linux-kernel - [PATCH RFC 2/3] timekeeping: Don't align frequency adjustments to ticks

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Message-Id: <20170517161317.19557-3-mlichvar@redhat.com>
Date:   Wed, 17 May 2017 18:13:16 +0200
From:   Miroslav Lichvar <mlichvar@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     John Stultz <john.stultz@...aro.org>,
        Prarit Bhargava <prarit@...hat.com>,
        Richard Cochran <richardcochran@...il.com>
Subject: [PATCH RFC 2/3] timekeeping: Don't align frequency adjustments to ticks

When the timekeeping multiplier is adjusted, the NTP error is adjusted
to correct the clock for the misalignment of the update to the start of
the tick. This error is corrected in later updates and the clock appears
as if the frequency was changed exactly on the tick.

Remove this correction to keep the point where the frequency is
effectively changed at the time of the update. This removes a major
source of the NTP error.

Cc: John Stultz <john.stultz@...aro.org>
Cc: Prarit Bhargava <prarit@...hat.com>
Cc: Richard Cochran <richardcochran@...il.com>
Signed-off-by: Miroslav Lichvar <mlichvar@...hat.com>
---
 kernel/time/timekeeping.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index ff542dd..5ae6f27 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1760,8 +1760,6 @@ static __always_inline void timekeeping_apply_adjustment(struct timekeeper *tk,
 	 *	xtime_nsec_2 = xtime_nsec_1 - offset
 	 * Which simplfies to:
 	 *	xtime_nsec -= offset
-	 *
-	 * XXX - TODO: Doc ntp_error calculation.
 	 */
 	if ((mult_adj > 0) && (tk->tkr_mono.mult + mult_adj < mult_adj)) {
 		/* NTP adjustment caused clocksource mult overflow */
@@ -1772,7 +1770,6 @@ static __always_inline void timekeeping_apply_adjustment(struct timekeeper *tk,
 	tk->tkr_mono.mult += mult_adj;
 	tk->xtime_interval += interval;
 	tk->tkr_mono.xtime_nsec -= offset;
-	tk->ntp_error -= (interval - offset) << tk->ntp_error_shift;
 }
 
 /*
-- 
2.9.3