network.USBD_NCM arrived upstream in 1.29: deliver it as a costume, or record the exclusion

[bdbarnett]

MicroPython 1.29 adds network.USBD_NCM (Ethernet-over-USB as a USB device class; extmod/network_usbd_ncm.c, documented under network), off by default. usbif's costume roster is the org's statement of which USB device classes a board can wear, and it says nothing about NCM either way.

The pin-move checklist (micropython-pydevices#1) asks for one of two outcomes, and this issue is where it gets recorded:

• Deliver it: a costume that enables USBD_NCM alongside (or instead of) the existing classes, with the descriptor policy usbif's extension header already applies, proven on the S3 and the P4 the way the other classes were.
• Exclude it: a line in the roster saying NCM is upstream's own class, reachable by enabling it in a board's config without usbif, and why usbif does not wrap it.

Nothing is broken today; the class is simply unaccounted for. Found in the post-pin-move sweep, 2026-09-23.

[bdbarnett]

Still unaccounted for on 1.29. Nothing in the README, docs, src or examples mentions NCM.

One thing to weigh: usbif serves the configuration descriptor itself. #26 shows that machine.USBDevice's runtime interfaces never reach the host on a usbif image, and network.USBD_NCM very likely hits the same wall. So "reachable without usbif" would really mean "on an image built without usbif".

@bdbarnett: deliver it as a costume, or record the exclusion? I'd record the exclusion for now: one roster line saying NCM needs an image without usbif. Build the costume when someone actually wants Ethernet over USB.

[bdbarnett]

Brad's decision (2026-09-26): parked. The NCM (Ethernet over USB) costume waits until something in the program needs a wired-over-USB link. That's the trigger.