REPL slow to connect on nRF52

[dhalbert]

@hathach I have noticed that the REPL is slow to appear in CircuitPython on nRF52, after connecting via a terminal program. I have seen this in Linux and after power-up or hard rest. This has been true for a while now. I will try to characterize this better but am opening this issue at least as a placeholder.

[hathach]

is it always slow or just start to be slow recently. I will try to narrow down the scope to analyze the issue.

[dhalbert]

@hathach I did a bisect and narrowed this down to:

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commit 5351a93
Author: hathach thach@tinyusb.org
Date: Mon Nov 23 13:39:14 2020 +0700

update tinyusb to fix cdc connection race

issue is fixed in hathach/tinyusb#557

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Before this commit, connecting to the REPL on a Feather nRF52840 responds immediately. Afterwards, it takes about five seconds after connecting to print the initial prompt. I looked in /var/log/syslog and nothing is printed during that time, so it seems all internal.

[hathach]

thanks @dhalbert I will look at this next week, there is also a relevant fix for dcd of nrf52 OUT transfer that could cause drop of packet recently hathach/tinyusb#733. I will also test if that also fix this as well.

[hathach]

@dhalbert I couldn't reproduce the issue with latest 0539b88, the REPL connected immediately, i am using minicom on ubuntu 20.04. How is your testing,

[dhalbert]

I am still seeing this on Linux with Ubuntu 20.04. I have an Intel-based motherboard. (I think yours may be AMD?) Seen on both a Feather Sense and a CPB. There is no code.py on the board, so it goes straight to the REPL.
Here's a Beagle trace:
nrf-repl-5-second-delay.tdc.zip
Steps:

1. Start trace
2. plug in board
3. wait a few seconds for enumeration to finish; confirmed by watching /var/log/syslog
4. connect via picocom (issue seen with minicom also)
5. it takes 5 seconds for Auto-reload is on. Simply save files over USB to run them or enter REPL to disable. prompt to appear. During and after that time, there is no extra output in /var/log/syslog.

[dhalbert]

I tried this on Windows also, with Tera Term, and it never actually seems to connect, which I haven't seen before. I could get a trace there too, if you want, but you have a Windows machine also.

[hathach]

Thanks @dhalbert I will pull out my windows machine to test with

update: I am able to reproduce the issue with another windows 10 intel laptop. Doing further work to examine and troubleshoot it. Also try the stock tinyusb cdc_msc exmaple, but it works normally. It is probably a race condition bug specifically to nrf52.