Based on kernel version 3.16. Page generated on 2014-08-06 21:38 EST.
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> 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> 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 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> 74 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> 78 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> 85 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> 92 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> 107 108 <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> 113 114 <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> 118 119 <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> 124 125 <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> 128 129 <section> 130 <title>Format Negotiation</title> 131 132 <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> 137 138 <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> 146 147 <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> 152 153 <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> 159 160 <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> 165 166 <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> 181 182 <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> 188 189 <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> 193 194 <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" /> 199 <colspec colname="frontend-0" /> 200 <colspec colname="frontend-1" /> 201 <colspec colname="scaler-0" /> 202 <colspec colname="scaler-1" /> 203 <thead> 204 <row> 205 <entry></entry> 206 <entry>Sensor/0</entry> 207 <entry>Frontend/0</entry> 208 <entry>Frontend/1</entry> 209 <entry>Scaler/0</entry> 210 <entry>Scaler/1</entry> 211 </row> 212 </thead> 213 <tbody valign="top"> 214 <row> 215 <entry>Initial state</entry> 216 <entry>2048x1536</entry> 217 <entry>-</entry> 218 <entry>-</entry> 219 <entry>-</entry> 220 <entry>-</entry> 221 </row> 222 <row> 223 <entry>Configure frontend input</entry> 224 <entry>2048x1536</entry> 225 <entry><emphasis>2048x1536</emphasis></entry> 226 <entry><emphasis>2046x1534</emphasis></entry> 227 <entry>-</entry> 228 <entry>-</entry> 229 </row> 230 <row> 231 <entry>Configure scaler input</entry> 232 <entry>2048x1536</entry> 233 <entry>2048x1536</entry> 234 <entry>2046x1534</entry> 235 <entry><emphasis>2046x1534</emphasis></entry> 236 <entry><emphasis>2046x1534</emphasis></entry> 237 </row> 238 <row> 239 <entry>Configure scaler output</entry> 240 <entry>2048x1536</entry> 241 <entry>2048x1536</entry> 242 <entry>2046x1534</entry> 243 <entry>2046x1534</entry> 244 <entry><emphasis>1280x960</emphasis></entry> 245 </row> 246 </tbody> 247 </tgroup> 248 </table> 249 250 <para> 251 <orderedlist> 252 <listitem><para>Initial state. The sensor output is set to its native 3MP 253 resolution. Resolutions on the host frontend and scaler input and output 254 pads are undefined.</para></listitem> 255 <listitem><para>The application configures the frontend input pad resolution to 256 2048x1536. The driver propagates the format to the frontend output pad. 257 Note that the propagated output format can be different, as in this case, 258 than the input format, as the hardware might need to crop pixels (for 259 instance when converting a Bayer filter pattern to RGB or YUV).</para></listitem> 260 <listitem><para>The application configures the scaler input pad resolution to 261 2046x1534 to match the frontend output resolution. The driver propagates 262 the format to the scaler output pad.</para></listitem> 263 <listitem><para>The application configures the scaler output pad resolution to 264 1280x960.</para></listitem> 265 </orderedlist> 266 </para> 267 268 <para>When satisfied with the try results, applications can set the active 269 formats by setting the <structfield>which</structfield> argument to 270 <constant>V4L2_SUBDEV_FORMAT_ACTIVE</constant>. Active formats are changed 271 exactly as try formats by drivers. To avoid modifying the hardware state 272 during format negotiation, applications should negotiate try formats first 273 and then modify the active settings using the try formats returned during 274 the last negotiation iteration. This guarantees that the active format 275 will be applied as-is by the driver without being modified. 276 </para> 277 </section> 278 279 <section id="v4l2-subdev-selections"> 280 <title>Selections: cropping, scaling and composition</title> 281 282 <para>Many sub-devices support cropping frames on their input or output 283 pads (or possible even on both). Cropping is used to select the area of 284 interest in an image, typically on an image sensor or a video decoder. It can 285 also be used as part of digital zoom implementations to select the area of 286 the image that will be scaled up.</para> 287 288 <para>Crop settings are defined by a crop rectangle and represented in a 289 &v4l2-rect; by the coordinates of the top left corner and the rectangle 290 size. Both the coordinates and sizes are expressed in pixels.</para> 291 292 <para>As for pad formats, drivers store try and active 293 rectangles for the selection targets <xref 294 linkend="v4l2-selections-common" />.</para> 295 296 <para>On sink pads, cropping is applied relative to the 297 current pad format. The pad format represents the image size as 298 received by the sub-device from the previous block in the 299 pipeline, and the crop rectangle represents the sub-image that 300 will be transmitted further inside the sub-device for 301 processing.</para> 302 303 <para>The scaling operation changes the size of the image by 304 scaling it to new dimensions. The scaling ratio isn't specified 305 explicitly, but is implied from the original and scaled image 306 sizes. Both sizes are represented by &v4l2-rect;.</para> 307 308 <para>Scaling support is optional. When supported by a subdev, 309 the crop rectangle on the subdev's sink pad is scaled to the 310 size configured using the &VIDIOC-SUBDEV-S-SELECTION; IOCTL 311 using <constant>V4L2_SEL_TGT_COMPOSE</constant> 312 selection target on the same pad. If the subdev supports scaling 313 but not composing, the top and left values are not used and must 314 always be set to zero.</para> 315 316 <para>On source pads, cropping is similar to sink pads, with the 317 exception that the source size from which the cropping is 318 performed, is the COMPOSE rectangle on the sink pad. In both 319 sink and source pads, the crop rectangle must be entirely 320 contained inside the source image size for the crop 321 operation.</para> 322 323 <para>The drivers should always use the closest possible 324 rectangle the user requests on all selection targets, unless 325 specifically told otherwise. 326 <constant>V4L2_SEL_FLAG_GE</constant> and 327 <constant>V4L2_SEL_FLAG_LE</constant> flags may be 328 used to round the image size either up or down. <xref 329 linkend="v4l2-selection-flags" /></para> 330 </section> 331 332 <section> 333 <title>Types of selection targets</title> 334 335 <section> 336 <title>Actual targets</title> 337 338 <para>Actual targets (without a postfix) reflect the actual 339 hardware configuration at any point of time. There is a BOUNDS 340 target corresponding to every actual target.</para> 341 </section> 342 343 <section> 344 <title>BOUNDS targets</title> 345 346 <para>BOUNDS targets is the smallest rectangle that contains all 347 valid actual rectangles. It may not be possible to set the actual 348 rectangle as large as the BOUNDS rectangle, however. This may be 349 because e.g. a sensor's pixel array is not rectangular but 350 cross-shaped or round. The maximum size may also be smaller than the 351 BOUNDS rectangle.</para> 352 </section> 353 354 </section> 355 356 <section> 357 <title>Order of configuration and format propagation</title> 358 359 <para>Inside subdevs, the order of image processing steps will 360 always be from the sink pad towards the source pad. This is also 361 reflected in the order in which the configuration must be 362 performed by the user: the changes made will be propagated to 363 any subsequent stages. If this behaviour is not desired, the 364 user must set 365 <constant>V4L2_SEL_FLAG_KEEP_CONFIG</constant> flag. This 366 flag causes no propagation of the changes are allowed in any 367 circumstances. This may also cause the accessed rectangle to be 368 adjusted by the driver, depending on the properties of the 369 underlying hardware.</para> 370 371 <para>The coordinates to a step always refer to the actual size 372 of the previous step. The exception to this rule is the source 373 compose rectangle, which refers to the sink compose bounds 374 rectangle --- if it is supported by the hardware.</para> 375 376 <orderedlist> 377 <listitem><para>Sink pad format. The user configures the sink pad 378 format. This format defines the parameters of the image the 379 entity receives through the pad for further processing.</para></listitem> 380 381 <listitem><para>Sink pad actual crop selection. The sink pad crop 382 defines the crop performed to the sink pad format.</para></listitem> 383 384 <listitem><para>Sink pad actual compose selection. The size of the 385 sink pad compose rectangle defines the scaling ratio compared 386 to the size of the sink pad crop rectangle. The location of 387 the compose rectangle specifies the location of the actual 388 sink compose rectangle in the sink compose bounds 389 rectangle.</para></listitem> 390 391 <listitem><para>Source pad actual crop selection. Crop on the source 392 pad defines crop performed to the image in the sink compose 393 bounds rectangle.</para></listitem> 394 395 <listitem><para>Source pad format. The source pad format defines the 396 output pixel format of the subdev, as well as the other 397 parameters with the exception of the image width and height. 398 Width and height are defined by the size of the source pad 399 actual crop selection.</para></listitem> 400 </orderedlist> 401 402 <para>Accessing any of the above rectangles not supported by the 403 subdev will return <constant>EINVAL</constant>. Any rectangle 404 referring to a previous unsupported rectangle coordinates will 405 instead refer to the previous supported rectangle. For example, 406 if sink crop is not supported, the compose selection will refer 407 to the sink pad format dimensions instead.</para> 408 409 <figure id="subdev-image-processing-crop"> 410 <title>Image processing in subdevs: simple crop example</title> 411 <mediaobject> 412 <imageobject> 413 <imagedata fileref="subdev-image-processing-crop.svg" 414 format="SVG" scale="200" /> 415 </imageobject> 416 </mediaobject> 417 </figure> 418 419 <para>In the above example, the subdev supports cropping on its 420 sink pad. To configure it, the user sets the media bus format on 421 the subdev's sink pad. Now the actual crop rectangle can be set 422 on the sink pad --- the location and size of this rectangle 423 reflect the location and size of a rectangle to be cropped from 424 the sink format. The size of the sink crop rectangle will also 425 be the size of the format of the subdev's source pad.</para> 426 427 <figure id="subdev-image-processing-scaling-multi-source"> 428 <title>Image processing in subdevs: scaling with multiple sources</title> 429 <mediaobject> 430 <imageobject> 431 <imagedata fileref="subdev-image-processing-scaling-multi-source.svg" 432 format="SVG" scale="200" /> 433 </imageobject> 434 </mediaobject> 435 </figure> 436 437 <para>In this example, the subdev is capable of first cropping, 438 then scaling and finally cropping for two source pads 439 individually from the resulting scaled image. The location of 440 the scaled image in the cropped image is ignored in sink compose 441 target. Both of the locations of the source crop rectangles 442 refer to the sink scaling rectangle, independently cropping an 443 area at location specified by the source crop rectangle from 444 it.</para> 445 446 <figure id="subdev-image-processing-full"> 447 <title>Image processing in subdevs: scaling and composition 448 with multiple sinks and sources</title> 449 <mediaobject> 450 <imageobject> 451 <imagedata fileref="subdev-image-processing-full.svg" 452 format="SVG" scale="200" /> 453 </imageobject> 454 </mediaobject> 455 </figure> 456 457 <para>The subdev driver supports two sink pads and two source 458 pads. The images from both of the sink pads are individually 459 cropped, then scaled and further composed on the composition 460 bounds rectangle. From that, two independent streams are cropped 461 and sent out of the subdev from the source pads.</para> 462 463 </section> 464 465 </section> 466 467 &sub-subdev-formats;