UDBC(4) Kernel Interfaces Manual UDBC(4) NAME udbc -- USB Debug Class device driver SYNOPSIS device usb device ucom device udbc In rc.conf(5): kld_list="udbc" DESCRIPTION The udbc driver provides support for USB Debug Class devices whose inter- face class is Diagnostic Class and the subclass is DbC.GP. The USB Debug Class is defined in the USB 3.1 Device Class Specification for Debug Devices. This is designed to provide a general-purpose communi- cation channel for debugging. It has also been widely implemented in USB xHCs (USB eXtensible Host Controllers), which can be found on many commod- ity computers, as an optional feature. Once this feature is enabled on a USB xHC, one of the USB ports will behave as a USB Debug Class device, not a host port, when a USB debug cable is connected. The supported class in USB xHCs is typically DbC.GP, while the specification defines several types of Debug Class devices. The DbC.GP uses IN and OUT endpoint pairs and re- alizes a single bidirectional serial communication channel. On most sys- tems, including FreeBSD, the DbC.GP is seen as a simple serial device. Most systems with USB xHC can be configured to provide DbC.GP access. The udbc is a driver that connects to DbC.GP-supported devices, offering tty(4) devices to connect to them via the ucom(4) device driver. HARDWARE CONFIGURATION A native DbC.GP device can be attached using the udbc driver in a straight- forward way. A USB xHC DbC.GP device on a target system needs a special hardware config- uration because all of the ports are supposed to be a USB Host. There is one method to expose DbC.GP is to use a USB 3.1 A-to-A cable (section 5.5.2 in USB 3.1 Legacy Cable and Connector Specification). When this cable is connected to a USB 3.1 port on the target system, the DbC-enabled USB xHC automatically switches the port as a USB Device. The udbc driver can find a DbC.GP device on that port. Note that a USB xHC with USB 3.2 support (USB Type-C connectors) is not compatible with the USB 3.1 A-to-A cable. Connecting a USB 3.2 C-to-C ca- ble or A-to-C cable does not automatically work, either, because it needs role configuration of the port, which is not supported on FreeBSD yet. FILES /dev/ttyU*.* for callin ports /dev/ttyU*.*.init /dev/ttyU*.*.lock corresponding callin initial-state and lock-state de- vices /dev/cuaU*.* for callout ports /dev/cuaU*.*.init /dev/cuaU*.*.lock corresponding callout initial-state and lock-state de- vices SEE ALSO tty(4), ucom(4), usb(4), xhci(4) STANDARDS eXtensible Host Controller Interface for Universal Serial Bus (XHCI), https://www.intel.com/content/dam/www/public/us/en/documents/technical- specifications/extensible-host-controler-interface-usb-xhci.pdf. USB 3.1 Device Class Specification for Debug Devices, https://www.usb.org/sites/de- fault/files/documents/usb_debug_class_rev_1_0_final_0.pdf. USB 3.1 Legacy Cable and Connector Specification, https://www.usb.org/document-li- brary/usb-31-legacy-cable-and-connector-revision-10. HISTORY The udbc driver first appeared FreeBSD 15.0. AUTHORS The udbc driver was written by Hiroki Sato <hrs@FreeBSD.org>. BUGS According to the XHCI specification the host side of USB Debug should work with any USB 3.0 port, whether connected directly to a controller or with a hub in between. Testing on some controllers has encountered issues when using a hub rather than a directly connected port on the controller. FreeBSD 15.1 September 3, 2025 UDBC(4)
NAME | SYNOPSIS | DESCRIPTION | HARDWARE CONFIGURATION | FILES | SEE ALSO | STANDARDS | HISTORY | AUTHORS | BUGS
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