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extmod/utime_mphal: ticks_diff(): switch arg order, return signed value.
Based on the earlier discussed RFC. Practice showed that the most natural order for arguments corresponds to mathematical subtraction: ticks_diff(x, y) <=> x - y Also, practice showed that in real life, it's hard to order events by time of occurance a priori, events tend to miss deadlines, etc. and the expected order breaks. And then there's a need to detect such cases. And ticks_diff can be used exactly for this purpose, if it returns a signed, instead of unsigned, value. E.g. if x is scheduled time for event, and y is the current time, then if ticks_diff(x, y) < 0 then event has missed a deadline (and e.g. needs to executed ASAP or skipped). Returning in this case a large unsigned number (like ticks_diff behaved previously) doesn't make sense, and such "large unsigned number" can't be reliably detected per our definition of ticks_* function (we don't expose to user level maximum value, it can be anything, relatively small or relatively large).
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Lines changed: 2 additions & 2 deletions

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‎extmod/utime_mphal.c‎

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -78,11 +78,11 @@ STATIC mp_obj_t time_ticks_cpu(void) {
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}
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MP_DEFINE_CONST_FUN_OBJ_0(mp_utime_ticks_cpu_obj, time_ticks_cpu);
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STATIC mp_obj_t time_ticks_diff(mp_obj_t start_in, mp_obj_t end_in) {
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STATIC mp_obj_t time_ticks_diff(mp_obj_t end_in, mp_obj_t start_in) {
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// we assume that the arguments come from ticks_xx so are small ints
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uint32_t start = MP_OBJ_SMALL_INT_VALUE(start_in);
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uint32_t end = MP_OBJ_SMALL_INT_VALUE(end_in);
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return MP_OBJ_NEW_SMALL_INT((end - start) & MP_SMALL_INT_POSITIVE_MASK);
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return MP_OBJ_NEW_SMALL_INT((int32_t)(end - start));
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}
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MP_DEFINE_CONST_FUN_OBJ_2(mp_utime_ticks_diff_obj, time_ticks_diff);
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