FAILED: patch "[PATCH] net: stmmac: ensure PTP time register reads are consistent" failed to apply to 4.9-stable tree

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 80d4609008e6d696a279e39ae7458c916fcd44c1 Mon Sep 17 00:00:00 2001
From: Yannick Vignon <[email protected]>
Date: Thu, 3 Feb 2022 17:00:25 +0100
Subject: [PATCH] net: stmmac: ensure PTP time register reads are consistent

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.

index 074e2cdfb0fa..a7ec9f4d46ce 100644
@@ -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)