[PATCH 3/7] timecounter: allow for non-power of two overflow

From: Martin Kaistra
Date: Thu Nov 04 2021 - 09:33:06 EST


Some hardware counters which are used as clocks have an overflow point
which is not a power of two. In order to be able to use the cycle
counter infrastructure with such hardware, add support for more generic
overflow logic.

Signed-off-by: Martin Kaistra <martin.kaistra@xxxxxxxxxxxxx>
---
include/linux/timecounter.h | 3 +++
kernel/time/timecounter.c | 3 +++
2 files changed, 6 insertions(+)

diff --git a/include/linux/timecounter.h b/include/linux/timecounter.h
index c6540ceea143..c71196a742b3 100644
--- a/include/linux/timecounter.h
+++ b/include/linux/timecounter.h
@@ -26,12 +26,15 @@
* see CYCLECOUNTER_MASK() helper macro
* @mult: cycle to nanosecond multiplier
* @shift: cycle to nanosecond divisor (power of two)
+ * @overflow_point: non-power of two overflow point (optional),
+ * smaller than mask
*/
struct cyclecounter {
u64 (*read)(const struct cyclecounter *cc);
u64 mask;
u32 mult;
u32 shift;
+ u64 overflow_point;
};

/**
diff --git a/kernel/time/timecounter.c b/kernel/time/timecounter.c
index e6285288d765..afd2910a9724 100644
--- a/kernel/time/timecounter.c
+++ b/kernel/time/timecounter.c
@@ -39,6 +39,9 @@ static u64 timecounter_read_delta(struct timecounter *tc)
/* calculate the delta since the last timecounter_read_delta(): */
cycle_delta = (cycle_now - tc->cycle_last) & tc->cc->mask;

+ if (tc->cc->overflow_point && (cycle_now - tc->cycle_last) > tc->cc->mask)
+ cycle_delta -= tc->cc->mask - tc->cc->overflow_point;
+
/* convert to nanoseconds: */
ns_offset = cyclecounter_cyc2ns(tc->cc, cycle_delta,
tc->mask, &tc->frac);
--
2.20.1