usb-device-midi: Fix midi class driver descriptors.

Missing custom USB audio class standard descriptor extensions, as required
by Windows usbaudio.sys (it seems).

Also fixed the wTotalLength field which was incorrect for both the old and
new versions of this code.

This work was funded through GitHub Sponsors.

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

micropython/usb/usb-device-midi/usb/device/midi.py

@@ -11,6 +11,13 @@ _INTERFACE_CLASS_AUDIO = const(0x01)
 _INTERFACE_SUBCLASS_AUDIO_CONTROL = const(0x01)
 _INTERFACE_SUBCLASS_AUDIO_MIDISTREAMING = const(0x03)
 
+# Audio subclass extends the standard endpoint descriptor
+# with two extra bytes
+_STD_DESC_AUDIO_ENDPOINT_LEN = const(9)
+_CLASS_DESC_ENDPOINT_LEN = const(5)
+
+_STD_DESC_ENDPOINT_TYPE = const(0x5)
+
 _JACK_TYPE_EMBEDDED = const(0x01)
 _JACK_TYPE_EXTERNAL = const(0x02)
 
@@ -192,8 +199,15 @@ class MIDIInterface(Interface):
             0x24,  # bDescriptorType CS_INTERFACE
             0x01,  # bDescriptorSubtype MS_HEADER
             0x0100,  # BcdADC
-            # wTotalLength: this descriptor, plus length of all Jack descriptors
-            7 + 4 * (_JACK_IN_DESC_LEN + _JACK_OUT_DESC_LEN),
+            # wTotalLength: of all class-specific descriptors
+            7
+            + 2
+            * (
+                _JACK_IN_DESC_LEN
+                + _JACK_OUT_DESC_LEN
+                + _STD_DESC_AUDIO_ENDPOINT_LEN
+                + _CLASS_DESC_ENDPOINT_LEN
+            ),
         )
 
         # The USB MIDI standard 1.0 allows modelling a baffling range of MIDI
@@ -222,26 +236,10 @@ class MIDIInterface(Interface):
         self.ep_in = ep_num | _EP_IN_FLAG
 
         # rx side, USB "in" endpoint and embedded MIDI IN Jacks
-        desc.endpoint(self.ep_in, "bulk", 64, 0)
-        desc.pack(
-            "BBBBB",
-            5,  # bLength
-            0x25,  # bDescriptorType CS_ENDPOINT
-            0x01,  # bDescriptorSubtype MS_GENERAL
-            1,  # bNumEmbMIDIJack
-            _EMB_OUT_JACK_ID,
-        )
+        _audio_endpoint(desc, self.ep_in, _EMB_OUT_JACK_ID)
 
         # tx side, USB "out" endpoint and embedded MIDI OUT jacks
-        desc.endpoint(self.ep_out, "bulk", 64, 0)
-        desc.pack(
-            "BBBBB",
-            5,  # bLength
-            0x25,  # bDescriptorType CS_ENDPOINT
-            0x01,  # bDescriptorSubtype MS_GENERAL
-            1,  # bNumEmbMIDIJack
-            _EMB_IN_JACK_ID,
-        )
+        _audio_endpoint(desc, self.ep_out, _EMB_IN_JACK_ID)
 
     def num_itfs(self):
         return 2
@@ -279,3 +277,30 @@ def _jack_out_desc(desc, bJackType, bJackID, bSourceId, bSourcePin):
         bSourcePin,  # baSourcePin(1)
         0x00,  # iJack, no string descriptor support yet
     )
+
+
+def _audio_endpoint(desc, bEndpointAddress, emb_jack_id):
+    # Append a standard USB endpoint descriptor and the USB class endpoint descriptor
+    # for this endpoint.
+    #
+    # Audio Class devices extend the standard endpoint descriptor with two extra bytes,
+    # so we can't easily call desc.endpoint() for the first part.
+    desc.pack(
+        # Standard USB endpoint descriptor (plus audio tweaks)
+        "<BBBBHBBB"
+        # Class endpoint descriptor
+        "BBBBB",
+        _STD_DESC_AUDIO_ENDPOINT_LEN,  # wLength
+        _STD_DESC_ENDPOINT_TYPE,  # bDescriptorType
+        bEndpointAddress,
+        2,  # bmAttributes, bulk
+        64,  # wMaxPacketSize
+        0,  # bInterval
+        0,  # bRefresh (unused)
+        0,  # bSynchInterval (unused)
+        _CLASS_DESC_ENDPOINT_LEN,  # bLength
+        0x25,  # bDescriptorType CS_ENDPOINT
+        0x01,  # bDescriptorSubtype MS_GENERAL
+        1,  # bNumEmbMIDIJack
+        emb_jack_id,  # BaAssocJackID(1)
+    )