usbd: Refactor descriptor generation

- Use a single static buffer for descriptor results.
- Wrap the descriptor functionality in a Descriptor class.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

Author: projectgus

micropython/usbd/cdc.py

@@ -62,6 +62,8 @@
 # Other definitions
 _CDC_VERSION = const(0x0120)  # release number in binary-coded decimal
 
+_CDC_CONTROL_EP_NUM = const(1)
+_CDC_DATA_EP_NUM = const(2)
 
 # CDC data interface definitions
 _CDC_ITF_DATA_CLASS = const(0xA)
@@ -216,7 +218,9 @@ class CDCControlInterface(USBInterface):
     # Implements the CDC Control Interface
 
     def __init__(self):
-        super().__init__(_INTERFACE_CLASS_CDC, _INTERFACE_SUBCLASS_CDC, _PROTOCOL_NONE)
+        super().__init__(
+            _INTERFACE_CLASS_CDC, _INTERFACE_SUBCLASS_CDC, _PROTOCOL_NONE, _CDC_CONTROL_EP_NUM
+        )
 
         # Callbacks for particular changes initiated by the host
         self.break_cb = None  # Host sent a "break" condition
@@ -229,26 +233,27 @@ def __init__(self):
 
         self.ep_in = None  # Set when enumeration happens
 
-    def get_itf_descriptor(self, num_eps, itf_idx, str_idx):
+    def descriptor_config_cb(self, desc, itf_num, str_num, ep_num):
         # CDC needs a Interface Association Descriptor (IAD)
         # two interfaces in total
-        desc = ustruct.pack(
+        desc.pack(
             "<BBBBBBBB",
             8,
             _ITF_ASSOCIATION_DESC_TYPE,
-            itf_idx,
+            itf_num,
             2,
             _INTERFACE_CLASS_CDC,
             _INTERFACE_SUBCLASS_CDC,
             _PROTOCOL_NONE,
             0,
         )
 
-        itf, strs = super().get_itf_descriptor(num_eps, itf_idx, str_idx)
-        desc += itf
+        # Now add the standard interface descriptor
+        strs = super().descriptor_config_cb(desc, itf_num, str_num, ep_num)
+
         # Append the CDC class-specific interface descriptor
         # see CDC120-track, p20
-        desc += ustruct.pack(
+        desc.pack(
             "<BBBH",
             5,  # bFunctionLength
             _CS_DESC_TYPE,  # bDescriptorType
@@ -258,7 +263,7 @@ def get_itf_descriptor(self, num_eps, itf_idx, str_idx):
 
         # CDC-PSTN table3 "Call Management"
         # set to No
-        desc += ustruct.pack(
+        desc.pack(
             "<BBBBB",
             5,  # bFunctionLength
             _CS_DESC_TYPE,  # bDescriptorType
@@ -269,29 +274,30 @@ def get_itf_descriptor(self, num_eps, itf_idx, str_idx):
 
         # CDC-PSTN table4 "Abstract Control"
         # set to support line_coding and send_break
-        desc += ustruct.pack(
+        desc.pack(
             "<BBBB",
             4,  # bFunctionLength
             _CS_DESC_TYPE,  # bDescriptorType
             _CDC_FUNC_DESC_ABSTRACT_CONTROL,  # bDescriptorSubtype
             0x6,
         )  # bmCapabilities D1, D2
+
         # CDC-PSTN "Union"
         # set control interface / data interface number
-        desc += ustruct.pack(
+        desc.pack(
             "<BBBBB",
             5,  # bFunctionLength
             _CS_DESC_TYPE,  # bDescriptorType
             _CDC_FUNC_DESC_UNION,  # bDescriptorSubtype
-            itf_idx,  # bControlInterface
-            itf_idx + 1,
+            itf_num,  # bControlInterface
+            itf_num + 1,  # data interface, assume this one will be assigned next
         )  # bSubordinateInterface0 (data class itf number)
-        return desc, strs
 
-    def get_endpoint_descriptors(self, ep_addr, str_idx):
-        self.ep_in = ep_addr | EP_IN_FLAG
-        desc = endpoint_descriptor(self.ep_in, "interrupt", 8, 16)
-        return (desc, [], (self.ep_in,))
+        # Descriptor for a single endpoint
+        self.ep_in = ep_num | EP_IN_FLAG
+        desc.endpoint(self.ep_in, "interrupt", 8, 16)
+
+        return strs
 
     def handle_interface_control_xfer(self, stage, request):
         # Handle class-specific interface control transfers
@@ -331,20 +337,26 @@ class CDCDataInterface(USBInterface):
     # Implements the CDC Data Interface
 
     def __init__(self):
-        super().__init__(_CDC_ITF_DATA_CLASS, _CDC_ITF_DATA_SUBCLASS, _CDC_ITF_DATA_PROT)
+        super().__init__(
+            _CDC_ITF_DATA_CLASS, _CDC_ITF_DATA_SUBCLASS, _CDC_ITF_DATA_PROT, _CDC_DATA_EP_NUM
+        )
         self._wb = ()  # Optional write Buffer (IN endpoint), set by CDC.init()
         self._rb = ()  # Optional read Buffer (OUT endpoint), set by CDC.init()
         self._timeout = 1000  # set from CDC.init() as well
 
         self.ep_in = self.ep_out = None  # Set when enumeration happens
 
-    def get_endpoint_descriptors(self, ep_addr, str_idx):
-        self.ep_in = ep_addr | EP_IN_FLAG
-        self.ep_out = ep_addr
-        # one IN / OUT Endpoint
-        e_out = endpoint_descriptor(self.ep_out, "bulk", _BULK_EP_LEN, 0)
-        e_in = endpoint_descriptor(self.ep_in, "bulk", _BULK_EP_LEN, 0)
-        return (e_out + e_in, [], (self.ep_out, self.ep_in))
+    def descriptor_config_cb(self, pack_fn, itf_num, str_num, ep_num):
+        # Add the standard interface descriptor
+        strs = super().descriptor_config_cb(pack_fn, itf_num, str_num, ep_num)
+
+        # Two endpoints, bulk IN and OUT
+        self.ep_in = ep_num | EP_IN_FLAG
+        self.ep_out = ep_num
+        endpoint_descriptor(pack_fn, self.ep_out, "bulk", _BULK_EP_LEN, 0)
+        endpoint_descriptor(pack_fn, self.ep_in, "bulk", _BULK_EP_LEN, 0)
+
+        return strs
 
     def write(self, buf):
         # use a memoryview to track how much of 'buf' we've written so far