usb-device: Keep the built-in driver's MaxPower value if not set.

Previously, if unset this value reverted to 100mA.

Now also respects the units provided in the argument.

This work was funded through GitHub Sponsors.

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

micropython/usb/usb-device/usb/device/impl.py

@@ -99,7 +99,7 @@ class _Device:
         device_subclass=None,
         device_protocol=None,
         config_str=None,
-        max_power_ma=50,
+        max_power_ma=None,
     ):
         # Configure the USB device with a set of interfaces, and optionally reconfiguring the
         # device and configuration descriptor fields
@@ -198,6 +198,13 @@ class _Device:
             iConfiguration = len(strs)
             strs.append(configuration_str)
 
+        if max_power_ma is None:
+            # Default to whatever value the builtin driver reports
+            max_power_ma = builtin.desc_cfg[8]
+        else:
+            # Otherwise, convert from mA to the units used in the descriptor
+            max_power_ma //= 2
+
         desc.pack_into(
             "<BBHBBBBB",
             0,