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 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 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> 91 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> 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> 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> 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> 187 188 <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> 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> 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> 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_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> 277 278 <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> 311 </section> 312 313 &sub-subdev-formats; 314
