[usb_gadget p00] Classes for encoding USB descriptors.

tools/usb_gadget/descriptor.py

Index: tools/usb_gadget/descriptor.py
diff --git a/tools/usb_gadget/descriptor.py b/tools/usb_gadget/descriptor.py
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+"""USB descriptor generation utilities.

[not at google - send to devlin, 2014/07/23 00:05:44]

copyright

[Reilly Grant (use Gerrit), 2014/07/23 00:43:54]

Done.

+
+Classes to represent and generate USB descriptors.
+"""

[Ken Rockot(use gerrit already), 2014/07/23 00:03:48]

Maybe this module should be usb_descriptors?

[Reilly Grant (use Gerrit), 2014/07/23 00:43:54]

Done.

+
+import struct
+
+import usb_const

[Ken Rockot(use gerrit already), 2014/07/23 00:03:47]

How about "usb_constants" for the module name?

[Reilly Grant (use Gerrit), 2014/07/23 00:43:54]

Done.

+
+
+class Field(object):
+

[Ken Rockot(use gerrit already), 2014/07/23 00:03:47]

A bit of documentation?

[Reilly Grant (use Gerrit), 2014/07/23 00:43:54]

Done.

+  def __init__(self, name, str_fmt, struct_fmt, required):
+    self.name = name
+    self.str_fmt = str_fmt
+    self.struct_fmt = struct_fmt
+    self.required = required
+
+  def Format(self, value):
+    return self.str_fmt.format(value)
+
+
+class Descriptor(object):
+  """Base class for USB descriptor types.
+
+  This class provides general functionality for creating object types that
+  represent USB descriptors. The AddField and related methods are used to
+  define the fields of each structure. Fields can then be set using keyword
+  arguments to the object constructor or by accessing properties on the object.
+  """
+
+  _fields = None
+
+  @classmethod
+  def AddField(cls, name, struct_fmt, str_fmt='{}', default=None):
+    """Adds a user-specified field to this descriptor.
+
+    Adds a field to the binary structure representing this descriptor. The field
+    can be set by passing a keyword argument name=... to the object constructor
+    will be accessible as foo.name on any instance.
+
+    If no default value is provided then the constructor will through an
+    exception if this field is not one of the provided keyword arguments.
+
+    Args:
+      name: String name of the field.
+      struct_fmt: Python 'struct' module format string for this field.
+      str_fmt: Python 'string' module format string for this field.
+      default: Default value.
+    """
+    if cls._fields is None:
+      cls._fields = []
+    cls._fields.append(Field(name, str_fmt, struct_fmt, default is None))
+
+    member_name = '_{}'.format(name)
+    def Setter(self, value):
+      setattr(self, member_name, value)
+    def Getter(self):
+      try:
+        return getattr(self, member_name)
+      except AttributeError:
+        assert default is not None
+        return default
+
+    setattr(cls, name, property(Getter, Setter))
+
+  @classmethod
+  def AddFixedField(cls, name, struct_fmt, value, str_fmt='{}'):
+    """Adds a constant field to this descriptor.
+
+    Adds a constant field to the binary structure representing this descriptor.
+    The field will be accessible as foo.name on any instance.
+
+    The value of this field may not be given as a constructor parameter or
+    set on an existing instance.
+
+    Args:
+      name: String name of the field.
+      struct_fmt: Python 'struct' module format string for this field.
+      value: Field value.
+      str_fmt: Python 'string' module format string for this field.
+    """
+    if cls._fields is None:
+      cls._fields = []
+    cls._fields.append(Field(name, str_fmt, struct_fmt, False))
+
+    def Setter(unused_self, unused_value):
+      raise RuntimeError('{} is a fixed field.'.format(name))
+    def Getter(unused_self):
+      return value
+
+    setattr(cls, name, property(Getter, Setter))
+
+  @classmethod
+  def AddComputedField(cls, name, struct_fmt, property_name, str_fmt='{}'):
+    """Adds a constant field to this descriptor.
+
+    Adds a field to the binary structure representing this descriptor whos value
+    is equal to an object property. The field will be accessible as foo.name on
+    any instance.
+
+    The value of this field may not be given as a constructor parameter or
+    set on an existing instance.
+
+    Args:
+      name: String name of the field.
+      struct_fmt: Python 'struct' module format string for this field.
+      property_name: Property to read.
+      str_fmt: Python 'string' module format string for this field.
+    """
+    if cls._fields is None:
+      cls._fields = []
+    cls._fields.append(Field(name, str_fmt, struct_fmt, False))
+
+    def Setter(unused_self, unused_value):
+      raise RuntimeError('{} is a computed field.'.format(name))
+    def Getter(self):
+      return getattr(self, property_name)
+
+    setattr(cls, name, property(Getter, Setter))
+
+  def __init__(self, **kwargs):
+    """Constructs a new instance of this descriptor.
+
+    All fields which do not have a default value and are not fixed or computed
+    from a property must be specified as keyword arguments.
+
+    Args:
+      **kwargs: Field values.
+
+    Raises:
+      TypeError: A required field was missing or an unexpected field was given.
+    """
+    fields = {field.name for field in self._fields}
+    required_fields = {field.name for field in self._fields if field.required}
+
+    for arg, value in kwargs.iteritems():
+      if arg not in fields:
+        raise TypeError('Unexpected field: {}'.format(arg))
+
+      setattr(self, arg, value)
+      required_fields.discard(arg)
+
+    if required_fields:
+      raise TypeError('Missing fields: {}'.format(', '.join(required_fields)))
+
+  @property
+  def fmt(self):
+    """Returns the Python 'struct' module format string for this descriptor."""
+    return '<{}'.format(''.join([field.struct_fmt for field in self._fields]))
+
+  @property
+  def struct_size(self):
+    """Returns the size of the struct defined by fmt."""
+    return struct.calcsize(self.fmt)
+
+  @property
+  def total_size(self):
+    """Returns the total size of this descriptor."""
+    return self.struct_size
+
+  def Encode(self):
+    """Returns the binary representation of this descriptor."""
+    values = [getattr(self, field.name) for field in self._fields]
+    return struct.pack(self.fmt, *values)
+
+  def __str__(self):
+    max_length = max(len(field.name) for field in self._fields)
+
+    return '{}:\n  {}'.format(
+        self.__class__.__name__,
+        '\n  '.join('{} {}'.format(
+            '{}:'.format(field.name).ljust(max_length+1),
+            field.Format(getattr(self, field.name))
+        ) for field in self._fields)
+    )
+
+
+class DeviceDescriptor(Descriptor):
+  """Represents a USB device descriptor."""
+
+  pass
+
+DeviceDescriptor.AddComputedField('bLength', 'B', 'struct_size')

[Ken Rockot(use gerrit already), 2014/07/23 00:03:47]

Might as well link to / reference any relevant section(s) of the USB spec from
usb.org here. Here and other places where you build descriptor types.

[Reilly Grant (use Gerrit), 2014/07/23 00:43:54]

Done.

+DeviceDescriptor.AddFixedField('bDescriptorType', 'B',
+                               usb_const.DescriptorType.DEVICE)
+DeviceDescriptor.AddField('bcdUSB', 'H', default=0x0200, str_fmt='0x{:04X}')
+DeviceDescriptor.AddField('bDeviceClass', 'B',
+                          default=usb_const.DeviceClass.PER_INTERFACE)
+DeviceDescriptor.AddField('bDeviceSubClass', 'B',
+                          default=usb_const.DeviceSubClass.PER_INTERFACE)
+DeviceDescriptor.AddField('bDeviceProtocol', 'B',
+                          default=usb_const.DeviceProtocol.PER_INTERFACE)
+DeviceDescriptor.AddField('bMaxPacketSize0', 'B', default=64)
+DeviceDescriptor.AddField('idVendor', 'H', str_fmt='0x{:04X}')
+DeviceDescriptor.AddField('idProduct', 'H', str_fmt='0x{:04X}')
+DeviceDescriptor.AddField('bcdDevice', 'H', str_fmt='0x{:04X}')
+DeviceDescriptor.AddField('iManufacturer', 'B', default=0)
+DeviceDescriptor.AddField('iProduct', 'B', default=0)
+DeviceDescriptor.AddField('iSerialNumber', 'B', default=0)
+DeviceDescriptor.AddField('bNumConfigurations', 'B', default=1)
+
+
+class DescriptorContainer(Descriptor):

[Ken Rockot(use gerrit already), 2014/07/23 00:03:48]

If this is established nomenclature in USB land, I'm fine with it; otherwise I
think I'd want to call this "CompositeDescriptor." What do you think?

[Reilly Grant (use Gerrit), 2014/07/23 00:43:54]

"CompositeDescriptor" may confusing because "USB Composite Device" means a
device with multiple interfaces each implementing a different protocol.

+  """Super-class for descriptors which contain more descriptors.
+
+  This class adds the ability for a descriptor to have an array of additional
+  descriptors which follow it.
+  """
+
+  def __init__(self, **kwargs):
+    super(DescriptorContainer, self).__init__(**kwargs)
+    self._descriptors = []
+
+  @property
+  def total_size(self):
+    return self.struct_size + sum([descriptor.total_size
+                                   for descriptor in self._descriptors])
+
+  def Add(self, descriptor):
+    self._descriptors.append(descriptor)
+
+  def Encode(self):
+    bufs = [super(DescriptorContainer, self).Encode()]
+    bufs.extend(descriptor.Encode() for descriptor in self._descriptors)
+    return ''.join(bufs)
+
+  def __str__(self):
+    return '{}\n{}'.format(super(DescriptorContainer, self).__str__(),
+                           '\n'.join(str(descriptor)
+                                     for descriptor in self._descriptors))
+
+
+class ConfigurationDescriptor(DescriptorContainer):
+  """Represents a USB device configuration descriptor.
+
+  Configuration descriptors may have a number of interface descriptors.
+  """
+
+  def __init__(self, **kwargs):
+    super(ConfigurationDescriptor, self).__init__(**kwargs)
+    self._interfaces = {}
+
+  @property
+  def num_interfaces(self):
+    interface_numbers = {key[0] for key in self._interfaces.iterkeys()}
+    return len(interface_numbers)
+
+  def AddInterface(self, interface):
+    key = (interface.bInterfaceNumber, interface.bAlternateSetting)
+    if key in self._interfaces:
+      raise RuntimeError('Interface {} (alternate {}) already defined.'
+                         .format(key[0], key[1]))
+    self._interfaces[key] = interface
+    self.Add(interface)
+
+  def GetInterfaces(self):
+    return self._interfaces.values()
+
+ConfigurationDescriptor.AddComputedField('bLength', 'B', 'struct_size')
+ConfigurationDescriptor.AddFixedField('bDescriptorType', 'B',
+                                      usb_const.DescriptorType.CONFIGURATION)
+ConfigurationDescriptor.AddComputedField('wTotalLength', 'H', 'total_size')
+ConfigurationDescriptor.AddComputedField('bNumInterfaces', 'B',
+                                         'num_interfaces')
+ConfigurationDescriptor.AddField('bConfigurationValue', 'B', default=1)
+ConfigurationDescriptor.AddField('iConfiguration', 'B', default=0)
+ConfigurationDescriptor.AddField('bmAttributes', 'B', str_fmt='0x{:02X}')
+ConfigurationDescriptor.AddField('MaxPower', 'B')
+
+
+class InterfaceDescriptor(DescriptorContainer):
+  """Represents a USB interface descriptor.
+
+  Interface descriptors may have a number of endpoint descriptors.
+  """
+
+  def __init__(self, **kwargs):
+    super(InterfaceDescriptor, self).__init__(**kwargs)
+    self._endpoints = {}
+
+  @property
+  def num_endpoints(self):
+    return len(self._endpoints)
+
+  def AddEndpoint(self, endpoint):
+    if endpoint.bEndpointAddress in self._endpoints:
+      raise RuntimeError('Endpoint 0x{:02X} already defined on this interface.'
+                         .format(endpoint.bEndpointAddress))
+    self._endpoints[endpoint.bEndpointAddress] = endpoint
+    self.Add(endpoint)
+
+  def GetEndpoints(self):
+    return self._endpoints.values()
+
+InterfaceDescriptor.AddComputedField('bLength', 'B', 'struct_size')
+InterfaceDescriptor.AddFixedField('bDescriptorType', 'B',
+                                  usb_const.DescriptorType.INTERFACE)
+InterfaceDescriptor.AddField('bInterfaceNumber', 'B')
+InterfaceDescriptor.AddField('bAlternateSetting', 'B', default=0)
+InterfaceDescriptor.AddComputedField('bNumEndpoints', 'B', 'num_endpoints')
+InterfaceDescriptor.AddField('bInterfaceClass', 'B',
+                             default=usb_const.InterfaceClass.VENDOR)
+InterfaceDescriptor.AddField('bInterfaceSubClass', 'B',
+                             default=usb_const.InterfaceSubClass.VENDOR)
+InterfaceDescriptor.AddField('bInterfaceProtocol', 'B',
+                             default=usb_const.InterfaceProtocol.VENDOR)
+InterfaceDescriptor.AddField('iInterface', 'B', default=0)
+
+
+class EndpointDescriptor(Descriptor):
+  """Represents a USB endpoint descriptor."""
+
+  pass
+
+EndpointDescriptor.AddComputedField('bLength', 'B', 'struct_size')
+EndpointDescriptor.AddFixedField('bDescriptorType', 'B',
+                                 usb_const.DescriptorType.ENDPOINT)
+EndpointDescriptor.AddField('bEndpointAddress', 'B', str_fmt='0x{:02X}')
+EndpointDescriptor.AddField('bmAttributes', 'B', str_fmt='0x{:02X}')
+EndpointDescriptor.AddField('wMaxPacketSize', 'H')
+EndpointDescriptor.AddField('bInterval', 'B')
+
+
+class HidDescriptor(Descriptor):
+  """Represents a USB HID descriptor."""
+
+  def __init__(self, **kwargs):
+    super(HidDescriptor, self).__init__(**kwargs)
+    self._descriptors = []
+
+  def AddDescriptor(self, typ, length):
+    self._descriptors.append((typ, length))
+
+  @property
+  def struct_size(self):
+    return super(HidDescriptor, self).struct_size + self.num_descriptors * 3
+
+  @property
+  def num_descriptors(self):
+    return len(self._descriptors)
+
+  def Encode(self):
+    bufs = [super(HidDescriptor, self).Encode()]
+    bufs.extend(struct.pack('<BH', typ, length)
+                for typ, length in self._descriptors)
+    return ''.join(bufs)
+
+  def __str__(self):
+    return '{}\n{}'.format(
+        super(HidDescriptor, self).__str__(),
+        '\n'.join('  bDescriptorType: 0x{:02X}\n  wDescriptorLength: {}'
+                  .format(typ, length) for typ, length in self._descriptors))
+
+HidDescriptor.AddComputedField('bLength', 'B', 'struct_size')
+HidDescriptor.AddFixedField('bDescriptorType', 'B', 33)
+HidDescriptor.AddField('bcdHID', 'H', default=0x0111, str_fmt='0x{:04X}')
+HidDescriptor.AddField('bCountryCode', 'B', default=0)
+HidDescriptor.AddComputedField('bNumDescriptors', 'B', 'num_descriptors')