353 строки
12 KiB
ReStructuredText
353 строки
12 KiB
ReStructuredText
=======================
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Linux UVC Gadget Driver
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=======================
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Overview
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--------
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The UVC Gadget driver is a driver for hardware on the *device* side of a USB
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connection. It is intended to run on a Linux system that has USB device-side
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hardware such as boards with an OTG port.
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On the device system, once the driver is bound it appears as a V4L2 device with
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the output capability.
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On the host side (once connected via USB cable), a device running the UVC Gadget
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driver *and controlled by an appropriate userspace program* should appear as a UVC
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specification compliant camera, and function appropriately with any program
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designed to handle them. The userspace program running on the device system can
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queue image buffers from a variety of sources to be transmitted via the USB
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connection. Typically this would mean forwarding the buffers from a camera sensor
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peripheral, but the source of the buffer is entirely dependent on the userspace
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companion program.
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Configuring the device kernel
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-----------------------------
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The Kconfig options USB_CONFIGFS, USB_LIBCOMPOSITE, USB_CONFIGFS_F_UVC and
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USB_F_UVC must be selected to enable support for the UVC gadget.
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Configuring the gadget through configfs
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---------------------------------------
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The UVC Gadget expects to be configured through configfs using the UVC function.
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This allows a significant degree of flexibility, as many of a UVC device's
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settings can be controlled this way.
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Not all of the available attributes are described here. For a complete enumeration
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see Documentation/ABI/testing/configfs-usb-gadget-uvc
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Assumptions
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~~~~~~~~~~~
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This section assumes that you have mounted configfs at `/sys/kernel/config` and
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created a gadget as `/sys/kernel/config/usb_gadget/g1`.
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The UVC Function
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~~~~~~~~~~~~~~~~
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The first step is to create the UVC function:
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.. code-block:: bash
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# These variables will be assumed throughout the rest of the document
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CONFIGFS="/sys/kernel/config"
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GADGET="$CONFIGFS/usb_gadget/g1"
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FUNCTION="$GADGET/functions/uvc.0"
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mkdir -p $FUNCTION
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Formats and Frames
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~~~~~~~~~~~~~~~~~~
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You must configure the gadget by telling it which formats you support, as well
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as the frame sizes and frame intervals that are supported for each format. In
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the current implementation there is no way for the gadget to refuse to set a
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format that the host instructs it to set, so it is important that this step is
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completed *accurately* to ensure that the host never asks for a format that
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can't be provided.
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Formats are created under the streaming/uncompressed and streaming/mjpeg configfs
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groups, with the framesizes created under the formats in the following
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structure:
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::
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uvc.0 +
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+ streaming +
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+ mjpeg +
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| + mjpeg +
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| + 720p
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+ uncompressed +
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+ yuyv +
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+ 720p
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+ 1080p
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Each frame can then be configured with a width and height, plus the maximum
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buffer size required to store a single frame, and finally with the supported
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frame intervals for that format and framesize. Width and height are enumerated in
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units of pixels, frame interval in units of 100ns. To create the structure
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above with 2, 15 and 100 fps frameintervals for each framesize for example you
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might do:
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.. code-block:: bash
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create_frame() {
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# Example usage:
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# create_frame <width> <height> <group> <format name>
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WIDTH=$1
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HEIGHT=$2
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FORMAT=$3
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NAME=$4
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wdir=$FUNCTION/streaming/$FORMAT/$NAME/${HEIGHT}p
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mkdir -p $wdir
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echo $WIDTH > $wdir/wWidth
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echo $HEIGHT > $wdir/wHeight
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echo $(( $WIDTH * $HEIGHT * 2 )) > $wdir/dwMaxVideoFrameBufferSize
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cat <<EOF > $wdir/dwFrameInterval
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666666
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100000
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5000000
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EOF
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}
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create_frame 1280 720 mjpeg mjpeg
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create_frame 1920 1080 mjpeg mjpeg
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create_frame 1280 720 uncompressed yuyv
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create_frame 1920 1080 uncompressed yuyv
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The only uncompressed format currently supported is YUYV, which is detailed at
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Documentation/userspace-api/media/v4l/pixfmt-packed.yuv.rst.
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Color Matching Descriptors
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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It's possible to specify some colometry information for each format you create.
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This step is optional, and default information will be included if this step is
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skipped; those default values follow those defined in the Color Matching Descriptor
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section of the UVC specification.
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To create a Color Matching Descriptor, create a configfs item and set its three
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attributes to your desired settings and then link to it from the format you wish
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it to be associated with:
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.. code-block:: bash
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# Create a new Color Matching Descriptor
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mkdir $FUNCTION/streaming/color_matching/yuyv
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pushd $FUNCTION/streaming/color_matching/yuyv
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echo 1 > bColorPrimaries
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echo 1 > bTransferCharacteristics
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echo 4 > bMatrixCoefficients
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popd
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# Create a symlink to the Color Matching Descriptor from the format's config item
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ln -s $FUNCTION/streaming/color_matching/yuyv $FUNCTION/streaming/uncompressed/yuyv
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For details about the valid values, consult the UVC specification. Note that a
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default color matching descriptor exists and is used by any format which does
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not have a link to a different Color Matching Descriptor. It's possible to
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change the attribute settings for the default descriptor, so bear in mind that if
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you do that you are altering the defaults for any format that does not link to
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a different one.
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Header linking
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~~~~~~~~~~~~~~
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The UVC specification requires that Format and Frame descriptors be preceded by
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Headers detailing things such as the number and cumulative size of the different
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Format descriptors that follow. This and similar operations are acheived in
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configfs by linking between the configfs item representing the header and the
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config items representing those other descriptors, in this manner:
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.. code-block:: bash
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mkdir $FUNCTION/streaming/header/h
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# This section links the format descriptors and their associated frames
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# to the header
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cd $FUNCTION/streaming/header/h
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ln -s ../../uncompressed/yuyv
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ln -s ../../mjpeg/mjpeg
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# This section ensures that the header will be transmitted for each
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# speed's set of descriptors. If support for a particular speed is not
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# needed then it can be skipped here.
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cd ../../class/fs
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ln -s ../../header/h
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cd ../../class/hs
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ln -s ../../header/h
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cd ../../class/ss
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ln -s ../../header/h
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cd ../../../control
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mkdir header/h
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ln -s header/h class/fs
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ln -s header/h class/ss
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Extension Unit Support
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~~~~~~~~~~~~~~~~~~~~~~
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A UVC Extension Unit (XU) basically provides a distinct unit to which control set
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and get requests can be addressed. The meaning of those control requests is
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entirely implementation dependent, but may be used to control settings outside
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of the UVC specification (for example enabling or disabling video effects). An
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XU can be inserted into the UVC unit chain or left free-hanging.
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Configuring an extension unit involves creating an entry in the appropriate
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directory and setting its attributes appropriately, like so:
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.. code-block:: bash
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mkdir $FUNCTION/control/extensions/xu.0
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pushd $FUNCTION/control/extensions/xu.0
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# Set the bUnitID of the Processing Unit as the source for this
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# Extension Unit
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echo 2 > baSourceID
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# Set this XU as the source of the default output terminal. This inserts
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# the XU into the UVC chain between the PU and OT such that the final
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# chain is IT > PU > XU.0 > OT
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cat bUnitID > ../../terminal/output/default/baSourceID
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# Flag some controls as being available for use. The bmControl field is
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# a bitmap with each bit denoting the availability of a particular
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# control. For example to flag the 0th, 2nd and 3rd controls available:
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echo 0x0d > bmControls
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# Set the GUID; this is a vendor-specific code identifying the XU.
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echo -e -n "\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f\x10" > guidExtensionCode
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popd
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The bmControls attribute and the baSourceID attribute are multi-value attributes.
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This means that you may write multiple newline separated values to them. For
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example to flag the 1st, 2nd, 9th and 10th controls as being available you would
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need to write two values to bmControls, like so:
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.. code-block:: bash
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cat << EOF > bmControls
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0x03
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0x03
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EOF
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The multi-value nature of the baSourceID attribute belies the fact that XUs can
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be multiple-input, though note that this currently has no significant effect.
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The bControlSize attribute reflects the size of the bmControls attribute, and
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similarly bNrInPins reflects the size of the baSourceID attributes. Both
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attributes are automatically increased / decreased as you set bmControls and
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baSourceID. It is also possible to manually increase or decrease bControlSize
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which has the effect of truncating entries to the new size, or padding entries
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out with 0x00, for example:
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::
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$ cat bmControls
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0x03
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0x05
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$ cat bControlSize
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2
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$ echo 1 > bControlSize
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$ cat bmControls
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0x03
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$ echo 2 > bControlSize
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$ cat bmControls
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0x03
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0x00
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bNrInPins and baSourceID function in the same way.
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Custom Strings Support
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~~~~~~~~~~~~~~~~~~~~~~
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String descriptors that provide a textual description for various parts of a
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USB device can be defined in the usual place within USB configfs, and may then
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be linked to from the UVC function root or from Extension Unit directories to
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assign those strings as descriptors:
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.. code-block:: bash
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# Create a string descriptor in us-EN and link to it from the function
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# root. The name of the link is significant here, as it declares this
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# descriptor to be intended for the Interface Association Descriptor.
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# Other significant link names at function root are vs0_desc and vs1_desc
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# For the VideoStreaming Interface 0/1 Descriptors.
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mkdir -p $GADGET/strings/0x409/iad_desc
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echo -n "Interface Associaton Descriptor" > $GADGET/strings/0x409/iad_desc/s
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ln -s $GADGET/strings/0x409/iad_desc $FUNCTION/iad_desc
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# Because the link to a String Descriptor from an Extension Unit clearly
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# associates the two, the name of this link is not significant and may
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# be set freely.
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mkdir -p $GADGET/strings/0x409/xu.0
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echo -n "A Very Useful Extension Unit" > $GADGET/strings/0x409/xu.0/s
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ln -s $GADGET/strings/0x409/xu.0 $FUNCTION/control/extensions/xu.0
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The interrupt endpoint
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~~~~~~~~~~~~~~~~~~~~~~
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The VideoControl interface has an optional interrupt endpoint which is by default
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disabled. This is intended to support delayed response control set requests for
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UVC (which should respond through the interrupt endpoint rather than tying up
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endpoint 0). At present support for sending data through this endpoint is missing
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and so it is left disabled to avoid confusion. If you wish to enable it you can
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do so through the configfs attribute:
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.. code-block:: bash
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echo 1 > $FUNCTION/control/enable_interrupt_ep
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Bandwidth configuration
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~~~~~~~~~~~~~~~~~~~~~~~
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There are three attributes which control the bandwidth of the USB connection.
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These live in the function root and can be set within limits:
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.. code-block:: bash
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# streaming_interval sets bInterval. Values range from 1..255
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echo 1 > $FUNCTION/streaming_interval
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# streaming_maxpacket sets wMaxPacketSize. Valid values are 1024/2048/3072
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echo 3072 > $FUNCTION/streaming_maxpacket
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# streaming_maxburst sets bMaxBurst. Valid values are 1..15
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echo 1 > $FUNCTION/streaming_maxburst
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The values passed here will be clamped to valid values according to the UVC
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specification (which depend on the speed of the USB connection). To understand
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how the settings influence bandwidth you should consult the UVC specifications,
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but a rule of thumb is that increasing the streaming_maxpacket setting will
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improve bandwidth (and thus the maximum possible framerate), whilst the same is
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true for streaming_maxburst provided the USB connection is running at SuperSpeed.
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Increasing streaming_interval will reduce bandwidth and framerate.
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The userspace application
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-------------------------
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By itself, the UVC Gadget driver cannot do anything particularly interesting. It
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must be paired with a userspace program that responds to UVC control requests and
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fills buffers to be queued to the V4L2 device that the driver creates. How those
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things are achieved is implementation dependent and beyond the scope of this
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document, but a reference application can be found at https://gitlab.freedesktop.org/camera/uvc-gadget
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