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authorFathi Boudra <fathi.boudra@linaro.org>2013-04-28 09:33:08 +0300
committerFathi Boudra <fathi.boudra@linaro.org>2013-04-28 09:33:08 +0300
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+ <title>Sub-device Interface</title>
+
+ <note>
+ <title>Experimental</title>
+ <para>This is an <link linkend="experimental">experimental</link>
+ interface and may change in the future.</para>
+ </note>
+
+ <para>The complex nature of V4L2 devices, where hardware is often made of
+ several integrated circuits that need to interact with each other in a
+ controlled way, leads to complex V4L2 drivers. The drivers usually reflect
+ the hardware model in software, and model the different hardware components
+ as software blocks called sub-devices.</para>
+
+ <para>V4L2 sub-devices are usually kernel-only objects. If the V4L2 driver
+ implements the media device API, they will automatically inherit from media
+ entities. Applications will be able to enumerate the sub-devices and discover
+ the hardware topology using the media entities, pads and links enumeration
+ API.</para>
+
+ <para>In addition to make sub-devices discoverable, drivers can also choose
+ to make them directly configurable by applications. When both the sub-device
+ driver and the V4L2 device driver support this, sub-devices will feature a
+ character device node on which ioctls can be called to
+ <itemizedlist>
+ <listitem><para>query, read and write sub-devices controls</para></listitem>
+ <listitem><para>subscribe and unsubscribe to events and retrieve them</para></listitem>
+ <listitem><para>negotiate image formats on individual pads</para></listitem>
+ </itemizedlist>
+ </para>
+
+ <para>Sub-device character device nodes, conventionally named
+ <filename>/dev/v4l-subdev*</filename>, use major number 81.</para>
+
+ <section>
+ <title>Controls</title>
+ <para>Most V4L2 controls are implemented by sub-device hardware. Drivers
+ usually merge all controls and expose them through video device nodes.
+ Applications can control all sub-devices through a single interface.</para>
+
+ <para>Complex devices sometimes implement the same control in different
+ pieces of hardware. This situation is common in embedded platforms, where
+ both sensors and image processing hardware implement identical functions,
+ such as contrast adjustment, white balance or faulty pixels correction. As
+ the V4L2 controls API doesn't support several identical controls in a single
+ device, all but one of the identical controls are hidden.</para>
+
+ <para>Applications can access those hidden controls through the sub-device
+ node with the V4L2 control API described in <xref linkend="control" />. The
+ ioctls behave identically as when issued on V4L2 device nodes, with the
+ exception that they deal only with controls implemented in the sub-device.
+ </para>
+
+ <para>Depending on the driver, those controls might also be exposed through
+ one (or several) V4L2 device nodes.</para>
+ </section>
+
+ <section>
+ <title>Events</title>
+ <para>V4L2 sub-devices can notify applications of events as described in
+ <xref linkend="event" />. The API behaves identically as when used on V4L2
+ device nodes, with the exception that it only deals with events generated by
+ the sub-device. Depending on the driver, those events might also be reported
+ on one (or several) V4L2 device nodes.</para>
+ </section>
+
+ <section id="pad-level-formats">
+ <title>Pad-level Formats</title>
+
+ <warning><para>Pad-level formats are only applicable to very complex device that
+ need to expose low-level format configuration to user space. Generic V4L2
+ applications do <emphasis>not</emphasis> need to use the API described in
+ this section.</para></warning>
+
+ <note><para>For the purpose of this section, the term
+ <wordasword>format</wordasword> means the combination of media bus data
+ format, frame width and frame height.</para></note>
+
+ <para>Image formats are typically negotiated on video capture and
+ output devices using the format and <link
+ linkend="vidioc-subdev-g-selection">selection</link> ioctls. The
+ driver is responsible for configuring every block in the video
+ pipeline according to the requested format at the pipeline input
+ and/or output.</para>
+
+ <para>For complex devices, such as often found in embedded systems,
+ identical image sizes at the output of a pipeline can be achieved using
+ different hardware configurations. One such example is shown on
+ <xref linkend="pipeline-scaling" />, where
+ image scaling can be performed on both the video sensor and the host image
+ processing hardware.</para>
+
+ <figure id="pipeline-scaling">
+ <title>Image Format Negotiation on Pipelines</title>
+ <mediaobject>
+ <imageobject>
+ <imagedata fileref="pipeline.pdf" format="PS" />
+ </imageobject>
+ <imageobject>
+ <imagedata fileref="pipeline.png" format="PNG" />
+ </imageobject>
+ <textobject>
+ <phrase>High quality and high speed pipeline configuration</phrase>
+ </textobject>
+ </mediaobject>
+ </figure>
+
+ <para>The sensor scaler is usually of less quality than the host scaler, but
+ scaling on the sensor is required to achieve higher frame rates. Depending
+ on the use case (quality vs. speed), the pipeline must be configured
+ differently. Applications need to configure the formats at every point in
+ the pipeline explicitly.</para>
+
+ <para>Drivers that implement the <link linkend="media-controller-intro">media
+ API</link> can expose pad-level image format configuration to applications.
+ When they do, applications can use the &VIDIOC-SUBDEV-G-FMT; and
+ &VIDIOC-SUBDEV-S-FMT; ioctls. to negotiate formats on a per-pad basis.</para>
+
+ <para>Applications are responsible for configuring coherent parameters on
+ the whole pipeline and making sure that connected pads have compatible
+ formats. The pipeline is checked for formats mismatch at &VIDIOC-STREAMON;
+ time, and an &EPIPE; is then returned if the configuration is
+ invalid.</para>
+
+ <para>Pad-level image format configuration support can be tested by calling
+ the &VIDIOC-SUBDEV-G-FMT; ioctl on pad 0. If the driver returns an &EINVAL;
+ pad-level format configuration is not supported by the sub-device.</para>
+
+ <section>
+ <title>Format Negotiation</title>
+
+ <para>Acceptable formats on pads can (and usually do) depend on a number
+ of external parameters, such as formats on other pads, active links, or
+ even controls. Finding a combination of formats on all pads in a video
+ pipeline, acceptable to both application and driver, can't rely on formats
+ enumeration only. A format negotiation mechanism is required.</para>
+
+ <para>Central to the format negotiation mechanism are the get/set format
+ operations. When called with the <structfield>which</structfield> argument
+ set to <constant>V4L2_SUBDEV_FORMAT_TRY</constant>, the
+ &VIDIOC-SUBDEV-G-FMT; and &VIDIOC-SUBDEV-S-FMT; ioctls operate on a set of
+ formats parameters that are not connected to the hardware configuration.
+ Modifying those 'try' formats leaves the device state untouched (this
+ applies to both the software state stored in the driver and the hardware
+ state stored in the device itself).</para>
+
+ <para>While not kept as part of the device state, try formats are stored
+ in the sub-device file handles. A &VIDIOC-SUBDEV-G-FMT; call will return
+ the last try format set <emphasis>on the same sub-device file
+ handle</emphasis>. Several applications querying the same sub-device at
+ the same time will thus not interact with each other.</para>
+
+ <para>To find out whether a particular format is supported by the device,
+ applications use the &VIDIOC-SUBDEV-S-FMT; ioctl. Drivers verify and, if
+ needed, change the requested <structfield>format</structfield> based on
+ device requirements and return the possibly modified value. Applications
+ can then choose to try a different format or accept the returned value and
+ continue.</para>
+
+ <para>Formats returned by the driver during a negotiation iteration are
+ guaranteed to be supported by the device. In particular, drivers guarantee
+ that a returned format will not be further changed if passed to an
+ &VIDIOC-SUBDEV-S-FMT; call as-is (as long as external parameters, such as
+ formats on other pads or links' configuration are not changed).</para>
+
+ <para>Drivers automatically propagate formats inside sub-devices. When a
+ try or active format is set on a pad, corresponding formats on other pads
+ of the same sub-device can be modified by the driver. Drivers are free to
+ modify formats as required by the device. However, they should comply with
+ the following rules when possible:
+ <itemizedlist>
+ <listitem><para>Formats should be propagated from sink pads to source pads.
+ Modifying a format on a source pad should not modify the format on any
+ sink pad.</para></listitem>
+ <listitem><para>Sub-devices that scale frames using variable scaling factors
+ should reset the scale factors to default values when sink pads formats
+ are modified. If the 1:1 scaling ratio is supported, this means that
+ source pads formats should be reset to the sink pads formats.</para></listitem>
+ </itemizedlist>
+ </para>
+
+ <para>Formats are not propagated across links, as that would involve
+ propagating them from one sub-device file handle to another. Applications
+ must then take care to configure both ends of every link explicitly with
+ compatible formats. Identical formats on the two ends of a link are
+ guaranteed to be compatible. Drivers are free to accept different formats
+ matching device requirements as being compatible.</para>
+
+ <para><xref linkend="sample-pipeline-config" />
+ shows a sample configuration sequence for the pipeline described in
+ <xref linkend="pipeline-scaling" /> (table
+ columns list entity names and pad numbers).</para>
+
+ <table pgwide="0" frame="none" id="sample-pipeline-config">
+ <title>Sample Pipeline Configuration</title>
+ <tgroup cols="3">
+ <colspec colname="what"/>
+ <colspec colname="sensor-0" />
+ <colspec colname="frontend-0" />
+ <colspec colname="frontend-1" />
+ <colspec colname="scaler-0" />
+ <colspec colname="scaler-1" />
+ <thead>
+ <row>
+ <entry></entry>
+ <entry>Sensor/0</entry>
+ <entry>Frontend/0</entry>
+ <entry>Frontend/1</entry>
+ <entry>Scaler/0</entry>
+ <entry>Scaler/1</entry>
+ </row>
+ </thead>
+ <tbody valign="top">
+ <row>
+ <entry>Initial state</entry>
+ <entry>2048x1536</entry>
+ <entry>-</entry>
+ <entry>-</entry>
+ <entry>-</entry>
+ <entry>-</entry>
+ </row>
+ <row>
+ <entry>Configure frontend input</entry>
+ <entry>2048x1536</entry>
+ <entry><emphasis>2048x1536</emphasis></entry>
+ <entry><emphasis>2046x1534</emphasis></entry>
+ <entry>-</entry>
+ <entry>-</entry>
+ </row>
+ <row>
+ <entry>Configure scaler input</entry>
+ <entry>2048x1536</entry>
+ <entry>2048x1536</entry>
+ <entry>2046x1534</entry>
+ <entry><emphasis>2046x1534</emphasis></entry>
+ <entry><emphasis>2046x1534</emphasis></entry>
+ </row>
+ <row>
+ <entry>Configure scaler output</entry>
+ <entry>2048x1536</entry>
+ <entry>2048x1536</entry>
+ <entry>2046x1534</entry>
+ <entry>2046x1534</entry>
+ <entry><emphasis>1280x960</emphasis></entry>
+ </row>
+ </tbody>
+ </tgroup>
+ </table>
+
+ <para>
+ <orderedlist>
+ <listitem><para>Initial state. The sensor output is set to its native 3MP
+ resolution. Resolutions on the host frontend and scaler input and output
+ pads are undefined.</para></listitem>
+ <listitem><para>The application configures the frontend input pad resolution to
+ 2048x1536. The driver propagates the format to the frontend output pad.
+ Note that the propagated output format can be different, as in this case,
+ than the input format, as the hardware might need to crop pixels (for
+ instance when converting a Bayer filter pattern to RGB or YUV).</para></listitem>
+ <listitem><para>The application configures the scaler input pad resolution to
+ 2046x1534 to match the frontend output resolution. The driver propagates
+ the format to the scaler output pad.</para></listitem>
+ <listitem><para>The application configures the scaler output pad resolution to
+ 1280x960.</para></listitem>
+ </orderedlist>
+ </para>
+
+ <para>When satisfied with the try results, applications can set the active
+ formats by setting the <structfield>which</structfield> argument to
+ <constant>V4L2_SUBDEV_FORMAT_ACTIVE</constant>. Active formats are changed
+ exactly as try formats by drivers. To avoid modifying the hardware state
+ during format negotiation, applications should negotiate try formats first
+ and then modify the active settings using the try formats returned during
+ the last negotiation iteration. This guarantees that the active format
+ will be applied as-is by the driver without being modified.
+ </para>
+ </section>
+
+ <section id="v4l2-subdev-selections">
+ <title>Selections: cropping, scaling and composition</title>
+
+ <para>Many sub-devices support cropping frames on their input or output
+ pads (or possible even on both). Cropping is used to select the area of
+ interest in an image, typically on an image sensor or a video decoder. It can
+ also be used as part of digital zoom implementations to select the area of
+ the image that will be scaled up.</para>
+
+ <para>Crop settings are defined by a crop rectangle and represented in a
+ &v4l2-rect; by the coordinates of the top left corner and the rectangle
+ size. Both the coordinates and sizes are expressed in pixels.</para>
+
+ <para>As for pad formats, drivers store try and active
+ rectangles for the selection targets <xref
+ linkend="v4l2-selections-common" />.</para>
+
+ <para>On sink pads, cropping is applied relative to the
+ current pad format. The pad format represents the image size as
+ received by the sub-device from the previous block in the
+ pipeline, and the crop rectangle represents the sub-image that
+ will be transmitted further inside the sub-device for
+ processing.</para>
+
+ <para>The scaling operation changes the size of the image by
+ scaling it to new dimensions. The scaling ratio isn't specified
+ explicitly, but is implied from the original and scaled image
+ sizes. Both sizes are represented by &v4l2-rect;.</para>
+
+ <para>Scaling support is optional. When supported by a subdev,
+ the crop rectangle on the subdev's sink pad is scaled to the
+ size configured using the &VIDIOC-SUBDEV-S-SELECTION; IOCTL
+ using <constant>V4L2_SEL_TGT_COMPOSE</constant>
+ selection target on the same pad. If the subdev supports scaling
+ but not composing, the top and left values are not used and must
+ always be set to zero.</para>
+
+ <para>On source pads, cropping is similar to sink pads, with the
+ exception that the source size from which the cropping is
+ performed, is the COMPOSE rectangle on the sink pad. In both
+ sink and source pads, the crop rectangle must be entirely
+ contained inside the source image size for the crop
+ operation.</para>
+
+ <para>The drivers should always use the closest possible
+ rectangle the user requests on all selection targets, unless
+ specifically told otherwise.
+ <constant>V4L2_SEL_FLAG_GE</constant> and
+ <constant>V4L2_SEL_FLAG_LE</constant> flags may be
+ used to round the image size either up or down. <xref
+ linkend="v4l2-selection-flags" /></para>
+ </section>
+
+ <section>
+ <title>Types of selection targets</title>
+
+ <section>
+ <title>Actual targets</title>
+
+ <para>Actual targets (without a postfix) reflect the actual
+ hardware configuration at any point of time. There is a BOUNDS
+ target corresponding to every actual target.</para>
+ </section>
+
+ <section>
+ <title>BOUNDS targets</title>
+
+ <para>BOUNDS targets is the smallest rectangle that contains all
+ valid actual rectangles. It may not be possible to set the actual
+ rectangle as large as the BOUNDS rectangle, however. This may be
+ because e.g. a sensor's pixel array is not rectangular but
+ cross-shaped or round. The maximum size may also be smaller than the
+ BOUNDS rectangle.</para>
+ </section>
+
+ </section>
+
+ <section>
+ <title>Order of configuration and format propagation</title>
+
+ <para>Inside subdevs, the order of image processing steps will
+ always be from the sink pad towards the source pad. This is also
+ reflected in the order in which the configuration must be
+ performed by the user: the changes made will be propagated to
+ any subsequent stages. If this behaviour is not desired, the
+ user must set
+ <constant>V4L2_SEL_FLAG_KEEP_CONFIG</constant> flag. This
+ flag causes no propagation of the changes are allowed in any
+ circumstances. This may also cause the accessed rectangle to be
+ adjusted by the driver, depending on the properties of the
+ underlying hardware.</para>
+
+ <para>The coordinates to a step always refer to the actual size
+ of the previous step. The exception to this rule is the source
+ compose rectangle, which refers to the sink compose bounds
+ rectangle --- if it is supported by the hardware.</para>
+
+ <orderedlist>
+ <listitem><para>Sink pad format. The user configures the sink pad
+ format. This format defines the parameters of the image the
+ entity receives through the pad for further processing.</para></listitem>
+
+ <listitem><para>Sink pad actual crop selection. The sink pad crop
+ defines the crop performed to the sink pad format.</para></listitem>
+
+ <listitem><para>Sink pad actual compose selection. The size of the
+ sink pad compose rectangle defines the scaling ratio compared
+ to the size of the sink pad crop rectangle. The location of
+ the compose rectangle specifies the location of the actual
+ sink compose rectangle in the sink compose bounds
+ rectangle.</para></listitem>
+
+ <listitem><para>Source pad actual crop selection. Crop on the source
+ pad defines crop performed to the image in the sink compose
+ bounds rectangle.</para></listitem>
+
+ <listitem><para>Source pad format. The source pad format defines the
+ output pixel format of the subdev, as well as the other
+ parameters with the exception of the image width and height.
+ Width and height are defined by the size of the source pad
+ actual crop selection.</para></listitem>
+ </orderedlist>
+
+ <para>Accessing any of the above rectangles not supported by the
+ subdev will return <constant>EINVAL</constant>. Any rectangle
+ referring to a previous unsupported rectangle coordinates will
+ instead refer to the previous supported rectangle. For example,
+ if sink crop is not supported, the compose selection will refer
+ to the sink pad format dimensions instead.</para>
+
+ <figure id="subdev-image-processing-crop">
+ <title>Image processing in subdevs: simple crop example</title>
+ <mediaobject>
+ <imageobject>
+ <imagedata fileref="subdev-image-processing-crop.svg"
+ format="SVG" scale="200" />
+ </imageobject>
+ </mediaobject>
+ </figure>
+
+ <para>In the above example, the subdev supports cropping on its
+ sink pad. To configure it, the user sets the media bus format on
+ the subdev's sink pad. Now the actual crop rectangle can be set
+ on the sink pad --- the location and size of this rectangle
+ reflect the location and size of a rectangle to be cropped from
+ the sink format. The size of the sink crop rectangle will also
+ be the size of the format of the subdev's source pad.</para>
+
+ <figure id="subdev-image-processing-scaling-multi-source">
+ <title>Image processing in subdevs: scaling with multiple sources</title>
+ <mediaobject>
+ <imageobject>
+ <imagedata fileref="subdev-image-processing-scaling-multi-source.svg"
+ format="SVG" scale="200" />
+ </imageobject>
+ </mediaobject>
+ </figure>
+
+ <para>In this example, the subdev is capable of first cropping,
+ then scaling and finally cropping for two source pads
+ individually from the resulting scaled image. The location of
+ the scaled image in the cropped image is ignored in sink compose
+ target. Both of the locations of the source crop rectangles
+ refer to the sink scaling rectangle, independently cropping an
+ area at location specified by the source crop rectangle from
+ it.</para>
+
+ <figure id="subdev-image-processing-full">
+ <title>Image processing in subdevs: scaling and composition
+ with multiple sinks and sources</title>
+ <mediaobject>
+ <imageobject>
+ <imagedata fileref="subdev-image-processing-full.svg"
+ format="SVG" scale="200" />
+ </imageobject>
+ </mediaobject>
+ </figure>
+
+ <para>The subdev driver supports two sink pads and two source
+ pads. The images from both of the sink pads are individually
+ cropped, then scaled and further composed on the composition
+ bounds rectangle. From that, two independent streams are cropped
+ and sent out of the subdev from the source pads.</para>
+
+ </section>
+
+ </section>
+
+ &sub-subdev-formats;