PTP_SYS_OFFSET_EXTENDED returns system timestamps that do not correctly
bracket the PHC register read on MACB/GEM. On a Raspberry Pi 5, the
returned interval can be as short as 37 ns, while an ordered register
read takes approximately 1 us. This biases the midpoint used by phc2sys,
causing CLOCK_REALTIME to run approximately 0.5 us ahead when synchronized
to the PHC.

gem_tsu_get_time() reads the nanoseconds register using the driver's
relaxed MMIO accessor. On weakly ordered systems, the subsequent system
timestamp can be taken before the register read completes. The internal
smp_rmb() in the pre-timestamp path also does not guarantee ordering
against the subsequent MMIO read.

Add rmb() before and after the bracketed nanoseconds read in both the
normal and seconds rollover paths so the system timestamps bracket the
PHC read. Adding the post-read barrier increases the minimum interval on
the same Raspberry Pi 5 to approximately 1 us.

Fixes: e51bb5c2784c ("net: macb: ptp: Switch to gettimex64() interface")
Tested-by: Nicolai Buchwitz <nb@tipi-net.de> # Raspberry Pi CM5, min bracket 37 ns -> 981 ns
Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>
Signed-off-by: James Clark <jjc@jclark.com>
---
Changes in v2:
- Add rmb() before the PHC read in both paths, following Théo Lebrun's
  feedback. Explain why the pre-timestamp path's smp_rmb() is insufficient
  for MMIO ordering.
- Wrap the post-read barrier comments.
- Add Nicolai Buchwitz's Tested-by and Reviewed-by tags.
- Drop RFC.

v1: https://lore.kernel.org/netdev/20260908053150.28694-1-jjc@jclark.com/

Nicolai's review and testing were on v1. The additional pre-read barriers
in v2 address Théo's feedback.

Reproducer:

#include <fcntl.h>
#include <linux/ptp_clock.h>
#include <stdio.h>
#include <sys/ioctl.h>

#define DEVICE "/dev/ptp0"

int main(void)
{
	struct ptp_sys_offset_extended ex = { .n_samples = 25 };
	long long min = -1;
	int fd = open(DEVICE, O_RDONLY);

	if (fd < 0) {
		perror(DEVICE);
		return 1;
	}
	for (int batch = 0; batch < 40; batch++) {
		if (ioctl(fd, PTP_SYS_OFFSET_EXTENDED, &ex) < 0) {
			perror("PTP_SYS_OFFSET_EXTENDED");
			return 1;
		}
		for (unsigned int i = 0; i < ex.n_samples; i++) {
			long long bracket = (ex.ts[i][2].sec - ex.ts[i][0].sec) * 1000000000LL
				+ (long long)ex.ts[i][2].nsec - ex.ts[i][0].nsec;
			if (min < 0 || bracket < min)
				min = bracket;
		}
	}
	printf("min bracket: %lld ns\n", min);
	return 0;
}

 drivers/net/ethernet/cadence/macb_ptp.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/drivers/net/ethernet/cadence/macb_ptp.c b/drivers/net/ethernet/cadence/macb_ptp.c
index e5195d7da..4fb287608 100644
--- a/drivers/net/ethernet/cadence/macb_ptp.c
+++ b/drivers/net/ethernet/cadence/macb_ptp.c
@@ -50,7 +50,15 @@ static int gem_tsu_get_time(struct ptp_clock_info *ptp, struct timespec64 *ts,
 
 	spin_lock_irqsave(&bp->tsu_clk_lock, flags);
 	ptp_read_system_prets(sts);
+	/* ptp_read_system_prets() uses smp_rmb() internally,
+	 * which does not guarantee ordering against MMIO reads.
+	 */
+	rmb();
 	first = gem_readl(bp, TN);
+	/* Ensure the PHC read completes before taking
+	 * the post timestamp.
+	 */
+	rmb();
 	ptp_read_system_postts(sts);
 	secl = gem_readl(bp, TSL);
 	sech = gem_readl(bp, TSH);
@@ -62,7 +70,15 @@ static int gem_tsu_get_time(struct ptp_clock_info *ptp, struct timespec64 *ts,
 		 * (assume all done within 1s)
 		 */
 		ptp_read_system_prets(sts);
+		/* ptp_read_system_prets() uses smp_rmb() internally,
+		 * which does not guarantee ordering against MMIO reads.
+		 */
+		rmb();
 		ts->tv_nsec = gem_readl(bp, TN);
+		/* Ensure the PHC read completes before taking
+		 * the post timestamp.
+		 */
+		rmb();
 		ptp_read_system_postts(sts);
 		secl = gem_readl(bp, TSL);
 		sech = gem_readl(bp, TSH);
-- 
2.47.3