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ztimer: convert_frac: use self->scale_now for max_value

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When using convert_frac to slow down a timer, the extension machinery is
used to prevent converting values that would be larger than, after
conversion, 2**32. In order to calculate the maximum value, UINT32_MAX
is converted.

Previously, this was mistakenly converted by multiplying with the
fraction used to scale up a value. E.g., when using convert_frac to
slow down a 1us timer to provide a 1ms timer, UINT32_MAX was multiplied
by 1000. As the result is used % UINT32_MAX, this lead to a wrong value
close to UINT32_MAX, as ((UINT32_MAX * N) % UINT32_MAX = (UINT32_MAX - N - 1)).

This PR fixes the calculation so max_value is determined by multiplying
with the fraction used to scale down a value.
This commit is contained in:
Kaspar Schleiser 2020-06-16 12:24:11 +02:00
parent 1992f57765
commit 1ff609167e
1 changed files with 1 additions and 1 deletions
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2
sys/ztimer/convert_frac.c
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@ -100,7 +100,7 @@ void ztimer_convert_frac_init(ztimer_convert_frac_t *self,
ztimer_convert_frac_compute_scale(self, freq_self, freq_lower);
if (freq_self < freq_lower) {
self->super.super.max_value = frac_scale(&self->scale_set, UINT32_MAX);
self->super.super.max_value = frac_scale(&self->scale_now, UINT32_MAX);
ztimer_init_extend(&self->super.super);
}
else {