Add Path to DeviceDesc (#101)

Populates information about device port path in the device descriptor.

device.go

@@ -24,11 +24,22 @@ import (
 
 // DeviceDesc is a representation of a USB device descriptor.
 type DeviceDesc struct {
-	// Bus information
-	Bus     int   // The bus on which the device was detected
-	Address int   // The address of the device on the bus
-	Speed   Speed // The negotiated operating speed for the device
-	Port    int   // The usb port on which the device was detected
+	// The bus on which the device was detected
+	Bus int
+	// The address of the device on the bus
+	Address int
+	// The negotiated operating speed for the device
+	Speed Speed
+	// The pyhsical port on the parent hub on which the device is connected.
+	// Ports are numbered from 1, excepting root hub devices which are always 0.
+	Port int
+	// Physical path of connected parent ports, starting at the root hub device.
+	// A path length of 0 represents a root hub device,
+	// a path length of 1 represents a device directly connected to a root hub,
+	// a path length of 2 or more are connected to intermediate hub devices.
+	// e.g. [1,2,3] represents a device connected to port 3 of a hub connected
+	// to port 2 of a hub connected to port 1 of a root hub.
+	Path []int
 
 	// Version information
 	Spec   BCD // USB Specification Release Number

libusb.go

@@ -224,14 +224,26 @@ func (libusbImpl) setDebug(c *libusbContext, lvl int) {
 }
 
 func (libusbImpl) getDeviceDesc(d *libusbDevice) (*DeviceDesc, error) {
-	var desc C.struct_libusb_device_descriptor
+	var (
+		desc     C.struct_libusb_device_descriptor
+		pathData [8]uint8
+		path     []int
+		port     int
+	)
 	if err := fromErrNo(C.libusb_get_device_descriptor((*C.libusb_device)(d), &desc)); err != nil {
 		return nil, err
 	}
+	pathLen := int(C.libusb_get_port_numbers((*C.libusb_device)(d), (*C.uint8_t)(&pathData[0]), 8))
+	for _, nPort := range pathData[:pathLen] {
+		port = int(nPort)
+		path = append(path, port)
+	}
+	// Defaults to port = 0, path = [] for root device
 	dev := &DeviceDesc{
 		Bus:                  int(C.libusb_get_bus_number((*C.libusb_device)(d))),
 		Address:              int(C.libusb_get_device_address((*C.libusb_device)(d))),
-		Port:                 int(C.libusb_get_port_number((*C.libusb_device)(d))),
+		Port:                 port,
+		Path:                 path,
 		Speed:                Speed(C.libusb_get_device_speed((*C.libusb_device)(d))),
 		Spec:                 BCD(desc.bcdUSB),
 		Device:               BCD(desc.bcdDevice),