[Bug] Computing time intervals with millis

[dpgeorge]

Describe the bug
When computing time intervals care must be taken with wrap around. Eg:

static inline void board_delay(uint32_t ms)
{
  uint32_t start_ms = board_millis();
  while( board_millis() < start_ms + ms) {}
}

If start_ms is close to 0xffffffff such that adding ms wraps it around then the function will most likely return straightaway.

[hathach]

thanks again, PR #73 should fix this. Feel free to re-open & comment if you think it doesn't

[dpgeorge]

A simple way to fix it, due to 2's complement arithmetic overflow which is well-defined behaviour (at least in C99), is like this:

static inline void board_delay(uint32_t ms)
{
  uint32_t start_ms = board_millis();
  while( board_millis() - start_ms < ms) {}
}

Also, there are many places in the code that use this pattern, eg in examples/device/cdc_msc_hid/src/main.c:hid_task():

if ( board_millis() < start_ms + interval_ms) return;

[hathach]

Ah thank you, that is very neat 😄 . That proves how amazing Math is 🔢