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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 in- terface class is Diagnostic Class and the subclass is DbC.GP. The USB Debug Class is defined in the USB 3.1 Device Class Specifica- tion for Debug Devices. This is designed to provide a general-purpose communication channel for debugging. It has also been widely imple- mented in USB xHCs (USB eXtensible Host Controllers), which can be found on many commodity 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 con- nected. 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 realizes a single bidirectional ser- ial communication channel. On most systems, 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, offer- ing 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 straightforward way. A USB xHC DbC.GP device on a target system needs a special hardware configuration 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 De- vice. 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 cable 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 devices /dev/cuaU*.* for callout ports /dev/cuaU*.*.init /dev/cuaU*.*.lock corresponding callout initial-state and lock-state devices 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/default/files/documents/usb_debug_class_rev_1_0_final_0.pdf. USB 3.1 Legacy Cable and Connector Specification, https://www.usb.org/document-library/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 15.0 September 3, 2025 UDBC(4)
NAME | SYNOPSIS | DESCRIPTION | HARDWARE CONFIGURATION | FILES | SEE ALSO | STANDARDS | HISTORY | AUTHORS | BUGS
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