feat(usb): add CDC/ACM suspend/resume check

[dakejahl]

Summary

Adds ntfc usb-suspend, a host-side check that a NuttX USB CDC/ACM link survives Linux USB runtime suspend. Moved here from apache/nuttx#19936 at @xiaoxiang781216's request; that PR now carries the driver fix alone.

Problem

A USB device driver that reports CLASS_SUSPEND but never CLASS_RESUME leaves cdcacm_suspend()'s uart_connected(&priv->serdev, false) latched, after which serial.c refuses every board-side open() and write() on the CDC port with -ENOTCONN for the rest of the boot. The device stays enumerated throughout and the host is perfectly happy, so nothing about the symptom points at suspend.

Linux hosts reach that state unprompted — with power/control=auto and the usual autosuspend_delay_ms=2000, closing the tty is enough — which is why this reads as an intermittent USB wedge rather than a deterministic bug, and why it needs a check that forces the transition instead of waiting for it.

Solution

Each cycle re-arms runtime PM and waits for runtime_status to actually reach suspended before judging anything: a resumed device will not idle out again on its own until PM is toggled, so a naive loop measures nothing after the first cycle. Opening the port is what resumes the device.

Bytes are counted both over the read window and over its trailing second. The tail count is what separates a working link from one that only flushed its stale CDC TX buffer on resume — on STM32H7 that stale buffer is a convincing 12 kB. With --console on a second port, the board is also asked directly whether its CDC port is writable again.

Exit codes: 0 recovered from every suspend, 1 did not, 2 the host never suspended so nothing was exercised.

Needs a Linux host, sudo for two sysfs power attributes, a closed port, and a board that transmits unprompted. Takes no --confpath.

Sample output against an ARK FMU v6X (STM32H743, CONFIG_STM32H7_OTGFS=y, CONFIG_CDCACM=y), before the driver fix:

[1] suspended=True   read=12179   tail=0       board-side open failed: -ENOTCONN
[2] suspended=True   read=0       tail=0       board-side open failed: -ENOTCONN

FAIL: the link died after suspend and did not come back.

and after:

[1] suspended=True   read=40898   tail=23316   board-side open ok
[2] suspended=True   read=42441   tail=22020   board-side open ok

PASS: the link recovered from every suspend.

tox -e lint,flake8,type,py passes, coverage stays at 100%.

[raiden00pl]

this code doesnt belong to this repo. Test cases are in https://github.com/apache/nuttx-ntfc-testing

[dakejahl]

Agreed, thanks — reopened against nuttx-ntfc-testing as apache/nuttx-ntfc-testing#4, reworked into a pytest case rather than an ntfc subcommand. Closing this one.