5. USB Control Proxy Interface
The control proxy interface is identified by bInterfaceClass = 0xff and bInterfaceSubclass = 0x58 ('X').
Certain operating systems (e.g. Windows) disallows issuing control requests to an interface that’s already claimed for bulk transfer by another process. To allow e.g. configuring the USB Trace Interface while it’s already opened for capture, this interface is provided as a workaround.
5.1. Control Requests
Control requests are vendor-specific interface-directed, i.e. with bmRequestType = 0x41 or 0xc1
and the lower half of wIndex containing the bInterfaceNumber of this interface.
bRequest has a range for each supported target interface with an associated offset.
When a request is handled, the offset is subtracted from bRequest and the request is forwarded to the target interface’s handler.
bRequest range |
Offset |
Target interface |
|---|---|---|
0x01 - 0x0f |
0x00 |