Start decoding descriptor in Go, rather than relying on libusb helper

functions. Descriptor format is defined explicitly by the USB spec.

descriptors.go

@@ -0,0 +1,107 @@
+// Copyright 2017 the gousb Authors.  All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package gousb
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+)
+
+// Descriptor structs based on USB 2.0 spec, section 9.6
+
+const (
+	deviceDescLen = 18
+	configDescLen = 9
+	intfDescLen   = 9
+	epDescLen     = 7
+)
+
+// decoded device descriptor, 18 bytes
+type usbDeviceDescriptor struct {
+	BLength            uint8  // 0
+	BDescriptorType    uint8  // 1
+	BCDUSB             uint16 // 2:3
+	BDeviceClass       uint8  // 4
+	BDeviceSubClass    uint8  // 5
+	BDeviceProtocol    uint8  // 6
+	BMaxPacketSize0    uint8  // 7
+	IDVendor           uint16 // 8:9
+	IDProduct          uint16 // 10:11
+	BCDDevice          uint16 // 12:13
+	IManufacturer      uint8  // 14
+	IProduct           uint8  // 15
+	ISerialNumber      uint8  // 16
+	BNumConfigurations uint8  // 17
+}
+
+func deviceDescFromBytes(descBytes []byte) (*DeviceDesc, error) {
+	if len(descBytes) < deviceDescLen {
+		return nil, fmt.Errorf("device descriptor is %d bytes, got only %d bytes", descBytes, len(descBytes))
+	}
+	d := &usbDeviceDescriptor{}
+	b := bytes.NewReader(descBytes[:deviceDescLen])
+	err := binary.Read(b, binary.LittleEndian, d)
+	if err != nil {
+		return nil, fmt.Errorf("failed to build the device descriptor: %v", err)
+	}
+	dev := &DeviceDesc{
+		Spec:                 BCD(d.BCDUSB),
+		Class:                Class(d.BDeviceClass),
+		SubClass:             Class(d.BDeviceSubClass),
+		Protocol:             Protocol(d.BDeviceProtocol),
+		MaxControlPacketSize: int(d.BMaxPacketSize0),
+		Vendor:               ID(d.IDVendor),
+		Product:              ID(d.IDProduct),
+		Device:               BCD(d.BCDDevice),
+		Configs:              make(map[int]ConfigDesc, d.BNumConfigurations),
+	}
+	return dev, nil
+}
+
+// decoded configuration descriptor, 9 bytes followed by bNumInterfaces interface descriptors
+type usbConfigDescriptor struct {
+	bLength             uint8  // 0
+	bDescriptorType     uint8  // 1
+	wTotalLength        uint16 // 2:3
+	bNumInterfaces      uint8  // 4
+	bConfigurationValue uint8  // 5
+	iConfiguration      uint8  // 6
+	bmAttributes        uint8  // 7
+	bMaxPower           uint8  // 8
+}
+
+// decoded interface descriptor, 9 bytes followed by bNumEndpoints endpoint descriptors.
+type usbInterfaceDescriptor struct {
+	bLength            uint8 // 0
+	bDescriptorType    uint8 // 1
+	bInterfaceNumber   uint8 // 2
+	bAlternateSetting  uint8 // 3
+	bNumEndpoints      uint8 // 4
+	bInterfaceClass    uint8 // 5
+	bInterfaceSubClass uint8 // 6
+	bInterfaceProtocol uint8 // 7
+	iInterface         uint8 // 8
+}
+
+// decoded endpoint descriptor, 7 bytes.
+type usbEndpointDescriptor struct {
+	bLength          uint8  // 0
+	bDescriptorType  uint8  // 1
+	bEndpointAddress uint8  // 2
+	bmAttributes     uint8  // 3
+	wMaxPacketSize   uint16 // 4:5
+	bInterval        uint8  // 6
+}