zephyr-cp: CIRCUITPY_USB_MSC is off, so storage USB drive control is missing

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Feature
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Tech-Stack
c
Bereich
backend, embedded-iot

Rechercherichtung

Start with ports/zephyr-cp/supervisor/usb.c and the USB MSC implementation in subsys/usb/device_next/class/usbd_msc_scsi.c; compare the needed lock and remount behavior with supervisor/shared/usb/usb_msc_flash.c. Check how _zephyr_disk_status() reports a disabled drive and how the build enables USB. Done means MSC is enabled for zephyr-cp, storage enable/disable and remount behavior work, and the block-device lock is released appropriately; Zephyr's missing read-only and eject support are noted limitations.

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Beschreibung

storage usb zephyr

Claude wrote this, with edits by @dhalbert. These are notes for whoever works on this later.

Problem

zephyr-cp presents CIRCUITPY over USB with its own MSC code in ports/zephyr-cp/supervisor/usb.c. Its build never defines CIRCUITPY_USB_MSC, so the USB parts of storage are compiled out. This causes several problems:

  • disable_usb_drive(), unsafe_disable_usb_drive(), and enable_usb_drive() always raise RuntimeError: Cannot change USB devices now.
  • remount() is allowed while the host has write access. It should raise "Cannot remount path when visible via USB." instead.
  • remount() clears the block device lock that USB took. zephyr-cp's locked[0] stays true, so USB never takes the lock again.
  • Nothing releases locked[], so the lock is not dropped on eject or unmount.

Seen on a Feather nRF52840 running zephyr-cp built from 314c0be79f.

What is needed
  • Define CIRCUITPY_USB_MSC for zephyr-cp when USB is enabled.
  • Implement LUN enabling and disabling: report CIRCUITPY not ready when storage_usb_enabled() is false.
  • Provide usb_msc_release_circuitpy_lock(), usb_msc_holds_lock(), and usb_msc_remount(), as supervisor/shared/usb/usb_msc_flash.c does (the first two are added in #11530).
Zephyr USB MSC support

Unfortunately, some features are missing from Zephyr USB MSC that we need. Checked in adafruit/zephyr at a4d81519a1, subsys/usb/device_next/class/usbd_msc_scsi.c:

  • LUN not ready: possible. There is no API for it. But TEST UNIT READY, READ, and WRITE all answer "medium not present" when the disk's status is not DISK_STATUS_OK (L425, L749, L803). zephyr-cp's _zephyr_disk_status() can return DISK_STATUS_NOMEDIA while the drive is disabled.
  • Read-only to the host: not supported. MODE SENSE always reports no write protection (L600). DISK_STATUS_WR_PROTECT is treated as medium not present. So when CIRCUITPY is writable by Python, as after remount("/", readonly=False) in boot.py, the drive disappears from the host instead of appearing read-only. This needs a Zephyr change.
  • Eject: no notification. START STOP UNIT is handled inside the class and does not call the disk driver (L610). zephyr-cp cannot release its lock when the host ejects the drive.
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