linux-kernel - [PATCH net] net: stmmac: ensure PTP time register reads are consistent

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Message-Id: <20220203160025.750632-1-yannick.vignon@oss.nxp.com>
Date:   Thu,  3 Feb 2022 17:00:25 +0100
From:   Yannick Vignon <yannick.vignon@....nxp.com>
To:     Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Rayagond Kokatanur <rayagond@...avyalabs.com>,
        netdev@...r.kernel.org, linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        sebastien.laveze@....nxp.com, Vladimir Oltean <olteanv@...il.com>,
        Xiaoliang Yang <xiaoliang.yang_1@....com>, mingkai.hu@....com,
        Joakim Zhang <qiangqing.zhang@....com>
Cc:     Yannick Vignon <yannick.vignon@....com>
Subject: [PATCH net] net: stmmac: ensure PTP time register reads are consistent

From: Yannick Vignon <yannick.vignon@....com>

Even if protected from preemption and interrupts, a small time window
remains when the 2 register reads could return inconsistent values,
each time the "seconds" register changes. This could lead to an about
1-second error in the reported time.

Add logic to ensure the "seconds" and "nanoseconds" values are consistent.

Fixes: 92ba6888510c ("stmmac: add the support for PTP hw clock driver")
Signed-off-by: Yannick Vignon <yannick.vignon@....com>
---
 .../ethernet/stmicro/stmmac/stmmac_hwtstamp.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_hwtstamp.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_hwtstamp.c
index 074e2cdfb0fa..a7ec9f4d46ce 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_hwtstamp.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_hwtstamp.c
@@ -145,15 +145,20 @@ static int adjust_systime(void __iomem *ioaddr, u32 sec, u32 nsec,
 
 static void get_systime(void __iomem *ioaddr, u64 *systime)
 {
-	u64 ns;
-
-	/* Get the TSSS value */
-	ns = readl(ioaddr + PTP_STNSR);
-	/* Get the TSS and convert sec time value to nanosecond */
-	ns += readl(ioaddr + PTP_STSR) * 1000000000ULL;
+	u64 ns, sec0, sec1;
+
+	/* Get the TSS value */
+	sec1 = readl_relaxed(ioaddr + PTP_STSR);
+	do {
+		sec0 = sec1;
+		/* Get the TSSS value */
+		ns = readl_relaxed(ioaddr + PTP_STNSR);
+		/* Get the TSS value */
+		sec1 = readl_relaxed(ioaddr + PTP_STSR);
+	} while (sec0 != sec1);
 
 	if (systime)
-		*systime = ns;
+		*systime = ns + (sec1 * 1000000000ULL);
 }
 
 static void get_ptptime(void __iomem *ptpaddr, u64 *ptp_time)
-- 
2.25.1