[PATCH v5 7/7] ptp: ptp_vmclock: return TAI not UTC

Chalios, Babis
2026-01-07T13:28:11Z

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@amazon.co.uk>
Signed-off-by: Babis Chalios <bchalios@amazon.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 9f17f4a1b2be..ef5c2a66d9b8 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;
}

--
2.34.1