feat(core): make usb endpoints registration nicer

core/src/apps/webauthn/__init__.py

@@ -20,4 +20,5 @@ def boot() -> None:
     )
     import usb
 
+    if usb.ENABLE_IFACE_WEBAUTHN:
         loop.schedule(handle_reports(usb.iface_webauthn))

core/src/usb.py

@@ -1,19 +1,57 @@
 from storage.device import get_device_id
 from trezor import io, utils
 
+_iface_iter = iter(range(5))
+
+ENABLE_IFACE_DEBUG = __debug__
+# change to False to enable VCP, see below
+ENABLE_IFACE_WEBAUTHN = not utils.BITCOIN_ONLY
+
+# We only have 10 available USB endpoints on real HW, of which 2 are taken up by the USB descriptor.
+# iface_wire, iface_debug and iface_webauthn also consume 2 each, iface_vcp uses 3.
+# That is a grand total of 11. That means that we can't enable everything at the same time.
+# By default, iface_vcp is only enabled on bitcoin_only firmware, where iface_webauthn
+# is disabled. Implementation-wise, we check if any of the previous ifaces is disabled
+# in order to enable VCP.
+ENABLE_IFACE_VCP = __debug__ and (
+    utils.EMULATOR or not ENABLE_IFACE_DEBUG or not ENABLE_IFACE_WEBAUTHN
+)
+
 # interface used for trezor wire protocol
-iface_wire = io.WebUSB(iface_num=0, ep_in=0x81, ep_out=0x01)
+id_wire = next(_iface_iter)
+iface_wire = io.WebUSB(
+    iface_num=id_wire,
+    ep_in=0x81 + id_wire,
+    ep_out=0x01 + id_wire,
+)
 
-if __debug__:
+# XXXXXXXXXXXXXXXXXXX
+#
+# We want the following branches present only in their respective firmwares. To achieve
+# that, we are taking advantage of the upy compiler static optimization: when an
+# if-expression statically evaluates to False, the branch is excluded from the bytecode.
+# This works magically for the __debug__ builtin, and `utils.BITCOIN_ONLY` is replaced
+# by a literal True/False by us in the build step.
+#
+# Therefore, each of the following needs to include the respective static expression
+# so that it can be correctly excluded from the resulting build.
+
+if __debug__ and ENABLE_IFACE_DEBUG:
     # interface used for debug messages with trezor wire protocol
-    iface_debug = io.WebUSB(iface_num=1, ep_in=0x82, ep_out=0x02)
+    id_debug = next(_iface_iter)
+    iface_debug = io.WebUSB(
+        iface_num=id_debug,
+        ep_in=0x81 + id_debug,
+        ep_out=0x01 + id_debug,
+    )
 
-if not utils.BITCOIN_ONLY:
+if not utils.BITCOIN_ONLY and ENABLE_IFACE_WEBAUTHN:
     # interface used for FIDO/U2F and FIDO2/WebAuthn HID transport
+    id_webauthn = next(_iface_iter)
     iface_webauthn = io.HID(
-        iface_num=2 if __debug__ else 1,
-        ep_in=0x83 if __debug__ else 0x82,
-        ep_out=0x03 if __debug__ else 0x02,
+        iface_num=id_webauthn,
+        ep_in=0x81 + id_webauthn,
+        ep_out=0x01 + id_webauthn,
         # fmt: off
         report_desc=bytes([
             0x06, 0xd0, 0xf1,  # USAGE_PAGE (FIDO Alliance)
@@ -36,19 +74,16 @@ if not utils.BITCOIN_ONLY:
         # fmt: on
     )
 
-if __debug__:
-    # We cannot use this on real device simultaneously with the iface_webauthn
-    # interface, because we have only limited number of endpoints (10).
-    # We start this only for bitcoin-only firmware or for emulator.
-    ENABLE_VCP_IFACE = utils.EMULATOR or utils.BITCOIN_ONLY
-    if ENABLE_VCP_IFACE:
+if __debug__ and ENABLE_IFACE_VCP:
     # interface used for cdc/vcp console emulation (debug messages)
+    id_vcp = next(_iface_iter)
+    id_vcp_data = next(_iface_iter)
     iface_vcp = io.VCP(
-            iface_num=2 if utils.BITCOIN_ONLY else 3,
-            data_iface_num=3 if utils.BITCOIN_ONLY else 4,
-            ep_in=0x83 if utils.BITCOIN_ONLY else 0x84,
-            ep_out=0x03 if utils.BITCOIN_ONLY else 0x04,
-            ep_cmd=0x84 if utils.BITCOIN_ONLY else 0x85,
+        iface_num=id_vcp,
+        data_iface_num=id_vcp_data,
+        ep_in=0x81 + id_vcp,
+        ep_out=0x01 + id_vcp,
+        ep_cmd=0x81 + id_vcp_data,
     )
 
 bus = io.USB(
@@ -62,10 +97,9 @@ bus = io.USB(
     usb21_landing=False,
 )
 bus.add(iface_wire)
-if __debug__:
+if __debug__ and ENABLE_IFACE_DEBUG:
     bus.add(iface_debug)
-if not utils.BITCOIN_ONLY:
+if not utils.BITCOIN_ONLY and ENABLE_IFACE_WEBAUTHN:
     bus.add(iface_webauthn)
-if __debug__:
-    if ENABLE_VCP_IFACE:
+if __debug__ and ENABLE_IFACE_VCP:
     bus.add(iface_vcp)