Documentation/media/uapi/v4l/dev-subdev.rst - linux - Git at Google

 .. -*- coding: utf-8; mode: rst -*-

 .. _subdev:

 ********************
 Sub-device Interface
 ********************

 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.

 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.

 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

 -  query, read and write sub-devices controls

 -  subscribe and unsubscribe to events and retrieve them

 -  negotiate image formats on individual pads

 Sub-device character device nodes, conventionally named
 ``/dev/v4l-subdev*``, use major number 81.


 Controls
 ========

 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.

 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.

 Applications can access those hidden controls through the sub-device
 node with the V4L2 control API described in :ref:`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.

 Depending on the driver, those controls might also be exposed through
 one (or several) V4L2 device nodes.


 Events
 ======

 V4L2 sub-devices can notify applications of events as described in
 :ref:`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.


 .. _pad-level-formats:

 Pad-level Formats
 =================

 .. warning::

     Pad-level formats are only applicable to very complex devices that
     need to expose low-level format configuration to user space. Generic
     V4L2 applications do *not* need to use the API described in this
     section.

 .. note::

     For the purpose of this section, the term *format* means the
     combination of media bus data format, frame width and frame height.

 Image formats are typically negotiated on video capture and output
 devices using the format and
 :ref:`selection <VIDIOC_SUBDEV_G_SELECTION>` 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.

 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
 :ref:`pipeline-scaling`, where image scaling can be performed on both
 the video sensor and the host image processing hardware.


 .. _pipeline-scaling:

 .. kernel-figure:: pipeline.dot
     :alt:   pipeline.dot
     :align: center

     Image Format Negotiation on Pipelines

     High quality and high speed pipeline configuration


 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.

 Drivers that implement the :ref:`media API <media-controller-intro>`
 can expose pad-level image format configuration to applications. When
 they do, applications can use the
 :ref:`VIDIOC_SUBDEV_G_FMT <VIDIOC_SUBDEV_G_FMT>` and
 :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` ioctls. to
 negotiate formats on a per-pad basis.

 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
 :ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` time, and an ``EPIPE`` error
 code is then returned if the configuration is invalid.

 Pad-level image format configuration support can be tested by calling
 the :ref:`VIDIOC_SUBDEV_G_FMT` ioctl on pad
 0. If the driver returns an ``EINVAL`` error code pad-level format
 configuration is not supported by the sub-device.


 Format Negotiation
 ------------------

 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.

 Central to the format negotiation mechanism are the get/set format
 operations. When called with the ``which`` argument set to
 :ref:`V4L2_SUBDEV_FORMAT_TRY <VIDIOC_SUBDEV_G_FMT>`, the
 :ref:`VIDIOC_SUBDEV_G_FMT <VIDIOC_SUBDEV_G_FMT>` and
 :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_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).

 While not kept as part of the device state, try formats are stored in
 the sub-device file handles. A
 :ref:`VIDIOC_SUBDEV_G_FMT <VIDIOC_SUBDEV_G_FMT>` call will return
 the last try format set *on the same sub-device file handle*. Several
 applications querying the same sub-device at the same time will thus not
 interact with each other.

 To find out whether a particular format is supported by the device,
 applications use the
 :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` ioctl. Drivers
 verify and, if needed, change the requested ``format`` 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.

 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 :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` call as-is
 (as long as external parameters, such as formats on other pads or links'
 configuration are not changed).

 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:

 -  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.

 -  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.

 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.

 :ref:`sample-pipeline-config` shows a sample configuration sequence
 for the pipeline described in :ref:`pipeline-scaling` (table columns
 list entity names and pad numbers).


 .. raw:: latex

     \tiny

 .. tabularcolumns:: |p{2.0cm}|p{2.3cm}|p{2.3cm}|p{2.3cm}|p{2.3cm}|p{2.3cm}|p{2.3cm}|

 .. _sample-pipeline-config:

 .. flat-table:: Sample Pipeline Configuration
     :header-rows:  1
     :stub-columns: 0
     :widths: 5 5 5 5 5 5 5

     * -
       - Sensor/0 format
       - Frontend/0 format
       - Frontend/1 format
       - Scaler/0 format
       - Scaler/0 compose selection rectangle
       - Scaler/1 format
     * - Initial state
       - 2048x1536/SGRBG8_1X8
       - (default)
       - (default)
       - (default)
       - (default)
       - (default)
     * - Configure frontend sink format
       - 2048x1536/SGRBG8_1X8
       - *2048x1536/SGRBG8_1X8*
       - *2046x1534/SGRBG8_1X8*
       - (default)
       - (default)
       - (default)
     * - Configure scaler sink format
       - 2048x1536/SGRBG8_1X8
       - 2048x1536/SGRBG8_1X8
       - 2046x1534/SGRBG8_1X8
       - *2046x1534/SGRBG8_1X8*
       - *0,0/2046x1534*
       - *2046x1534/SGRBG8_1X8*
     * - Configure scaler sink compose selection
       - 2048x1536/SGRBG8_1X8
       - 2048x1536/SGRBG8_1X8
       - 2046x1534/SGRBG8_1X8
       - 2046x1534/SGRBG8_1X8
       - *0,0/1280x960*
       - *1280x960/SGRBG8_1X8*

 .. raw:: latex

     \normalsize

 1. Initial state. The sensor source pad format is set to its native 3MP
    size and V4L2_MBUS_FMT_SGRBG8_1X8 media bus code. Formats on the
    host frontend and scaler sink and source pads have the default
    values, as well as the compose rectangle on the scaler's sink pad.

 2. The application configures the frontend sink pad format's size to
    2048x1536 and its media bus code to V4L2_MBUS_FMT_SGRBG_1X8. The
    driver propagates the format to the frontend source pad.

 3. The application configures the scaler sink pad format's size to
    2046x1534 and the media bus code to V4L2_MBUS_FMT_SGRBG_1X8 to
    match the frontend source size and media bus code. The media bus code
    on the sink pad is set to V4L2_MBUS_FMT_SGRBG_1X8. The driver
    propagates the size to the compose selection rectangle on the
    scaler's sink pad, and the format to the scaler source pad.

 4. The application configures the size of the compose selection
    rectangle of the scaler's sink pad 1280x960. The driver propagates
    the size to the scaler's source pad format.

 When satisfied with the try results, applications can set the active
 formats by setting the ``which`` argument to
 ``V4L2_SUBDEV_FORMAT_ACTIVE``. 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.


 .. _v4l2-subdev-selections:

 Selections: cropping, scaling and composition
 ---------------------------------------------

 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.

 Crop settings are defined by a crop rectangle and represented in a
 struct :c:type:`v4l2_rect` by the coordinates of the top
 left corner and the rectangle size. Both the coordinates and sizes are
 expressed in pixels.

 As for pad formats, drivers store try and active rectangles for the
 selection targets :ref:`v4l2-selections-common`.

 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.

 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
 struct :c:type:`v4l2_rect`.

 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
 :ref:`VIDIOC_SUBDEV_S_SELECTION <VIDIOC_SUBDEV_G_SELECTION>` IOCTL
 using ``V4L2_SEL_TGT_COMPOSE`` 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.

 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.

 The drivers should always use the closest possible rectangle the user
 requests on all selection targets, unless specifically told otherwise.
 ``V4L2_SEL_FLAG_GE`` and ``V4L2_SEL_FLAG_LE`` flags may be used to round
 the image size either up or down. :ref:`v4l2-selection-flags`


 Types of selection targets
 --------------------------


 Actual targets
 ^^^^^^^^^^^^^^

 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.


 BOUNDS targets
 ^^^^^^^^^^^^^^

 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.


 Order of configuration and format propagation
 ---------------------------------------------

 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 ``V4L2_SEL_FLAG_KEEP_CONFIG`` 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.

 The coordinates to a step always refer to the actual size of the
 previous step. The exception to this rule is the sink compose
 rectangle, which refers to the sink compose bounds rectangle --- if it
 is supported by the hardware.

 1. 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.

 2. Sink pad actual crop selection. The sink pad crop defines the crop
    performed to the sink pad format.

 3. 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.

 4. Source pad actual crop selection. Crop on the source pad defines crop
    performed to the image in the sink compose bounds rectangle.

 5. 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.

 Accessing any of the above rectangles not supported by the subdev will
 return ``EINVAL``. 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.


 .. _subdev-image-processing-crop:

 .. kernel-figure:: subdev-image-processing-crop.svg
     :alt:   subdev-image-processing-crop.svg
     :align: center

     **Figure 4.5. Image processing in subdevs: simple crop example**

 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.


 .. _subdev-image-processing-scaling-multi-source:

 .. kernel-figure:: subdev-image-processing-scaling-multi-source.svg
     :alt:   subdev-image-processing-scaling-multi-source.svg
     :align: center

     **Figure 4.6. Image processing in subdevs: scaling with multiple sources**

 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.


 .. _subdev-image-processing-full:

 .. kernel-figure:: subdev-image-processing-full.svg
     :alt:    subdev-image-processing-full.svg
     :align:  center

     **Figure 4.7. Image processing in subdevs: scaling and composition with multiple sinks and sources**

 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.


 .. toctree::
     :maxdepth: 1

     subdev-formats
	.. -- coding: utf-8; mode: rst --

	.. _subdev:

	********************
	Sub-device Interface
	********************

	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.

	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.

	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

	- query, read and write sub-devices controls

	- subscribe and unsubscribe to events and retrieve them

	- negotiate image formats on individual pads

	Sub-device character device nodes, conventionally named
	``/dev/v4l-subdev*``, use major number 81.


	Controls
	========

	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.

	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.

	Applications can access those hidden controls through the sub-device
	node with the V4L2 control API described in :ref:`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.

	Depending on the driver, those controls might also be exposed through
	one (or several) V4L2 device nodes.


	Events
	======

	V4L2 sub-devices can notify applications of events as described in
	:ref:`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.


	.. _pad-level-formats:

	Pad-level Formats
	=================

	.. warning::

	Pad-level formats are only applicable to very complex devices that
	need to expose low-level format configuration to user space. Generic
	V4L2 applications do not need to use the API described in this
	section.

	.. note::

	For the purpose of this section, the term format means the
	combination of media bus data format, frame width and frame height.

	Image formats are typically negotiated on video capture and output
	devices using the format and
	:ref:`selection <VIDIOC_SUBDEV_G_SELECTION>` 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.

	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
	:ref:`pipeline-scaling`, where image scaling can be performed on both
	the video sensor and the host image processing hardware.


	.. _pipeline-scaling:

	.. kernel-figure:: pipeline.dot
	:alt: pipeline.dot
	:align: center

	Image Format Negotiation on Pipelines

	High quality and high speed pipeline configuration



	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.

	Drivers that implement the :ref:`media API <media-controller-intro>`
	can expose pad-level image format configuration to applications. When
	they do, applications can use the
	:ref:`VIDIOC_SUBDEV_G_FMT <VIDIOC_SUBDEV_G_FMT>` and
	:ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` ioctls. to
	negotiate formats on a per-pad basis.

	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
	:ref:`VIDIOC_STREAMON <VIDIOC_STREAMON>` time, and an ``EPIPE`` error
	code is then returned if the configuration is invalid.

	Pad-level image format configuration support can be tested by calling
	the :ref:`VIDIOC_SUBDEV_G_FMT` ioctl on pad
	0. If the driver returns an ``EINVAL`` error code pad-level format
	configuration is not supported by the sub-device.


	Format Negotiation
	------------------

	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.

	Central to the format negotiation mechanism are the get/set format
	operations. When called with the ``which`` argument set to
	:ref:`V4L2_SUBDEV_FORMAT_TRY <VIDIOC_SUBDEV_G_FMT>`, the
	:ref:`VIDIOC_SUBDEV_G_FMT <VIDIOC_SUBDEV_G_FMT>` and
	:ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_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).

	While not kept as part of the device state, try formats are stored in
	the sub-device file handles. A
	:ref:`VIDIOC_SUBDEV_G_FMT <VIDIOC_SUBDEV_G_FMT>` call will return
	the last try format set on the same sub-device file handle. Several
	applications querying the same sub-device at the same time will thus not
	interact with each other.

	To find out whether a particular format is supported by the device,
	applications use the
	:ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` ioctl. Drivers
	verify and, if needed, change the requested ``format`` 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.

	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 :ref:`VIDIOC_SUBDEV_S_FMT <VIDIOC_SUBDEV_G_FMT>` call as-is
	(as long as external parameters, such as formats on other pads or links'
	configuration are not changed).

	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:

	- 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.

	- 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.

	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.

	:ref:`sample-pipeline-config` shows a sample configuration sequence
	for the pipeline described in :ref:`pipeline-scaling` (table columns
	list entity names and pad numbers).


	.. raw:: latex

	\tiny

	.. tabularcolumns:: \|p{2.0cm}\|p{2.3cm}\|p{2.3cm}\|p{2.3cm}\|p{2.3cm}\|p{2.3cm}\|p{2.3cm}\|

	.. _sample-pipeline-config:

	.. flat-table:: Sample Pipeline Configuration
	:header-rows: 1
	:stub-columns: 0
	:widths: 5 5 5 5 5 5 5

	* -
	- Sensor/0 format
	- Frontend/0 format
	- Frontend/1 format
	- Scaler/0 format
	- Scaler/0 compose selection rectangle
	- Scaler/1 format
	* - Initial state
	- 2048x1536/SGRBG8_1X8
	- (default)
	- (default)
	- (default)
	- (default)
	- (default)
	* - Configure frontend sink format
	- 2048x1536/SGRBG8_1X8
	- 2048x1536/SGRBG8_1X8
	- 2046x1534/SGRBG8_1X8
	- (default)
	- (default)
	- (default)
	* - Configure scaler sink format
	- 2048x1536/SGRBG8_1X8
	- 2048x1536/SGRBG8_1X8
	- 2046x1534/SGRBG8_1X8
	- 2046x1534/SGRBG8_1X8
	- 0,0/2046x1534
	- 2046x1534/SGRBG8_1X8
	* - Configure scaler sink compose selection
	- 2048x1536/SGRBG8_1X8
	- 2048x1536/SGRBG8_1X8
	- 2046x1534/SGRBG8_1X8
	- 2046x1534/SGRBG8_1X8
	- 0,0/1280x960
	- 1280x960/SGRBG8_1X8

	.. raw:: latex

	\normalsize

	1. Initial state. The sensor source pad format is set to its native 3MP
	size and V4L2_MBUS_FMT_SGRBG8_1X8 media bus code. Formats on the
	host frontend and scaler sink and source pads have the default
	values, as well as the compose rectangle on the scaler's sink pad.

	2. The application configures the frontend sink pad format's size to
	2048x1536 and its media bus code to V4L2_MBUS_FMT_SGRBG_1X8. The
	driver propagates the format to the frontend source pad.

	3. The application configures the scaler sink pad format's size to
	2046x1534 and the media bus code to V4L2_MBUS_FMT_SGRBG_1X8 to
	match the frontend source size and media bus code. The media bus code
	on the sink pad is set to V4L2_MBUS_FMT_SGRBG_1X8. The driver
	propagates the size to the compose selection rectangle on the
	scaler's sink pad, and the format to the scaler source pad.

	4. The application configures the size of the compose selection
	rectangle of the scaler's sink pad 1280x960. The driver propagates
	the size to the scaler's source pad format.

	When satisfied with the try results, applications can set the active
	formats by setting the ``which`` argument to
	``V4L2_SUBDEV_FORMAT_ACTIVE``. 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.


	.. _v4l2-subdev-selections:

	Selections: cropping, scaling and composition
	---------------------------------------------

	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.

	Crop settings are defined by a crop rectangle and represented in a
	struct :c:type:`v4l2_rect` by the coordinates of the top
	left corner and the rectangle size. Both the coordinates and sizes are
	expressed in pixels.

	As for pad formats, drivers store try and active rectangles for the
	selection targets :ref:`v4l2-selections-common`.

	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.

	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
	struct :c:type:`v4l2_rect`.

	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
	:ref:`VIDIOC_SUBDEV_S_SELECTION <VIDIOC_SUBDEV_G_SELECTION>` IOCTL
	using ``V4L2_SEL_TGT_COMPOSE`` 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.

	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.

	The drivers should always use the closest possible rectangle the user
	requests on all selection targets, unless specifically told otherwise.
	``V4L2_SEL_FLAG_GE`` and ``V4L2_SEL_FLAG_LE`` flags may be used to round
	the image size either up or down. :ref:`v4l2-selection-flags`


	Types of selection targets
	--------------------------


	Actual targets
	^^^^^^^^^^^^^^

	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.


	BOUNDS targets
	^^^^^^^^^^^^^^

	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.


	Order of configuration and format propagation
	---------------------------------------------

	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 ``V4L2_SEL_FLAG_KEEP_CONFIG`` 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.

	The coordinates to a step always refer to the actual size of the
	previous step. The exception to this rule is the sink compose
	rectangle, which refers to the sink compose bounds rectangle --- if it
	is supported by the hardware.

	1. 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.

	2. Sink pad actual crop selection. The sink pad crop defines the crop
	performed to the sink pad format.

	3. 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.

	4. Source pad actual crop selection. Crop on the source pad defines crop
	performed to the image in the sink compose bounds rectangle.

	5. 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.

	Accessing any of the above rectangles not supported by the subdev will
	return ``EINVAL``. 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.


	.. _subdev-image-processing-crop:

	.. kernel-figure:: subdev-image-processing-crop.svg
	:alt: subdev-image-processing-crop.svg
	:align: center

	Figure 4.5. Image processing in subdevs: simple crop example

	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.


	.. _subdev-image-processing-scaling-multi-source:

	.. kernel-figure:: subdev-image-processing-scaling-multi-source.svg
	:alt: subdev-image-processing-scaling-multi-source.svg
	:align: center

	Figure 4.6. Image processing in subdevs: scaling with multiple sources

	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.


	.. _subdev-image-processing-full:

	.. kernel-figure:: subdev-image-processing-full.svg
	:alt: subdev-image-processing-full.svg
	:align: center

	Figure 4.7. Image processing in subdevs: scaling and composition with multiple sinks and sources

	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.


	.. toctree::
	:maxdepth: 1

	subdev-formats