Documentation/DocBook/v4l/dev-subdev.xml

   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_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
 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;