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

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Date:   Thu,  4 Nov 2021 14:31:57 +0100
From:   Martin Kaistra <martin.kaistra@...utronix.de>
To:     Florian Fainelli <f.fainelli@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>
Cc:     martin.kaistra@...utronix.de,
        Richard Cochran <richardcochran@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: [PATCH 3/7] timecounter: allow for non-power of two overflow

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@...utronix.de>
---
 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