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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 output
80 devices using the <link linkend="crop">cropping and scaling</link> ioctls.
81 The driver is responsible for configuring every block in the video pipeline
82 according to the requested format at the pipeline input and/or
83 output.</para>
85 <para>For complex devices, such as often found in embedded systems,
86 identical image sizes at the output of a pipeline can be achieved using
87 different hardware configurations. One such example is shown on
88 <xref linkend="pipeline-scaling" />, where
89 image scaling can be performed on both the video sensor and the host image
90 processing hardware.</para>
92 <figure id="pipeline-scaling">
93 <title>Image Format Negotiation on Pipelines</title>
94 <mediaobject>
95 <imageobject>
96 <imagedata fileref="pipeline.pdf" format="PS" />
97 </imageobject>
98 <imageobject>
99 <imagedata fileref="pipeline.png" format="PNG" />
100 </imageobject>
101 <textobject>
102 <phrase>High quality and high speed pipeline configuration</phrase>
103 </textobject>
104 </mediaobject>
105 </figure>
107 <para>The sensor scaler is usually of less quality than the host scaler, but
108 scaling on the sensor is required to achieve higher frame rates. Depending
109 on the use case (quality vs. speed), the pipeline must be configured
110 differently. Applications need to configure the formats at every point in
111 the pipeline explicitly.</para>
113 <para>Drivers that implement the <link linkend="media-controller-intro">media
114 API</link> can expose pad-level image format configuration to applications.
115 When they do, applications can use the &VIDIOC-SUBDEV-G-FMT; and
116 &VIDIOC-SUBDEV-S-FMT; ioctls. to negotiate formats on a per-pad basis.</para>
118 <para>Applications are responsible for configuring coherent parameters on
119 the whole pipeline and making sure that connected pads have compatible
120 formats. The pipeline is checked for formats mismatch at &VIDIOC-STREAMON;
121 time, and an &EPIPE; is then returned if the configuration is
122 invalid.</para>
124 <para>Pad-level image format configuration support can be tested by calling
125 the &VIDIOC-SUBDEV-G-FMT; ioctl on pad 0. If the driver returns an &EINVAL;
126 pad-level format configuration is not supported by the sub-device.</para>
128 <section>
129 <title>Format Negotiation</title>
131 <para>Acceptable formats on pads can (and usually do) depend on a number
132 of external parameters, such as formats on other pads, active links, or
133 even controls. Finding a combination of formats on all pads in a video
134 pipeline, acceptable to both application and driver, can't rely on formats
135 enumeration only. A format negotiation mechanism is required.</para>
137 <para>Central to the format negotiation mechanism are the get/set format
138 operations. When called with the <structfield>which</structfield> argument
139 set to <constant>V4L2_SUBDEV_FORMAT_TRY</constant>, the
140 &VIDIOC-SUBDEV-G-FMT; and &VIDIOC-SUBDEV-S-FMT; ioctls operate on a set of
141 formats parameters that are not connected to the hardware configuration.
142 Modifying those 'try' formats leaves the device state untouched (this
143 applies to both the software state stored in the driver and the hardware
144 state stored in the device itself).</para>
146 <para>While not kept as part of the device state, try formats are stored
147 in the sub-device file handles. A &VIDIOC-SUBDEV-G-FMT; call will return
148 the last try format set <emphasis>on the same sub-device file
149 handle</emphasis>. Several applications querying the same sub-device at
150 the same time will thus not interact with each other.</para>
152 <para>To find out whether a particular format is supported by the device,
153 applications use the &VIDIOC-SUBDEV-S-FMT; ioctl. Drivers verify and, if
154 needed, change the requested <structfield>format</structfield> based on
155 device requirements and return the possibly modified value. Applications
156 can then choose to try a different format or accept the returned value and
157 continue.</para>
159 <para>Formats returned by the driver during a negotiation iteration are
160 guaranteed to be supported by the device. In particular, drivers guarantee
161 that a returned format will not be further changed if passed to an
162 &VIDIOC-SUBDEV-S-FMT; call as-is (as long as external parameters, such as
163 formats on other pads or links' configuration are not changed).</para>
165 <para>Drivers automatically propagate formats inside sub-devices. When a
166 try or active format is set on a pad, corresponding formats on other pads
167 of the same sub-device can be modified by the driver. Drivers are free to
168 modify formats as required by the device. However, they should comply with
169 the following rules when possible:
170 <itemizedlist>
171 <listitem><para>Formats should be propagated from sink pads to source pads.
172 Modifying a format on a source pad should not modify the format on any
173 sink pad.</para></listitem>
174 <listitem><para>Sub-devices that scale frames using variable scaling factors
175 should reset the scale factors to default values when sink pads formats
176 are modified. If the 1:1 scaling ratio is supported, this means that
177 source pads formats should be reset to the sink pads formats.</para></listitem>
178 </itemizedlist>
179 </para>
181 <para>Formats are not propagated across links, as that would involve
182 propagating them from one sub-device file handle to another. Applications
183 must then take care to configure both ends of every link explicitly with
184 compatible formats. Identical formats on the two ends of a link are
185 guaranteed to be compatible. Drivers are free to accept different formats
186 matching device requirements as being compatible.</para>
188 <para><xref linkend="sample-pipeline-config" />
189 shows a sample configuration sequence for the pipeline described in
190 <xref linkend="pipeline-scaling" /> (table
191 columns list entity names and pad numbers).</para>
193 <table pgwide="0" frame="none" id="sample-pipeline-config">
194 <title>Sample Pipeline Configuration</title>
195 <tgroup cols="3">
196 <colspec colname="what"/>
197 <colspec colname="sensor-0" />
198 <colspec colname="frontend-0" />
199 <colspec colname="frontend-1" />
200 <colspec colname="scaler-0" />
201 <colspec colname="scaler-1" />
202 <thead>
203 <row>
204 <entry></entry>
205 <entry>Sensor/0</entry>
206 <entry>Frontend/0</entry>
207 <entry>Frontend/1</entry>
208 <entry>Scaler/0</entry>
209 <entry>Scaler/1</entry>
210 </row>
211 </thead>
212 <tbody valign="top">
213 <row>
214 <entry>Initial state</entry>
215 <entry>2048x1536</entry>
216 <entry>-</entry>
217 <entry>-</entry>
218 <entry>-</entry>
219 <entry>-</entry>
220 </row>
221 <row>
222 <entry>Configure frontend input</entry>
223 <entry>2048x1536</entry>
224 <entry><emphasis>2048x1536</emphasis></entry>
225 <entry><emphasis>2046x1534</emphasis></entry>
226 <entry>-</entry>
227 <entry>-</entry>
228 </row>
229 <row>
230 <entry>Configure scaler input</entry>
231 <entry>2048x1536</entry>
232 <entry>2048x1536</entry>
233 <entry>2046x1534</entry>
234 <entry><emphasis>2046x1534</emphasis></entry>
235 <entry><emphasis>2046x1534</emphasis></entry>
236 </row>
237 <row>
238 <entry>Configure scaler output</entry>
239 <entry>2048x1536</entry>
240 <entry>2048x1536</entry>
241 <entry>2046x1534</entry>
242 <entry>2046x1534</entry>
243 <entry><emphasis>1280x960</emphasis></entry>
244 </row>
245 </tbody>
246 </tgroup>
247 </table>
249 <para>
250 <orderedlist>
251 <listitem><para>Initial state. The sensor output is set to its native 3MP
252 resolution. Resolutions on the host frontend and scaler input and output
253 pads are undefined.</para></listitem>
254 <listitem><para>The application configures the frontend input pad resolution to
255 2048x1536. The driver propagates the format to the frontend output pad.
256 Note that the propagated output format can be different, as in this case,
257 than the input format, as the hardware might need to crop pixels (for
258 instance when converting a Bayer filter pattern to RGB or YUV).</para></listitem>
259 <listitem><para>The application configures the scaler input pad resolution to
260 2046x1534 to match the frontend output resolution. The driver propagates
261 the format to the scaler output pad.</para></listitem>
262 <listitem><para>The application configures the scaler output pad resolution to
263 1280x960.</para></listitem>
264 </orderedlist>
265 </para>
267 <para>When satisfied with the try results, applications can set the active
268 formats by setting the <structfield>which</structfield> argument to
269 <constant>V4L2_SUBDEV_FORMAT_ACTIVE</constant>. Active formats are changed
270 exactly as try formats by drivers. To avoid modifying the hardware state
271 during format negotiation, applications should negotiate try formats first
272 and then modify the active settings using the try formats returned during
273 the last negotiation iteration. This guarantees that the active format
274 will be applied as-is by the driver without being modified.
275 </para>
276 </section>
278 <section>
279 <title>Cropping and scaling</title>
281 <para>Many sub-devices support cropping frames on their input or output
282 pads (or possible even on both). Cropping is used to select the area of
283 interest in an image, typically on a video sensor or video decoder. It can
284 also be used as part of digital zoom implementations to select the area of
285 the image that will be scaled up.</para>
287 <para>Crop settings are defined by a crop rectangle and represented in a
288 &v4l2-rect; by the coordinates of the top left corner and the rectangle
289 size. Both the coordinates and sizes are expressed in pixels.</para>
291 <para>The crop rectangle is retrieved and set using the
292 &VIDIOC-SUBDEV-G-CROP; and &VIDIOC-SUBDEV-S-CROP; ioctls. Like for pad
293 formats, drivers store try and active crop rectangles. The format
294 negotiation mechanism applies to crop settings as well.</para>
296 <para>On input pads, cropping is applied relatively to the current pad
297 format. The pad format represents the image size as received by the
298 sub-device from the previous block in the pipeline, and the crop rectangle
299 represents the sub-image that will be transmitted further inside the
300 sub-device for processing. The crop rectangle be entirely containted
301 inside the input image size.</para>
303 <para>Input crop rectangle are reset to their default value when the input
304 image format is modified. Drivers should use the input image size as the
305 crop rectangle default value, but hardware requirements may prevent this.
306 </para>
308 <para>Cropping behaviour on output pads is not defined.</para>
310 </section>
311 </section>
313 &sub-subdev-formats;