refactor(core): improve clarity in main.py

core/src/main.py

@@ -1,25 +1,42 @@
 # isort:skip_file
 # fmt: off
 
-# Import always-active modules
-import storage
-import storage.device
-if __debug__:
-    import storage.debug
+# We are tightly controlling the memory layout. Order of imports is important.
+# Modules imported directly from here also must take care to have as few dependencies
+# as possible.
 
+# === Import always-active modules
+# trezor imports only C modules
 import trezor
-import trezor.pin  # noqa: F401
+# trezor.utils import only C modules
 from trezor import utils
-
+# we need space for 30 items in the trezor module
 utils.presize_module("trezor", 30)
+
+# storage imports storage.common, storage.cache and storage.device.
+# These import trezor, trezor.config (which is a C module), trezor.utils, and each other.
+import storage
+# we will need space for 12 items in the storage module
 utils.presize_module("storage", 12)
 
 if not utils.BITCOIN_ONLY:
+    # storage.fido2 only imports C modules
     import storage.fido2  # noqa: F401
 
-# Prepare the USB interfaces first. Do not connect to the host yet.
+if __debug__:
+    # storage.debug only imports C modules
+    import storage.debug
+
+# trezor.pin imports trezor.utils
+# We need it as an always-active module because trezor.pin.show_pin_timeout is used
+# as a UI callback for storage, which can be invoked at any time
+import trezor.pin  # noqa: F401
+
+# === Prepare the USB interfaces first. Do not connect to the host yet.
+# usb imports trezor.utils and trezor.io which is a C module
 import usb
 
+# create an unimport manager that will be reused in the main loop
 unimport_manager = utils.unimport()
 
 # unlock the device, unload the boot module afterwards
@@ -28,8 +45,11 @@ with unimport_manager:
     del boot
 
 # start the USB
+import storage.device
+
 usb.bus.open(storage.device.get_device_id())
 
+# run the endless loop
 while True:
     with unimport_manager:
         import session  # noqa: F401

core/src/session.py

@@ -1,6 +1,5 @@
 from trezor import loop, utils, wire, workflow
 
-# load applications
 import apps.base
 import usb