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1 | <title>Sub-device Interface</title> |
2 | ||
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> | |
8 | ||
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> | |
14 | ||
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> | |
20 | ||
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> | |
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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> | |
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29 | </itemizedlist> |
30 | </para> | |
31 | ||
32 | <para>Sub-device character device nodes, conventionally named | |
33 | <filename>/dev/v4l-subdev*</filename>, use major number 81.</para> | |
34 | ||
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> | |
40 | ||
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> | |
47 | ||
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> | |
53 | ||
54 | <para>Depending on the driver, those controls might also be exposed through | |
55 | one (or several) V4L2 device nodes.</para> | |
56 | </section> | |
57 | ||
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> | |
66 | ||
67 | <section id="pad-level-formats"> | |
68 | <title>Pad-level Formats</title> | |
69 | ||
665bf368 | 70 | <warning><para>Pad-level formats are only applicable to very complex device that |
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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 | |
665bf368 | 73 | this section.</para></warning> |
333c8b97 | 74 | |
665bf368 | 75 | <note><para>For the purpose of this section, the term |
333c8b97 | 76 | <wordasword>format</wordasword> means the combination of media bus data |
665bf368 | 77 | format, frame width and frame height.</para></note> |
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78 | |
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> | |
84 | ||
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 | |
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87 | different hardware configurations. One such example is shown on |
88 | <xref linkend="pipeline-scaling" />, where | |
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89 | image scaling can be performed on both the video sensor and the host image |
90 | processing hardware.</para> | |
91 | ||
92 | <figure id="pipeline-scaling"> | |
25985edc | 93 | <title>Image Format Negotiation on Pipelines</title> |
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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> | |
106 | ||
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> | |
112 | ||
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> | |
117 | ||
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> | |
123 | ||
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> | |
127 | ||
128 | <section> | |
129 | <title>Format Negotiation</title> | |
130 | ||
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> | |
136 | ||
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> | |
145 | ||
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> | |
151 | ||
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> | |
158 | ||
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> | |
164 | ||
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> | |
665bf368 | 171 | <listitem><para>Formats should be propagated from sink pads to source pads. |
333c8b97 | 172 | Modifying a format on a source pad should not modify the format on any |
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173 | sink pad.</para></listitem> |
174 | <listitem><para>Sub-devices that scale frames using variable scaling factors | |
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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 | |
665bf368 | 177 | source pads formats should be reset to the sink pads formats.</para></listitem> |
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178 | </itemizedlist> |
179 | </para> | |
180 | ||
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> | |
187 | ||
665bf368 | 188 | <para><xref linkend="sample-pipeline-config" /> |
333c8b97 | 189 | shows a sample configuration sequence for the pipeline described in |
665bf368 | 190 | <xref linkend="pipeline-scaling" /> (table |
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191 | columns list entity names and pad numbers).</para> |
192 | ||
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> | |
248 | ||
249 | <para> | |
250 | <orderedlist> | |
665bf368 | 251 | <listitem><para>Initial state. The sensor output is set to its native 3MP |
333c8b97 | 252 | resolution. Resolutions on the host frontend and scaler input and output |
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253 | pads are undefined.</para></listitem> |
254 | <listitem><para>The application configures the frontend input pad resolution to | |
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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 | |
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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 | |
333c8b97 | 260 | 2046x1534 to match the frontend output resolution. The driver propagates |
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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> | |
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264 | </orderedlist> |
265 | </para> | |
266 | ||
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_TRY</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> | |
277 | ||
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278 | <section> |
279 | <title>Cropping and scaling</title> | |
280 | ||
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> | |
286 | ||
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> | |
290 | ||
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> | |
295 | ||
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> | |
302 | ||
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> | |
307 | ||
308 | <para>Cropping behaviour on output pads is not defined.</para> | |
309 | ||
310 | </section> | |
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311 | </section> |
312 | ||
313 | &sub-subdev-formats; |