netdev - [PATCH v4 7/7] ptp: ptp_vmclock: return TAI not UTC

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Message-ID: <20251218142408.8395-8-bchalios@amazon.es>
Date: Thu, 18 Dec 2025 14:25:29 +0000
From: "Chalios, Babis" <bchalios@...zon.es>
To: "robh@...nel.org" <robh@...nel.org>, "krzk+dt@...nel.org"
	<krzk+dt@...nel.org>, "conor+dt@...nel.org" <conor+dt@...nel.org>,
	"richardcochran@...il.com" <richardcochran@...il.com>, "dwmw2@...radead.org"
	<dwmw2@...radead.org>, "andrew+netdev@...n.ch" <andrew+netdev@...n.ch>,
	"davem@...emloft.net" <davem@...emloft.net>, "edumazet@...gle.com"
	<edumazet@...gle.com>, "kuba@...nel.org" <kuba@...nel.org>,
	"pabeni@...hat.com" <pabeni@...hat.com>
CC: "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Chalios,
 Babis" <bchalios@...zon.es>, "Graf (AWS), Alexander" <graf@...zon.de>,
	"mzxreary@...inter.de" <mzxreary@...inter.de>, "Cali, Marco"
	<xmarcalx@...zon.co.uk>, "Woodhouse, David" <dwmw@...zon.co.uk>
Subject: [PATCH v4 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 7f342e5a6a92..deab3205601b 100644
--- a/drivers/ptp/ptp_vmclock.c
+++ b/drivers/ptp/ptp_vmclock.c
@@ -82,13 +82,13 @@ 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;
@@ -349,9 +349,9 @@ 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;
 	}
 
-- 
2.34.1