linux-kernel - [RFC PATCH 1/7] timekeeping: Fix cross-timestamp interpolation on counter wrap

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Message-Id: <20230630171052.985577-2-peter.hilber@opensynergy.com>
Date:   Fri, 30 Jun 2023 19:10:44 +0200
From:   Peter Hilber <peter.hilber@...nsynergy.com>
To:     linux-kernel@...r.kernel.org
Cc:     Peter Hilber <peter.hilber@...nsynergy.com>,
        John Stultz <jstultz@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>,
        "Christopher S. Hall" <christopher.s.hall@...el.com>
Subject: [RFC PATCH 1/7] timekeeping: Fix cross-timestamp interpolation on counter wrap

cycle_between() decides whether get_device_system_crosststamp() will
interpolate for older counter readings.

cycle_between() yields wrong results for a counter wrap-around where after
< before < test, and for the case after < test < before.

Fix the comparison logic.

Fixes: 2c756feb18d9 ("time: Add history to cross timestamp interface supporting slower devices")
Signed-off-by: Peter Hilber <peter.hilber@...nsynergy.com>
---
 kernel/time/timekeeping.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 266d02809dbb..8f35455b6250 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1186,7 +1186,7 @@ static bool cycle_between(u64 before, u64 test, u64 after)
 {
 	if (test > before && test < after)
 		return true;
-	if (test < before && before > after)
+	if (before > after && (test > before || test < after))
 		return true;
 	return false;
 }
-- 
2.39.2