linux-kernel - [PATCH v6 7/7] ptp: ptp_vmclock: return TAI not UTC

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Message-ID: <20260121143402.3092-8-itazur@amazon.com>
Date: Wed, 21 Jan 2026 14:33:44 +0000
From: Takahiro Itazuri <itazur@...zon.com>
To: <robh@...nel.org>, <krzk+dt@...nel.org>, <conor+dt@...nel.org>,
	<richardcochran@...il.com>, <dwmw2@...radead.org>, <andrew+netdev@...n.ch>,
	<davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
	<pabeni@...hat.com>
CC: <devicetree@...r.kernel.org>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, Babis Chalios <mail@...alios.io>, "Alexander
 Graf" <graf@...zon.de>, <mzxreary@...inter.de>, Marco Cali
	<xmarcalx@...zon.co.uk>, David Woodhouse <dwmw@...zon.co.uk>, "Takahiro
 Itazuri" <itazur@...zon.co.uk>
Subject: [PATCH v6 7/7] ptp: ptp_vmclock: return TAI not UTC

From: David Woodhouse <dwmw@...zon.co.uk>

To output UTC would involve complex calculations about whether the time
elapsed since the reference time has crossed the end of the month when
a leap second takes effect. I've prototyped that, but it made me sad.

Much better to report TAI, which is what PHCs should do anyway.
And much much simpler.

Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
Signed-off-by: Babis Chalios <bchalios@...zon.es>
---
 drivers/ptp/ptp_vmclock.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/ptp/ptp_vmclock.c b/drivers/ptp/ptp_vmclock.c
index 55ec3cda3995..6cf63a86c997 100644
--- a/drivers/ptp/ptp_vmclock.c
+++ b/drivers/ptp/ptp_vmclock.c
@@ -83,13 +83,13 @@ static uint64_t mul_u64_u64_shr_add_u64(uint64_t *res_hi, uint64_t delta,
 
 static bool tai_adjust(struct vmclock_abi *clk, uint64_t *sec)
 {
-	if (likely(clk->time_type == VMCLOCK_TIME_UTC))
+	if (clk->time_type == VMCLOCK_TIME_TAI)
 		return true;
 
-	if (clk->time_type == VMCLOCK_TIME_TAI &&
+	if (clk->time_type == VMCLOCK_TIME_UTC &&
 	    (le64_to_cpu(clk->flags) & VMCLOCK_FLAG_TAI_OFFSET_VALID)) {
 		if (sec)
-			*sec += (int16_t)le16_to_cpu(clk->tai_offset_sec);
+			*sec -= (int16_t)le16_to_cpu(clk->tai_offset_sec);
 		return true;
 	}
 	return false;
@@ -350,9 +350,9 @@ static struct ptp_clock *vmclock_ptp_register(struct device *dev,
 		return NULL;
 	}
 
-	/* Only UTC, or TAI with offset */
+	/* Accept TAI directly, or UTC with valid offset for conversion to TAI */
 	if (!tai_adjust(st->clk, NULL)) {
-		dev_info(dev, "vmclock does not provide unambiguous UTC\n");
+		dev_info(dev, "vmclock does not provide unambiguous time\n");
 		return NULL;
 	}
 
-- 
2.47.3