Freeze of nrf52840 USB Dongle due to USB CDC-ACM data transfer

RE: Freeze of nrf52840 USB Dongle due to USB CDC-ACM data transfer

hmolesworth — Thu, 09 Apr 2020 21:45:40 GMT

I don't have an answer, but there does seem to be an inconsistency in that the first location is never used and the '\r' terminator doesn't terminate unless followed by another character which is then followed by inserting a 0 which could be past the end of the buffer as it is not checked:

This line pre-increments m_rx_buffer_pos before the call, so m_rx_buffer[0] is never populated
+ &m_rx_buffer[++m_rx_buffer_pos], READ_SIZE);

This line looks at the previous character received, not the most recent
+ if(m_rx_buffer[m_rx_buffer_pos-1] == '\r')
+ {
// This line replaces the chracter after the most recent with 0
+ m_rx_buffer[++m_rx_buffer_pos] = '\0';

So .. termination is by '\r',[anything], otherwise the buffer will eventually overflow. m_rx_buffer[0] will never be set but will contain whatever anything else writing to this buffer last put there.

\r will only terminate immediately when hit if the terminal is configured to append - say - \n after every \r.

Using an 8-bit index to a 9-bit (512) element array will also not work as intended:

+static uint8_t m_rx_buffer_pos = 0;
+static char m_rx_buffer[512];

RE: Freeze of nrf52840 USB Dongle due to USB CDC-ACM data transfer

daten — Thu, 09 Apr 2020 21:19:25 GMT

I allow myself to push a bit for a solution. This is a hard blocker for us and currently halting rollout.

RE: Freeze of nrf52840 USB Dongle due to USB CDC-ACM data transfer

daten — Wed, 01 Apr 2020 12:12:44 GMT

After rethinking the issue, its potential causes and occurrences, I don't think increasing READ_SIZE would help much, if at all:

a) Looking at the log output I posted the freeze in this very case already occurred after 2 loop cycles, which really isn't a lot, so even if I heavily increase READ_SIZE I most likely will still have more than 1 loop cycle.

b) The freeze seems not to happen within the loop, but when / after leaving it. Potentially related to re-setting the buffer array index to 0.

RE: Freeze of nrf52840 USB Dongle due to USB CDC-ACM data transfer

daten — Wed, 01 Apr 2020 09:58:12 GMT

Hello and thanks for your reply. I could change the code accordingly (and adjust READ_SIZE to sth. like 8 I suppose), however that rather sounds like the issue occurring "less likely" due to less loop cycles, as in hiding it instead of solving the issue.

The freeze is a state I don't know how to recover from and I can't risk it happening - even if it's "less likely" due to reading >1byte per loop cycle.

So I'd really like to know what's happening and how to get rid of the freeze for good.

RE: Freeze of nrf52840 USB Dongle due to USB CDC-ACM data transfer

hmolesworth — Wed, 01 Apr 2020 03:48:21 GMT

I am having issues with something similar, and pushed it to one side. In this case the example only reads a singe byte at a time (what a strange example ..) and I see you modified that to attempt to read more, however this is now an issue:

ret = app_usbd_cdc_acm_read(&m_app_cdc_acm, &m_rx_buffer[++m_rx_buffer_pos], READ_SIZE);

I would suggest trying:

m_rx_buffer_pos += READ_SIZE;
ret = app_usbd_cdc_acm_read(&m_app_cdc_acm, &m_rx_buffer[m_rx_buffer_pos], READ_SIZE);

or:

ret = app_usbd_cdc_acm_read(&m_app_cdc_acm, &m_rx_buffer[m_rx_buffer_pos], READ_SIZE);
m_rx_buffer_pos += READ_SIZE;

Assuming each invocation reads READ_SIZE bytes from the USN buffer