usb_gadget: Define a string descriptor type and use it.

This patch constructs string descriptors using the framework from the
usb_descriptors module instead of raw calls to struct.pack.

BUG=None

Review URL: https://codereview.chromium.org/1243143002

Cr-Commit-Position: refs/heads/master@{#339894}

2 files changed, 31 insertions, 5 deletions

diff --git a/tools/usb_gadget/gadget.py b/tools/usb_gadget/gadget.py
index abcf491..c290750 100644
--- a/tools/usb_gadget/gadget.py
+++ b/tools/usb_gadget/gadget.py
@@ -442,11 +442,9 @@ class Gadget(object):
     elif index not in self._strings[lang]:
       return None
     else:
-      string = self._strings[lang][index].encode('UTF-16LE')
-      header = struct.pack(
-          '<BB', 2 + len(string), usb_constants.DescriptorType.STRING)
-      buf = header + string
-      return buf[:length]
+      descriptor = usb_descriptors.StringDescriptor(
+          bString=self._strings[lang][index])
+      return descriptor.Encode()[:length]
 
   def GetMicrosoftOSDescriptorV1(self, recipient, value, index, length):
     """Handle a the Microsoft OS 1.0 Descriptor request from the host.
diff --git a/tools/usb_gadget/usb_descriptors.py b/tools/usb_gadget/usb_descriptors.py
index f5bf315..1ec5dbd 100644
--- a/tools/usb_gadget/usb_descriptors.py
+++ b/tools/usb_gadget/usb_descriptors.py
@@ -247,6 +247,34 @@ class DescriptorContainer(Descriptor):
                                      for descriptor in self._descriptors))
 
 
+class StringDescriptor(Descriptor):
+  """Standard String Descriptor.
+
+  See Universal Serial Bus Specification Revision 2.0 Table 9-16.
+  """
+
+  def __init__(self, **kwargs):
+    self.bString = kwargs.pop('bString', '')
+    super(StringDescriptor, self).__init__(**kwargs)
+
+  @property
+  def total_size(self):
+    return self.struct_size + len(self.bString.encode('UTF-16LE'))
+
+  def Encode(self):
+    return (
+        super(StringDescriptor, self).Encode() +
+        self.bString.encode('UTF-16LE'))
+
+  def __str__(self):
+    return '{}\n  bString:         "{}"'.format(
+        super(StringDescriptor, self).__str__(), self.bString)
+
+StringDescriptor.AddComputedField('bLength', 'B', 'total_size')
+StringDescriptor.AddFixedField(
+    'bDescriptorType', 'B', usb_constants.DescriptorType.STRING)
+
+
 class ConfigurationDescriptor(DescriptorContainer):
   """Standard Configuration Descriptor.