Linux UVC Gadget Driver¶
The UVC Gadget driver is a driver for hardware on the device side of a USB connection. It is intended to run on a Linux system that has USB device-side hardware such as boards with an OTG port.
On the device system, once the driver is bound it appears as a V4L2 device with the output capability.
On the host side (once connected via USB cable), a device running the UVC Gadget driver and controlled by an appropriate userspace program should appear as a UVC specification compliant camera, and function appropriately with any program designed to handle them. The userspace program running on the device system can queue image buffers from a variety of sources to be transmitted via the USB connection. Typically this would mean forwarding the buffers from a camera sensor peripheral, but the source of the buffer is entirely dependent on the userspace companion program.
Configuring the device kernel¶
The Kconfig options USB_CONFIGFS, USB_LIBCOMPOSITE, USB_CONFIGFS_F_UVC and USB_F_UVC must be selected to enable support for the UVC gadget.
Configuring the gadget through configfs¶
The UVC Gadget expects to be configured through configfs using the UVC function. This allows a significant degree of flexibility, as many of a UVC device's settings can be controlled this way.
Not all of the available attributes are described here. For a complete enumeration see Documentation/ABI/testing/configfs-usb-gadget-uvc
This section assumes that you have mounted configfs at /sys/kernel/config and created a gadget as /sys/kernel/config/usb_gadget/g1.
The UVC Function¶
The first step is to create the UVC function:
# These variables will be assumed throughout the rest of the document
mkdir -p $FUNCTION
Formats and Frames¶
You must configure the gadget by telling it which formats you support, as well as the frame sizes and frame intervals that are supported for each format. In the current implementation there is no way for the gadget to refuse to set a format that the host instructs it to set, so it is important that this step is completed accurately to ensure that the host never asks for a format that can't be provided.
Formats are created under the streaming/uncompressed and streaming/mjpeg configfs groups, with the framesizes created under the formats in the following structure:
+ streaming +
+ mjpeg +
| + mjpeg +
| + 720p
| + 1080p
+ uncompressed +
+ yuyv +
Each frame can then be configured with a width and height, plus the maximum buffer size required to store a single frame, and finally with the supported frame intervals for that format and framesize. Width and height are enumerated in units of pixels, frame interval in units of 100ns. To create the structure above with 2, 15 and 100 fps frameintervals for each framesize for example you might do:
# Example usage:
# create_frame <width> <height> <group> <format name>
mkdir -p $wdir
echo $WIDTH > $wdir/wWidth
echo $HEIGHT > $wdir/wHeight
echo $(( $WIDTH * $HEIGHT * 2 )) > $wdir/dwMaxVideoFrameBufferSize
cat <<EOF > $wdir/dwFrameInterval
create_frame 1280 720 mjpeg mjpeg
create_frame 1920 1080 mjpeg mjpeg
create_frame 1280 720 uncompressed yuyv
create_frame 1920 1080 uncompressed yuyv
The only uncompressed format currently supported is YUYV, which is detailed at Packed YUV formats.
Color Matching Descriptors¶
It's possible to specify some colometry information for each format you create. This step is optional, and default information will be included if this step is skipped; those default values follow those defined in the Color Matching Descriptor section of the UVC specification.
To create a Color Matching Descriptor, create a configfs item and set its three attributes to your desired settings and then link to it from the format you wish it to be associated with:
# Create a new Color Matching Descriptor
echo 1 > bColorPrimaries
echo 1 > bTransferCharacteristics
echo 4 > bMatrixCoefficients
# Create a symlink to the Color Matching Descriptor from the format's config item
ln -s $FUNCTION/streaming/color_matching/yuyv $FUNCTION/streaming/uncompressed/yuyv
For details about the valid values, consult the UVC specification. Note that a default color matching descriptor exists and is used by any format which does not have a link to a different Color Matching Descriptor. It's possible to change the attribute settings for the default descriptor, so bear in mind that if you do that you are altering the defaults for any format that does not link to a different one.
Extension Unit Support¶
A UVC Extension Unit (XU) basically provides a distinct unit to which control set and get requests can be addressed. The meaning of those control requests is entirely implementation dependent, but may be used to control settings outside of the UVC specification (for example enabling or disabling video effects). An XU can be inserted into the UVC unit chain or left free-hanging.
Configuring an extension unit involves creating an entry in the appropriate directory and setting its attributes appropriately, like so:
# Set the bUnitID of the Processing Unit as the source for this
# Extension Unit
echo 2 > baSourceID
# Set this XU as the source of the default output terminal. This inserts
# the XU into the UVC chain between the PU and OT such that the final
# chain is IT > PU > XU.0 > OT
cat bUnitID > ../../terminal/output/default/baSourceID
# Flag some controls as being available for use. The bmControl field is
# a bitmap with each bit denoting the availability of a particular
# control. For example to flag the 0th, 2nd and 3rd controls available:
echo 0x0d > bmControls
# Set the GUID; this is a vendor-specific code identifying the XU.
echo -e -n "\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f\x10" > guidExtensionCode
The bmControls attribute and the baSourceID attribute are multi-value attributes. This means that you may write multiple newline separated values to them. For example to flag the 1st, 2nd, 9th and 10th controls as being available you would need to write two values to bmControls, like so:
cat << EOF > bmControls
The multi-value nature of the baSourceID attribute belies the fact that XUs can be multiple-input, though note that this currently has no significant effect.
The bControlSize attribute reflects the size of the bmControls attribute, and similarly bNrInPins reflects the size of the baSourceID attributes. Both attributes are automatically increased / decreased as you set bmControls and baSourceID. It is also possible to manually increase or decrease bControlSize which has the effect of truncating entries to the new size, or padding entries out with 0x00, for example:
$ cat bmControls
$ cat bControlSize
$ echo 1 > bControlSize
$ cat bmControls
$ echo 2 > bControlSize
$ cat bmControls
bNrInPins and baSourceID function in the same way.
Configuring Supported Controls for Camera Terminal and Processing Unit¶
The Camera Terminal and Processing Units in the UVC chain also have bmControls attributes which function similarly to the same field in an Extension Unit. Unlike XUs however, the meaning of the bitflag for these units is defined in the UVC specification; you should consult the "Camera Terminal Descriptor" and "Processing Unit Descriptor" sections for an enumeration of the flags.
# Set the Processing Unit's bmControls, flagging Brightness, Contrast
# and Hue as available controls:
echo 0x05 > $FUNCTION/control/processing/default/bmControls
# Set the Camera Terminal's bmControls, flagging Focus Absolute and
# Focus Relative as available controls:
echo 0x60 > $FUNCTION/control/terminal/camera/default/bmControls
If you do not set these fields then by default the Auto-Exposure Mode control for the Camera Terminal and the Brightness control for the Processing Unit will be flagged as available; if they are not supported you should set the field to 0x00.
Note that the size of the bmControls field for a Camera Terminal or Processing Unit is fixed by the UVC specification, and so the bControlSize attribute is read-only here.
Custom Strings Support¶
String descriptors that provide a textual description for various parts of a USB device can be defined in the usual place within USB configfs, and may then be linked to from the UVC function root or from Extension Unit directories to assign those strings as descriptors:
# Create a string descriptor in us-EN and link to it from the function
# root. The name of the link is significant here, as it declares this
# descriptor to be intended for the Interface Association Descriptor.
# Other significant link names at function root are vs0_desc and vs1_desc
# For the VideoStreaming Interface 0/1 Descriptors.
mkdir -p $GADGET/strings/0x409/iad_desc
echo -n "Interface Associaton Descriptor" > $GADGET/strings/0x409/iad_desc/s
ln -s $GADGET/strings/0x409/iad_desc $FUNCTION/iad_desc
# Because the link to a String Descriptor from an Extension Unit clearly
# associates the two, the name of this link is not significant and may
# be set freely.
mkdir -p $GADGET/strings/0x409/xu.0
echo -n "A Very Useful Extension Unit" > $GADGET/strings/0x409/xu.0/s
ln -s $GADGET/strings/0x409/xu.0 $FUNCTION/control/extensions/xu.0
The interrupt endpoint¶
The VideoControl interface has an optional interrupt endpoint which is by default disabled. This is intended to support delayed response control set requests for UVC (which should respond through the interrupt endpoint rather than tying up endpoint 0). At present support for sending data through this endpoint is missing and so it is left disabled to avoid confusion. If you wish to enable it you can do so through the configfs attribute:
echo 1 > $FUNCTION/control/enable_interrupt_ep
There are three attributes which control the bandwidth of the USB connection. These live in the function root and can be set within limits:
# streaming_interval sets bInterval. Values range from 1..255
echo 1 > $FUNCTION/streaming_interval
# streaming_maxpacket sets wMaxPacketSize. Valid values are 1024/2048/3072
echo 3072 > $FUNCTION/streaming_maxpacket
# streaming_maxburst sets bMaxBurst. Valid values are 1..15
echo 1 > $FUNCTION/streaming_maxburst
The values passed here will be clamped to valid values according to the UVC specification (which depend on the speed of the USB connection). To understand how the settings influence bandwidth you should consult the UVC specifications, but a rule of thumb is that increasing the streaming_maxpacket setting will improve bandwidth (and thus the maximum possible framerate), whilst the same is true for streaming_maxburst provided the USB connection is running at SuperSpeed. Increasing streaming_interval will reduce bandwidth and framerate.
The userspace application¶
By itself, the UVC Gadget driver cannot do anything particularly interesting. It must be paired with a userspace program that responds to UVC control requests and fills buffers to be queued to the V4L2 device that the driver creates. How those things are achieved is implementation dependent and beyond the scope of this document, but a reference application can be found at https://gitlab.freedesktop.org/camera/uvc-gadget