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Added comment to wdog_start

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Add a comment briefly explaining why one timer tick is added to the watchdog delay.  This has confused a lot of people and has resulted in numerous awkward discussions and ill-informed attempts to modify the code to remove that addition.  Perhaps a rather lengthy comment will reduce that confusion and put an end to this thrashing.
This commit is contained in:
Gregory Nutt 2022-02-08 15:12:36 -06:00 committed by Xiang Xiao
parent b3d45fd6c0
commit 8af4d65c9a
1 changed files with 14 additions and 1 deletions
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15
sched/wdog/wd_start.c
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@ -204,7 +204,20 @@ int wd_start(FAR struct wdog_s *wdog, sclock_t delay,
up_getpicbase(&wdog->picbase);
wdog->arg = arg;
/* Calculate delay+1, forcing the delay into a range that we can handle */
/* Calculate delay+1, forcing the delay into a range that we can handle.
*
* NOTE that one is added to the delay. This is correct and must not be
* changed: The contract for the use wdog_start is that the wdog will
* delay FOR AT LEAST as long as requested, but may delay longer due to
* variety of factors. The wdog logic has no knowledge of the the phase
* of the system timer when it is started: The next timer interrupt may
* occur immediately or may be delayed for almost a full cycle. In order
* to meet the contract requirement, the requested time is also always
* incremented by one so that the delay is always at least as long as
* requested.
*
* This is extensive documentation about this time issue elsewhere.
*/
if (delay <= 0)
{