    format, frame width and frame height.</para></note>
79	    <para>Image formats are typically negotiated on video capture and
80	    output devices using the format and <link
81	    linkend="vidioc-subdev-g-selection">selection</link> ioctls. The
82	    driver is responsible for configuring every block in the video
83	    pipeline according to the requested format at the pipeline input
84	    and/or output.</para>
86	    <para>For complex devices, such as often found in embedded systems,
87	    identical image sizes at the output of a pipeline can be achieved using
88	    different hardware configurations. One such example is shown on
89	    <xref linkend="pipeline-scaling" />, where
90	    image scaling can be performed on both the video sensor and the host image
91	    processing hardware.</para>
93	    <figure id="pipeline-scaling">
94	      <title>Image Format Negotiation on Pipelines</title>
95	      <mediaobject>
96		<imageobject>
97		  <imagedata fileref="pipeline.pdf" format="PS" />
98		</imageobject>
99		<imageobject>
100		  <imagedata fileref="pipeline.png" format="PNG" />
101		</imageobject>
102		<textobject>
103		  <phrase>High quality and high speed pipeline configuration</phrase>
104		</textobject>
105	      </mediaobject>
106	    </figure>
108	    <para>The sensor scaler is usually of less quality than the host scaler, but
109	    scaling on the sensor is required to achieve higher frame rates. Depending
110	    on the use case (quality vs. speed), the pipeline must be configured
111	    differently. Applications need to configure the formats at every point in
112	    the pipeline explicitly.</para>
114	    <para>Drivers that implement the <link linkend="media-controller-intro">media
115	    API</link> can expose pad-level image format configuration to applications.
116	    When they do, applications can use the &VIDIOC-SUBDEV-G-FMT; and
117	    &VIDIOC-SUBDEV-S-FMT; ioctls. to negotiate formats on a per-pad basis.</para>
119	    <para>Applications are responsible for configuring coherent parameters on
120	    the whole pipeline and making sure that connected pads have compatible
121	    formats. The pipeline is checked for formats mismatch at &VIDIOC-STREAMON;
122	    time, and an &EPIPE; is then returned if the configuration is
123	    invalid.</para>
125	    <para>Pad-level image format configuration support can be tested by calling
126	    the &VIDIOC-SUBDEV-G-FMT; ioctl on pad 0. If the driver returns an &EINVAL;
127	    pad-level format configuration is not supported by the sub-device.</para>
129	    <section>
130	      <title>Format Negotiation</title>
132	      <para>Acceptable formats on pads can (and usually do) depend on a number
133	      of external parameters, such as formats on other pads, active links, or
134	      even controls. Finding a combination of formats on all pads in a video
135	      pipeline, acceptable to both application and driver, can't rely on formats
136	      enumeration only. A format negotiation mechanism is required.</para>
138	      <para>Central to the format negotiation mechanism are the get/set format
139	      operations. When called with the <structfield>which</structfield> argument
140	      set to <constant>V4L2_SUBDEV_FORMAT_TRY</constant>, the
141	      &VIDIOC-SUBDEV-G-FMT; and &VIDIOC-SUBDEV-S-FMT; ioctls operate on a set of
142	      formats parameters that are not connected to the hardware configuration.
143	      Modifying those 'try' formats leaves the device state untouched (this
144	      applies to both the software state stored in the driver and the hardware
145	      state stored in the device itself).</para>
147	      <para>While not kept as part of the device state, try formats are stored
148	      in the sub-device file handles. A &VIDIOC-SUBDEV-G-FMT; call will return
149	      the last try format set <emphasis>on the same sub-device file
150	      handle</emphasis>. Several applications querying the same sub-device at
151	      the same time will thus not interact with each other.</para>
153	      <para>To find out whether a particular format is supported by the device,
154	      applications use the &VIDIOC-SUBDEV-S-FMT; ioctl. Drivers verify and, if
155	      needed, change the requested <structfield>format</structfield> based on
156	      device requirements and return the possibly modified value. Applications
157	      can then choose to try a different format or accept the returned value and
158	      continue.</para>
160	      <para>Formats returned by the driver during a negotiation iteration are
161	      guaranteed to be supported by the device. In particular, drivers guarantee
162	      that a returned format will not be further changed if passed to an
163	      &VIDIOC-SUBDEV-S-FMT; call as-is (as long as external parameters, such as
164	      formats on other pads or links' configuration are not changed).</para>
166	      <para>Drivers automatically propagate formats inside sub-devices. When a
167	      try or active format is set on a pad, corresponding formats on other pads
168	      of the same sub-device can be modified by the driver. Drivers are free to
169	      modify formats as required by the device. However, they should comply with
170	      the following rules when possible:
171	      <itemizedlist>
172	        <listitem><para>Formats should be propagated from sink pads to source pads.
173		Modifying a format on a source pad should not modify the format on any
174		sink pad.</para></listitem>
175	        <listitem><para>Sub-devices that scale frames using variable scaling factors
176		should reset the scale factors to default values when sink pads formats
177		are modified. If the 1:1 scaling ratio is supported, this means that
178		source pads formats should be reset to the sink pads formats.</para></listitem>
179	      </itemizedlist>
180	      </para>
182	      <para>Formats are not propagated across links, as that would involve
183	      propagating them from one sub-device file handle to another. Applications
184	      must then take care to configure both ends of every link explicitly with
185	      compatible formats. Identical formats on the two ends of a link are
186	      guaranteed to be compatible. Drivers are free to accept different formats
187	      matching device requirements as being compatible.</para>
189	      <para><xref linkend="sample-pipeline-config" />
190	      shows a sample configuration sequence for the pipeline described in
191	      <xref linkend="pipeline-scaling" /> (table
192	      columns list entity names and pad numbers).</para>
194	      <table pgwide="0" frame="none" id="sample-pipeline-config">
195		<title>Sample Pipeline Configuration</title>
196		<tgroup cols="3">
197		  <colspec colname="what"/>
198		  <colspec colname="sensor-0 format" />
199		  <colspec colname="frontend-0 format" />
200		  <colspec colname="frontend-1 format" />
201		  <colspec colname="scaler-0 format" />
202		  <colspec colname="scaler-0 compose" />
203		  <colspec colname="scaler-1 format" />
204		  <thead>
205		    <row>
206		      <entry></entry>
207		      <entry>Sensor/0 format</entry>
208		      <entry>Frontend/0 format</entry>
209		      <entry>Frontend/1 format</entry>
210		      <entry>Scaler/0 format</entry>
211		      <entry>Scaler/0 compose selection rectangle</entry>
212		      <entry>Scaler/1 format</entry>
213		    </row>
214		  </thead>
215		  <tbody valign="top">
216		    <row>
217		      <entry>Initial state</entry>
218		      <entry>2048x1536/SGRBG8_1X8</entry>
219		      <entry>(default)</entry>
220		      <entry>(default)</entry>
221		      <entry>(default)</entry>
222		      <entry>(default)</entry>
223		      <entry>(default)</entry>
224		    </row>
225		    <row>
226		      <entry>Configure frontend sink format</entry>
227		      <entry>2048x1536/SGRBG8_1X8</entry>
228		      <entry><emphasis>2048x1536/SGRBG8_1X8</emphasis></entry>
229		      <entry><emphasis>2046x1534/SGRBG8_1X8</emphasis></entry>
230		      <entry>(default)</entry>
231		      <entry>(default)</entry>
232		      <entry>(default)</entry>
233		    </row>
234		    <row>
235		      <entry>Configure scaler sink format</entry>
236		      <entry>2048x1536/SGRBG8_1X8</entry>
237		      <entry>2048x1536/SGRBG8_1X8</entry>
238		      <entry>2046x1534/SGRBG8_1X8</entry>
239		      <entry><emphasis>2046x1534/SGRBG8_1X8</emphasis></entry>
240		      <entry><emphasis>0,0/2046x1534</emphasis></entry>
241		      <entry><emphasis>2046x1534/SGRBG8_1X8</emphasis></entry>
242		    </row>
243		    <row>
244		      <entry>Configure scaler sink compose selection</entry>
245		      <entry>2048x1536/SGRBG8_1X8</entry>
246		      <entry>2048x1536/SGRBG8_1X8</entry>
247		      <entry>2046x1534/SGRBG8_1X8</entry>
248		      <entry>2046x1534/SGRBG8_1X8</entry>
249		      <entry><emphasis>0,0/1280x960</emphasis></entry>
250		      <entry><emphasis>1280x960/SGRBG8_1X8</emphasis></entry>
251		    </row>
252		  </tbody>
253		</tgroup>
254	      </table>
256	      <para>
257	      <orderedlist>
258		<listitem><para>Initial state. The sensor source pad format is
259		set to its native 3MP size and V4L2_MBUS_FMT_SGRBG8_1X8
260		media bus code. Formats on the host frontend and scaler sink
261		and source pads have the default values, as well as the
262		compose rectangle on the scaler's sink pad.</para></listitem>
264		<listitem><para>The application configures the frontend sink
265		pad format's size to 2048x1536 and its media bus code to
266		V4L2_MBUS_FMT_SGRBG_1X8. The driver propagates the format to
267		the frontend source pad.</para></listitem>
269		<listitem><para>The application configures the scaler sink pad
270		format's size to 2046x1534 and the media bus code to
271		V4L2_MBUS_FMT_SGRBG_1X8 to match the frontend source size and
272		media bus code. The media bus code on the sink pad is set to
273		V4L2_MBUS_FMT_SGRBG_1X8. The driver propagates the size to the
274		compose selection rectangle on the scaler's sink pad, and the
275		format to the scaler source pad.</para></listitem>
277		<listitem><para>The application configures the size of the compose
278		selection rectangle of the scaler's sink pad 1280x960. The driver
279		propagates the size to the scaler's source pad
280		format.</para></listitem>
282	      </orderedlist>
283	      </para>
285	      <para>When satisfied with the try results, applications can set the active
286	      formats by setting the <structfield>which</structfield> argument to
287	      <constant>V4L2_SUBDEV_FORMAT_ACTIVE</constant>. Active formats are changed
288	      exactly as try formats by drivers. To avoid modifying the hardware state
289	      during format negotiation, applications should negotiate try formats first
290	      and then modify the active settings using the try formats returned during
291	      the last negotiation iteration. This guarantees that the active format
292	      will be applied as-is by the driver without being modified.
293	      </para>
294	    </section>
296	    <section id="v4l2-subdev-selections">
297	      <title>Selections: cropping, scaling and composition</title>
299	      <para>Many sub-devices support cropping frames on their input or output
300	      pads (or possible even on both). Cropping is used to select the area of
301	      interest in an image, typically on an image sensor or a video decoder. It can
302	      also be used as part of digital zoom implementations to select the area of
303	      the image that will be scaled up.</para>
305	      <para>Crop settings are defined by a crop rectangle and represented in a
306	      &v4l2-rect; by the coordinates of the top left corner and the rectangle
307	      size. Both the coordinates and sizes are expressed in pixels.</para>
309	      <para>As for pad formats, drivers store try and active
310	      rectangles for the selection targets <xref
311	      linkend="v4l2-selections-common" />.</para>
313	      <para>On sink pads, cropping is applied relative to the
314	      current pad format. The pad format represents the image size as
315	      received by the sub-device from the previous block in the
316	      pipeline, and the crop rectangle represents the sub-image that
317	      will be transmitted further inside the sub-device for
318	      processing.</para>
320	      <para>The scaling operation changes the size of the image by
321	      scaling it to new dimensions. The scaling ratio isn't specified
322	      explicitly, but is implied from the original and scaled image
323	      sizes. Both sizes are represented by &v4l2-rect;.</para>
325	      <para>Scaling support is optional. When supported by a subdev,
326	      the crop rectangle on the subdev's sink pad is scaled to the
327	      size configured using the &VIDIOC-SUBDEV-S-SELECTION; IOCTL
328	      using <constant>V4L2_SEL_TGT_COMPOSE</constant>
329	      selection target on the same pad. If the subdev supports scaling
330	      but not composing, the top and left values are not used and must
331	      always be set to zero.</para>
333	      <para>On source pads, cropping is similar to sink pads, with the
334	      exception that the source size from which the cropping is
335	      performed, is the COMPOSE rectangle on the sink pad. In both
336	      sink and source pads, the crop rectangle must be entirely
337	      contained inside the source image size for the crop
338	      operation.</para>
340	      <para>The drivers should always use the closest possible
341	      rectangle the user requests on all selection targets, unless
342	      specifically told otherwise.
343	      <constant>V4L2_SEL_FLAG_GE</constant> and
344	      <constant>V4L2_SEL_FLAG_LE</constant> flags may be
345	      used to round the image size either up or down. <xref
346	      linkend="v4l2-selection-flags" /></para>
347	    </section>
349	    <section>
350	      <title>Types of selection targets</title>
352	      <section>
353		<title>Actual targets</title>
355		<para>Actual targets (without a postfix) reflect the actual
356		hardware configuration at any point of time. There is a BOUNDS
357		target corresponding to every actual target.</para>
358	      </section>
360	      <section>
361		<title>BOUNDS targets</title>
363		<para>BOUNDS targets is the smallest rectangle that contains all
364		valid actual rectangles. It may not be possible to set the actual
365		rectangle as large as the BOUNDS rectangle, however. This may be
366		because e.g. a sensor's pixel array is not rectangular but
367		cross-shaped or round. The maximum size may also be smaller than the
368		BOUNDS rectangle.</para>
369	      </section>
371	    </section>
373	    <section>
374	      <title>Order of configuration and format propagation</title>
376	      <para>Inside subdevs, the order of image processing steps will
377	      always be from the sink pad towards the source pad. This is also
378	      reflected in the order in which the configuration must be
379	      performed by the user: the changes made will be propagated to
380	      any subsequent stages. If this behaviour is not desired, the
381	      user must set
382	      <constant>V4L2_SEL_FLAG_KEEP_CONFIG</constant> flag. This
383	      flag causes no propagation of the changes are allowed in any
384	      circumstances. This may also cause the accessed rectangle to be
385	      adjusted by the driver, depending on the properties of the
386	      underlying hardware.</para>
388	      <para>The coordinates to a step always refer to the actual size
389	      of the previous step. The exception to this rule is the source
390	      compose rectangle, which refers to the sink compose bounds
391	      rectangle --- if it is supported by the hardware.</para>
393	      <orderedlist>
394		<listitem><para>Sink pad format. The user configures the sink pad
395		format. This format defines the parameters of the image the
396		entity receives through the pad for further processing.</para></listitem>
398		<listitem><para>Sink pad actual crop selection. The sink pad crop
399		defines the crop performed to the sink pad format.</para></listitem>
401		<listitem><para>Sink pad actual compose selection. The size of the
402		sink pad compose rectangle defines the scaling ratio compared
403		to the size of the sink pad crop rectangle. The location of
404		the compose rectangle specifies the location of the actual
405		sink compose rectangle in the sink compose bounds
406		rectangle.</para></listitem>
408		<listitem><para>Source pad actual crop selection. Crop on the source
409		pad defines crop performed to the image in the sink compose
410		bounds rectangle.</para></listitem>
412		<listitem><para>Source pad format. The source pad format defines the
413		output pixel format of the subdev, as well as the other
414		parameters with the exception of the image width and height.
415		Width and height are defined by the size of the source pad
416		actual crop selection.</para></listitem>
417	      </orderedlist>
419	      <para>Accessing any of the above rectangles not supported by the
420	      subdev will return <constant>EINVAL</constant>. Any rectangle
421	      referring to a previous unsupported rectangle coordinates will
422	      instead refer to the previous supported rectangle. For example,
423	      if sink crop is not supported, the compose selection will refer
424	      to the sink pad format dimensions instead.</para>
426	      <figure id="subdev-image-processing-crop">
427		<title>Image processing in subdevs: simple crop example</title>
428		<mediaobject>
429		  <imageobject>
430		    <imagedata fileref="subdev-image-processing-crop.svg"
431		    format="SVG" scale="200" />
432		  </imageobject>
433		</mediaobject>
434	      </figure>
436	      <para>In the above example, the subdev supports cropping on its
437	      sink pad. To configure it, the user sets the media bus format on
438	      the subdev's sink pad. Now the actual crop rectangle can be set
439	      on the sink pad --- the location and size of this rectangle
440	      reflect the location and size of a rectangle to be cropped from
441	      the sink format. The size of the sink crop rectangle will also
442	      be the size of the format of the subdev's source pad.</para>
444	      <figure id="subdev-image-processing-scaling-multi-source">
445		<title>Image processing in subdevs: scaling with multiple sources</title>
446		<mediaobject>
447		  <imageobject>
448		    <imagedata fileref="subdev-image-processing-scaling-multi-source.svg"
449		    format="SVG" scale="200" />
450		  </imageobject>
451		</mediaobject>
452	      </figure>
454	      <para>In this example, the subdev is capable of first cropping,
455	      then scaling and finally cropping for two source pads
456	      individually from the resulting scaled image. The location of
457	      the scaled image in the cropped image is ignored in sink compose
458	      target. Both of the locations of the source crop rectangles
459	      refer to the sink scaling rectangle, independently cropping an
460	      area at location specified by the source crop rectangle from
461	      it.</para>
463	      <figure id="subdev-image-processing-full">
464		<title>Image processing in subdevs: scaling and composition
465		with multiple sinks and sources</title>
466		<mediaobject>
467		  <imageobject>
468		    <imagedata fileref="subdev-image-processing-full.svg"
469		    format="SVG" scale="200" />
470		  </imageobject>
471		</mediaobject>
472	      </figure>
474	      <para>The subdev driver supports two sink pads and two source
475	      pads. The images from both of the sink pads are individually
476	      cropped, then scaled and further composed on the composition
477	      bounds rectangle. From that, two independent streams are cropped
478	      and sent out of the subdev from the source pads.</para>
480	    </section>
482	  </section>
484	  &sub-subdev-formats;
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