Based on kernel version 4.7.2. Page generated on 2016-08-22 22:45 EST.
1 <title>Sub-device Interface</title> 2 3 <para>The complex nature of V4L2 devices, where hardware is often made of 4 several integrated circuits that need to interact with each other in a 5 controlled way, leads to complex V4L2 drivers. The drivers usually reflect 6 the hardware model in software, and model the different hardware components 7 as software blocks called sub-devices.</para> 8 9 <para>V4L2 sub-devices are usually kernel-only objects. If the V4L2 driver 10 implements the media device API, they will automatically inherit from media 11 entities. Applications will be able to enumerate the sub-devices and discover 12 the hardware topology using the media entities, pads and links enumeration 13 API.</para> 14 15 <para>In addition to make sub-devices discoverable, drivers can also choose 16 to make them directly configurable by applications. When both the sub-device 17 driver and the V4L2 device driver support this, sub-devices will feature a 18 character device node on which ioctls can be called to 19 <itemizedlist> 20 <listitem><para>query, read and write sub-devices controls</para></listitem> 21 <listitem><para>subscribe and unsubscribe to events and retrieve them</para></listitem> 22 <listitem><para>negotiate image formats on individual pads</para></listitem> 23 </itemizedlist> 24 </para> 25 26 <para>Sub-device character device nodes, conventionally named 27 <filename>/dev/v4l-subdev*</filename>, use major number 81.</para> 28 29 <section> 30 <title>Controls</title> 31 <para>Most V4L2 controls are implemented by sub-device hardware. Drivers 32 usually merge all controls and expose them through video device nodes. 33 Applications can control all sub-devices through a single interface.</para> 34 35 <para>Complex devices sometimes implement the same control in different 36 pieces of hardware. This situation is common in embedded platforms, where 37 both sensors and image processing hardware implement identical functions, 38 such as contrast adjustment, white balance or faulty pixels correction. As 39 the V4L2 controls API doesn't support several identical controls in a single 40 device, all but one of the identical controls are hidden.</para> 41 42 <para>Applications can access those hidden controls through the sub-device 43 node with the V4L2 control API described in <xref linkend="control" />. The 44 ioctls behave identically as when issued on V4L2 device nodes, with the 45 exception that they deal only with controls implemented in the sub-device. 46 </para> 47 48 <para>Depending on the driver, those controls might also be exposed through 49 one (or several) V4L2 device nodes.</para> 50 </section> 51 52 <section> 53 <title>Events</title> 54 <para>V4L2 sub-devices can notify applications of events as described in 55 <xref linkend="event" />. The API behaves identically as when used on V4L2 56 device nodes, with the exception that it only deals with events generated by 57 the sub-device. Depending on the driver, those events might also be reported 58 on one (or several) V4L2 device nodes.</para> 59 </section> 60 61 <section id="pad-level-formats"> 62 <title>Pad-level Formats</title> 63 64 <warning><para>Pad-level formats are only applicable to very complex device that 65 need to expose low-level format configuration to user space. Generic V4L2 66 applications do <emphasis>not</emphasis> need to use the API described in 67 this section.</para></warning> 68 69 <note><para>For the purpose of this section, the term 70 <wordasword>format</wordasword> means the combination of media bus data 71 format, frame width and frame height.</para></note> 72 73 <para>Image formats are typically negotiated on video capture and 74 output devices using the format and <link 75 linkend="vidioc-subdev-g-selection">selection</link> ioctls. The 76 driver is responsible for configuring every block in the video 77 pipeline according to the requested format at the pipeline input 78 and/or output.</para> 79 80 <para>For complex devices, such as often found in embedded systems, 81 identical image sizes at the output of a pipeline can be achieved using 82 different hardware configurations. One such example is shown on 83 <xref linkend="pipeline-scaling" />, where 84 image scaling can be performed on both the video sensor and the host image 85 processing hardware.</para> 86 87 <figure id="pipeline-scaling"> 88 <title>Image Format Negotiation on Pipelines</title> 89 <mediaobject> 90 <imageobject> 91 <imagedata fileref="pipeline.pdf" format="PS" /> 92 </imageobject> 93 <imageobject> 94 <imagedata fileref="pipeline.png" format="PNG" /> 95 </imageobject> 96 <textobject> 97 <phrase>High quality and high speed pipeline configuration</phrase> 98 </textobject> 99 </mediaobject> 100 </figure> 101 102 <para>The sensor scaler is usually of less quality than the host scaler, but 103 scaling on the sensor is required to achieve higher frame rates. Depending 104 on the use case (quality vs. speed), the pipeline must be configured 105 differently. Applications need to configure the formats at every point in 106 the pipeline explicitly.</para> 107 108 <para>Drivers that implement the <link linkend="media-controller-intro">media 109 API</link> can expose pad-level image format configuration to applications. 110 When they do, applications can use the &VIDIOC-SUBDEV-G-FMT; and 111 &VIDIOC-SUBDEV-S-FMT; ioctls. to negotiate formats on a per-pad basis.</para> 112 113 <para>Applications are responsible for configuring coherent parameters on 114 the whole pipeline and making sure that connected pads have compatible 115 formats. The pipeline is checked for formats mismatch at &VIDIOC-STREAMON; 116 time, and an &EPIPE; is then returned if the configuration is 117 invalid.</para> 118 119 <para>Pad-level image format configuration support can be tested by calling 120 the &VIDIOC-SUBDEV-G-FMT; ioctl on pad 0. If the driver returns an &EINVAL; 121 pad-level format configuration is not supported by the sub-device.</para> 122 123 <section> 124 <title>Format Negotiation</title> 125 126 <para>Acceptable formats on pads can (and usually do) depend on a number 127 of external parameters, such as formats on other pads, active links, or 128 even controls. Finding a combination of formats on all pads in a video 129 pipeline, acceptable to both application and driver, can't rely on formats 130 enumeration only. A format negotiation mechanism is required.</para> 131 132 <para>Central to the format negotiation mechanism are the get/set format 133 operations. When called with the <structfield>which</structfield> argument 134 set to <constant>V4L2_SUBDEV_FORMAT_TRY</constant>, the 135 &VIDIOC-SUBDEV-G-FMT; and &VIDIOC-SUBDEV-S-FMT; ioctls operate on a set of 136 formats parameters that are not connected to the hardware configuration. 137 Modifying those 'try' formats leaves the device state untouched (this 138 applies to both the software state stored in the driver and the hardware 139 state stored in the device itself).</para> 140 141 <para>While not kept as part of the device state, try formats are stored 142 in the sub-device file handles. A &VIDIOC-SUBDEV-G-FMT; call will return 143 the last try format set <emphasis>on the same sub-device file 144 handle</emphasis>. Several applications querying the same sub-device at 145 the same time will thus not interact with each other.</para> 146 147 <para>To find out whether a particular format is supported by the device, 148 applications use the &VIDIOC-SUBDEV-S-FMT; ioctl. Drivers verify and, if 149 needed, change the requested <structfield>format</structfield> based on 150 device requirements and return the possibly modified value. Applications 151 can then choose to try a different format or accept the returned value and 152 continue.</para> 153 154 <para>Formats returned by the driver during a negotiation iteration are 155 guaranteed to be supported by the device. In particular, drivers guarantee 156 that a returned format will not be further changed if passed to an 157 &VIDIOC-SUBDEV-S-FMT; call as-is (as long as external parameters, such as 158 formats on other pads or links' configuration are not changed).</para> 159 160 <para>Drivers automatically propagate formats inside sub-devices. When a 161 try or active format is set on a pad, corresponding formats on other pads 162 of the same sub-device can be modified by the driver. Drivers are free to 163 modify formats as required by the device. However, they should comply with 164 the following rules when possible: 165 <itemizedlist> 166 <listitem><para>Formats should be propagated from sink pads to source pads. 167 Modifying a format on a source pad should not modify the format on any 168 sink pad.</para></listitem> 169 <listitem><para>Sub-devices that scale frames using variable scaling factors 170 should reset the scale factors to default values when sink pads formats 171 are modified. If the 1:1 scaling ratio is supported, this means that 172 source pads formats should be reset to the sink pads formats.</para></listitem> 173 </itemizedlist> 174 </para> 175 176 <para>Formats are not propagated across links, as that would involve 177 propagating them from one sub-device file handle to another. Applications 178 must then take care to configure both ends of every link explicitly with 179 compatible formats. Identical formats on the two ends of a link are 180 guaranteed to be compatible. Drivers are free to accept different formats 181 matching device requirements as being compatible.</para> 182 183 <para><xref linkend="sample-pipeline-config" /> 184 shows a sample configuration sequence for the pipeline described in 185 <xref linkend="pipeline-scaling" /> (table 186 columns list entity names and pad numbers).</para> 187 188 <table pgwide="0" frame="none" id="sample-pipeline-config"> 189 <title>Sample Pipeline Configuration</title> 190 <tgroup cols="3"> 191 <colspec colname="what"/> 192 <colspec colname="sensor-0 format" /> 193 <colspec colname="frontend-0 format" /> 194 <colspec colname="frontend-1 format" /> 195 <colspec colname="scaler-0 format" /> 196 <colspec colname="scaler-0 compose" /> 197 <colspec colname="scaler-1 format" /> 198 <thead> 199 <row> 200 <entry></entry> 201 <entry>Sensor/0 format</entry> 202 <entry>Frontend/0 format</entry> 203 <entry>Frontend/1 format</entry> 204 <entry>Scaler/0 format</entry> 205 <entry>Scaler/0 compose selection rectangle</entry> 206 <entry>Scaler/1 format</entry> 207 </row> 208 </thead> 209 <tbody valign="top"> 210 <row> 211 <entry>Initial state</entry> 212 <entry>2048x1536/SGRBG8_1X8</entry> 213 <entry>(default)</entry> 214 <entry>(default)</entry> 215 <entry>(default)</entry> 216 <entry>(default)</entry> 217 <entry>(default)</entry> 218 </row> 219 <row> 220 <entry>Configure frontend sink format</entry> 221 <entry>2048x1536/SGRBG8_1X8</entry> 222 <entry><emphasis>2048x1536/SGRBG8_1X8</emphasis></entry> 223 <entry><emphasis>2046x1534/SGRBG8_1X8</emphasis></entry> 224 <entry>(default)</entry> 225 <entry>(default)</entry> 226 <entry>(default)</entry> 227 </row> 228 <row> 229 <entry>Configure scaler sink format</entry> 230 <entry>2048x1536/SGRBG8_1X8</entry> 231 <entry>2048x1536/SGRBG8_1X8</entry> 232 <entry>2046x1534/SGRBG8_1X8</entry> 233 <entry><emphasis>2046x1534/SGRBG8_1X8</emphasis></entry> 234 <entry><emphasis>0,0/2046x1534</emphasis></entry> 235 <entry><emphasis>2046x1534/SGRBG8_1X8</emphasis></entry> 236 </row> 237 <row> 238 <entry>Configure scaler sink compose selection</entry> 239 <entry>2048x1536/SGRBG8_1X8</entry> 240 <entry>2048x1536/SGRBG8_1X8</entry> 241 <entry>2046x1534/SGRBG8_1X8</entry> 242 <entry>2046x1534/SGRBG8_1X8</entry> 243 <entry><emphasis>0,0/1280x960</emphasis></entry> 244 <entry><emphasis>1280x960/SGRBG8_1X8</emphasis></entry> 245 </row> 246 </tbody> 247 </tgroup> 248 </table> 249 250 <para> 251 <orderedlist> 252 <listitem><para>Initial state. The sensor source pad format is 253 set to its native 3MP size and V4L2_MBUS_FMT_SGRBG8_1X8 254 media bus code. Formats on the host frontend and scaler sink 255 and source pads have the default values, as well as the 256 compose rectangle on the scaler's sink pad.</para></listitem> 257 258 <listitem><para>The application configures the frontend sink 259 pad format's size to 2048x1536 and its media bus code to 260 V4L2_MBUS_FMT_SGRBG_1X8. The driver propagates the format to 261 the frontend source pad.</para></listitem> 262 263 <listitem><para>The application configures the scaler sink pad 264 format's size to 2046x1534 and the media bus code to 265 V4L2_MBUS_FMT_SGRBG_1X8 to match the frontend source size and 266 media bus code. The media bus code on the sink pad is set to 267 V4L2_MBUS_FMT_SGRBG_1X8. The driver propagates the size to the 268 compose selection rectangle on the scaler's sink pad, and the 269 format to the scaler source pad.</para></listitem> 270 271 <listitem><para>The application configures the size of the compose 272 selection rectangle of the scaler's sink pad 1280x960. The driver 273 propagates the size to the scaler's source pad 274 format.</para></listitem> 275 276 </orderedlist> 277 </para> 278 279 <para>When satisfied with the try results, applications can set the active 280 formats by setting the <structfield>which</structfield> argument to 281 <constant>V4L2_SUBDEV_FORMAT_ACTIVE</constant>. Active formats are changed 282 exactly as try formats by drivers. To avoid modifying the hardware state 283 during format negotiation, applications should negotiate try formats first 284 and then modify the active settings using the try formats returned during 285 the last negotiation iteration. This guarantees that the active format 286 will be applied as-is by the driver without being modified. 287 </para> 288 </section> 289 290 <section id="v4l2-subdev-selections"> 291 <title>Selections: cropping, scaling and composition</title> 292 293 <para>Many sub-devices support cropping frames on their input or output 294 pads (or possible even on both). Cropping is used to select the area of 295 interest in an image, typically on an image sensor or a video decoder. It can 296 also be used as part of digital zoom implementations to select the area of 297 the image that will be scaled up.</para> 298 299 <para>Crop settings are defined by a crop rectangle and represented in a 300 &v4l2-rect; by the coordinates of the top left corner and the rectangle 301 size. Both the coordinates and sizes are expressed in pixels.</para> 302 303 <para>As for pad formats, drivers store try and active 304 rectangles for the selection targets <xref 305 linkend="v4l2-selections-common" />.</para> 306 307 <para>On sink pads, cropping is applied relative to the 308 current pad format. The pad format represents the image size as 309 received by the sub-device from the previous block in the 310 pipeline, and the crop rectangle represents the sub-image that 311 will be transmitted further inside the sub-device for 312 processing.</para> 313 314 <para>The scaling operation changes the size of the image by 315 scaling it to new dimensions. The scaling ratio isn't specified 316 explicitly, but is implied from the original and scaled image 317 sizes. Both sizes are represented by &v4l2-rect;.</para> 318 319 <para>Scaling support is optional. When supported by a subdev, 320 the crop rectangle on the subdev's sink pad is scaled to the 321 size configured using the &VIDIOC-SUBDEV-S-SELECTION; IOCTL 322 using <constant>V4L2_SEL_TGT_COMPOSE</constant> 323 selection target on the same pad. If the subdev supports scaling 324 but not composing, the top and left values are not used and must 325 always be set to zero.</para> 326 327 <para>On source pads, cropping is similar to sink pads, with the 328 exception that the source size from which the cropping is 329 performed, is the COMPOSE rectangle on the sink pad. In both 330 sink and source pads, the crop rectangle must be entirely 331 contained inside the source image size for the crop 332 operation.</para> 333 334 <para>The drivers should always use the closest possible 335 rectangle the user requests on all selection targets, unless 336 specifically told otherwise. 337 <constant>V4L2_SEL_FLAG_GE</constant> and 338 <constant>V4L2_SEL_FLAG_LE</constant> flags may be 339 used to round the image size either up or down. <xref 340 linkend="v4l2-selection-flags" /></para> 341 </section> 342 343 <section> 344 <title>Types of selection targets</title> 345 346 <section> 347 <title>Actual targets</title> 348 349 <para>Actual targets (without a postfix) reflect the actual 350 hardware configuration at any point of time. There is a BOUNDS 351 target corresponding to every actual target.</para> 352 </section> 353 354 <section> 355 <title>BOUNDS targets</title> 356 357 <para>BOUNDS targets is the smallest rectangle that contains all 358 valid actual rectangles. It may not be possible to set the actual 359 rectangle as large as the BOUNDS rectangle, however. This may be 360 because e.g. a sensor's pixel array is not rectangular but 361 cross-shaped or round. The maximum size may also be smaller than the 362 BOUNDS rectangle.</para> 363 </section> 364 365 </section> 366 367 <section> 368 <title>Order of configuration and format propagation</title> 369 370 <para>Inside subdevs, the order of image processing steps will 371 always be from the sink pad towards the source pad. This is also 372 reflected in the order in which the configuration must be 373 performed by the user: the changes made will be propagated to 374 any subsequent stages. If this behaviour is not desired, the 375 user must set 376 <constant>V4L2_SEL_FLAG_KEEP_CONFIG</constant> flag. This 377 flag causes no propagation of the changes are allowed in any 378 circumstances. This may also cause the accessed rectangle to be 379 adjusted by the driver, depending on the properties of the 380 underlying hardware.</para> 381 382 <para>The coordinates to a step always refer to the actual size 383 of the previous step. The exception to this rule is the source 384 compose rectangle, which refers to the sink compose bounds 385 rectangle --- if it is supported by the hardware.</para> 386 387 <orderedlist> 388 <listitem><para>Sink pad format. The user configures the sink pad 389 format. This format defines the parameters of the image the 390 entity receives through the pad for further processing.</para></listitem> 391 392 <listitem><para>Sink pad actual crop selection. The sink pad crop 393 defines the crop performed to the sink pad format.</para></listitem> 394 395 <listitem><para>Sink pad actual compose selection. The size of the 396 sink pad compose rectangle defines the scaling ratio compared 397 to the size of the sink pad crop rectangle. The location of 398 the compose rectangle specifies the location of the actual 399 sink compose rectangle in the sink compose bounds 400 rectangle.</para></listitem> 401 402 <listitem><para>Source pad actual crop selection. Crop on the source 403 pad defines crop performed to the image in the sink compose 404 bounds rectangle.</para></listitem> 405 406 <listitem><para>Source pad format. The source pad format defines the 407 output pixel format of the subdev, as well as the other 408 parameters with the exception of the image width and height. 409 Width and height are defined by the size of the source pad 410 actual crop selection.</para></listitem> 411 </orderedlist> 412 413 <para>Accessing any of the above rectangles not supported by the 414 subdev will return <constant>EINVAL</constant>. Any rectangle 415 referring to a previous unsupported rectangle coordinates will 416 instead refer to the previous supported rectangle. For example, 417 if sink crop is not supported, the compose selection will refer 418 to the sink pad format dimensions instead.</para> 419 420 <figure id="subdev-image-processing-crop"> 421 <title>Image processing in subdevs: simple crop example</title> 422 <mediaobject> 423 <imageobject> 424 <imagedata fileref="subdev-image-processing-crop.svg" 425 format="SVG" scale="200" /> 426 </imageobject> 427 </mediaobject> 428 </figure> 429 430 <para>In the above example, the subdev supports cropping on its 431 sink pad. To configure it, the user sets the media bus format on 432 the subdev's sink pad. Now the actual crop rectangle can be set 433 on the sink pad --- the location and size of this rectangle 434 reflect the location and size of a rectangle to be cropped from 435 the sink format. The size of the sink crop rectangle will also 436 be the size of the format of the subdev's source pad.</para> 437 438 <figure id="subdev-image-processing-scaling-multi-source"> 439 <title>Image processing in subdevs: scaling with multiple sources</title> 440 <mediaobject> 441 <imageobject> 442 <imagedata fileref="subdev-image-processing-scaling-multi-source.svg" 443 format="SVG" scale="200" /> 444 </imageobject> 445 </mediaobject> 446 </figure> 447 448 <para>In this example, the subdev is capable of first cropping, 449 then scaling and finally cropping for two source pads 450 individually from the resulting scaled image. The location of 451 the scaled image in the cropped image is ignored in sink compose 452 target. Both of the locations of the source crop rectangles 453 refer to the sink scaling rectangle, independently cropping an 454 area at location specified by the source crop rectangle from 455 it.</para> 456 457 <figure id="subdev-image-processing-full"> 458 <title>Image processing in subdevs: scaling and composition 459 with multiple sinks and sources</title> 460 <mediaobject> 461 <imageobject> 462 <imagedata fileref="subdev-image-processing-full.svg" 463 format="SVG" scale="200" /> 464 </imageobject> 465 </mediaobject> 466 </figure> 467 468 <para>The subdev driver supports two sink pads and two source 469 pads. The images from both of the sink pads are individually 470 cropped, then scaled and further composed on the composition 471 bounds rectangle. From that, two independent streams are cropped 472 and sent out of the subdev from the source pads.</para> 473 474 </section> 475 476 </section> 477 478 &sub-subdev-formats;