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diff --git a/Documentation/usb/gadget_uvc.rst b/Documentation/usb/gadget_uvc.rst new file mode 100644 index 000000000000..6d22faceb1a0 --- /dev/null +++ b/Documentation/usb/gadget_uvc.rst @@ -0,0 +1,352 @@ +======================= +Linux UVC Gadget Driver +======================= + +Overview +-------- +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 + +Assumptions +~~~~~~~~~~~ +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: + +.. code-block:: bash + + # These variables will be assumed throughout the rest of the document + CONFIGFS="/sys/kernel/config" + GADGET="$CONFIGFS/usb_gadget/g1" + FUNCTION="$GADGET/functions/uvc.0" + + 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: + +:: + + uvc.0 + + | + + streaming + + | + + mjpeg + + | | + | + mjpeg + + | | + | + 720p + | | + | + 1080p + | + + uncompressed + + | + + yuyv + + | + + 720p + | + + 1080p + +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: + +.. code-block:: bash + + create_frame() { + # Example usage: + # create_frame <width> <height> <group> <format name> + + WIDTH=$1 + HEIGHT=$2 + FORMAT=$3 + NAME=$4 + + wdir=$FUNCTION/streaming/$FORMAT/$NAME/${HEIGHT}p + + mkdir -p $wdir + echo $WIDTH > $wdir/wWidth + echo $HEIGHT > $wdir/wHeight + echo $(( $WIDTH * $HEIGHT * 2 )) > $wdir/dwMaxVideoFrameBufferSize + cat <<EOF > $wdir/dwFrameInterval + 666666 + 100000 + 5000000 + EOF + } + + 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 +Documentation/userspace-api/media/v4l/pixfmt-packed.yuv.rst. + +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: + +.. code-block:: bash + + # Create a new Color Matching Descriptor + + mkdir $FUNCTION/streaming/color_matching/yuyv + pushd $FUNCTION/streaming/color_matching/yuyv + + echo 1 > bColorPrimaries + echo 1 > bTransferCharacteristics + echo 4 > bMatrixCoefficients + + popd + + # 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. + + +Header linking +~~~~~~~~~~~~~~ + +The UVC specification requires that Format and Frame descriptors be preceded by +Headers detailing things such as the number and cumulative size of the different +Format descriptors that follow. This and similar operations are acheived in +configfs by linking between the configfs item representing the header and the +config items representing those other descriptors, in this manner: + +.. code-block:: bash + + mkdir $FUNCTION/streaming/header/h + + # This section links the format descriptors and their associated frames + # to the header + cd $FUNCTION/streaming/header/h + ln -s ../../uncompressed/yuyv + ln -s ../../mjpeg/mjpeg + + # This section ensures that the header will be transmitted for each + # speed's set of descriptors. If support for a particular speed is not + # needed then it can be skipped here. + cd ../../class/fs + ln -s ../../header/h + cd ../../class/hs + ln -s ../../header/h + cd ../../class/ss + ln -s ../../header/h + cd ../../../control + mkdir header/h + ln -s header/h class/fs + ln -s header/h class/ss + + +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: + +.. code-block:: bash + + mkdir $FUNCTION/control/extensions/xu.0 + pushd $FUNCTION/control/extensions/xu.0 + + # 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 + + popd + +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: + +.. code-block:: bash + + cat << EOF > bmControls + 0x03 + 0x03 + EOF + +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 + 0x03 + 0x05 + + $ cat bControlSize + 2 + + $ echo 1 > bControlSize + $ cat bmControls + 0x03 + + $ echo 2 > bControlSize + $ cat bmControls + 0x03 + 0x00 + +bNrInPins and baSourceID function in the same way. + +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: + +.. code-block:: bash + + # 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: + +.. code-block:: bash + + echo 1 > $FUNCTION/control/enable_interrupt_ep + +Bandwidth configuration +~~~~~~~~~~~~~~~~~~~~~~~ + +There are three attributes which control the bandwidth of the USB connection. +These live in the function root and can be set within limits: + +.. code-block:: bash + + # 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 |