netdev - [PATCH] igb: shorten maximum PHC timecounter update interval

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Message-Id: <20181012111339.1361-1-mlichvar@redhat.com>
Date:   Fri, 12 Oct 2018 13:13:39 +0200
From:   Miroslav Lichvar <mlichvar@...hat.com>
To:     intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org
Cc:     Miroslav Lichvar <mlichvar@...hat.com>,
        Jacob Keller <jacob.e.keller@...el.com>,
        Richard Cochran <richardcochran@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH] igb: shorten maximum PHC timecounter update interval

The timecounter needs to be updated at least once per ~550 seconds in
order to avoid a 40-bit SYSTIM timestamp to be misinterpreted as an old
timestamp.

Since commit 500462a9d ("timers: Switch to a non-cascading wheel"),
scheduling of delayed work seems to be less accurate and a requested
delay of 540 seconds may actually be longer than 550 seconds. Shorten
the delay to 480 seconds to be sure the timecounter is updated in time.

This fixes an issue with HW timestamps on 82580/I350/I354 being off by
~1100 seconds for few seconds every ~9 minutes.

Cc: Jacob Keller <jacob.e.keller@...el.com>
Cc: Richard Cochran <richardcochran@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Miroslav Lichvar <mlichvar@...hat.com>
---
 drivers/net/ethernet/intel/igb/igb_ptp.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/intel/igb/igb_ptp.c b/drivers/net/ethernet/intel/igb/igb_ptp.c
index 9f4d700e09df..29ced6b74d36 100644
--- a/drivers/net/ethernet/intel/igb/igb_ptp.c
+++ b/drivers/net/ethernet/intel/igb/igb_ptp.c
@@ -51,9 +51,15 @@
  *
  * The 40 bit 82580 SYSTIM overflows every
  *   2^40 * 10^-9 /  60  = 18.3 minutes.
+ *
+ * SYSTIM is converted to real time using a timecounter. As
+ * timecounter_cyc2time() allows old timestamps, the timecounter
+ * needs to be updated at least once per half of the SYSTIM interval.
+ * Scheduling of delayed work is not very accurate, so we aim for 8
+ * minutes to be sure the actual interval is shorter than 9.16 minutes.
  */
 
-#define IGB_SYSTIM_OVERFLOW_PERIOD	(HZ * 60 * 9)
+#define IGB_SYSTIM_OVERFLOW_PERIOD	(HZ * 60 * 8)
 #define IGB_PTP_TX_TIMEOUT		(HZ * 15)
 #define INCPERIOD_82576			BIT(E1000_TIMINCA_16NS_SHIFT)
 #define INCVALUE_82576_MASK		GENMASK(E1000_TIMINCA_16NS_SHIFT - 1, 0)
-- 
2.17.1