linux-kernel - [PATCH 5.10 43/74] net: stmmac: ensure PTP time register reads are consistent

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Message-Id: <20220207103758.633267782@linuxfoundation.org>
Date:   Mon,  7 Feb 2022 12:06:41 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Yannick Vignon <yannick.vignon@....com>,
        "Russell King (Oracle)" <rmk+kernel@...linux.org.uk>,
        Jakub Kicinski <kuba@...nel.org>
Subject: [PATCH 5.10 43/74] net: stmmac: ensure PTP time register reads are consistent

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

commit 80d4609008e6d696a279e39ae7458c916fcd44c1 upstream.

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>
Reviewed-by: Russell King (Oracle) <rmk+kernel@...linux.org.uk>
Link: https://lore.kernel.org/r/20220203160025.750632-1-yannick.vignon@oss.nxp.com
Signed-off-by: Jakub Kicinski <kuba@...nel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_hwtstamp.c |   17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_hwtstamp.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_hwtstamp.c
@@ -142,15 +142,20 @@ static int adjust_systime(void __iomem *
 
 static void get_systime(void __iomem *ioaddr, u64 *systime)
 {
-	u64 ns;
+	u64 ns, sec0, sec1;
 
-	/* 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;
+	/* 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);
 }
 
 const struct stmmac_hwtimestamp stmmac_ptp = {