Re: [PATCH v2 2/2] MAINTAINERS: update PTP maintainer entries after directory split

[Wen Gu]

On 2026/6/2 16:04, David Woodhouse wrote:
> On Mon, 2026-06-01 at 21:20 -0700, Richard Cochran wrote:
> > Still, I don't understand why these new (non-network related ) device
> > drivers can't be implemented in their own class using
> > posix_clock_register()
> > etc.
> >
> > Any of the useful bits (like sysfs interfaces) can be refactored out
> > of ptp_clock.c and shared as a common layer.
>
> The key is that userspace wants to get snapshots of the reference clock
> either precisely synchronized with the system clock where possible, or
> sandwiched as closely together as possible with ABA readings of the
> system, reference, system clock.
>
> Those *are* the PTP_SYS_OFFSET* ioctls, which tools like chrony already
> support with 'refclock PHC /dev/ptp…'.
>
> I'm not sure how factoring those out into separate POSIX AUX clocks
> would help? I think they do want to present as /dev/ptp*.

I agree. Both [1] and [2] independently support the same point:
chrony's PHC refclock relies on ptp-specific ioctls (in sys_linux.c [3]),
not just clock_gettime(), so the ioctl interface needs to stay.
A posix_clock-based class wouldn't provide that.

[1] https://lore.kernel.org/netdev/2a4c9a00-45f5-4f6a-90c4-492ea1d50b79@xxxxxxxxxxxxxxxxx/
[2] https://lore.kernel.org/all/vmwwnl3zv26lmmuqp2vqltg2fudalpc5jrw7k6ifg6l5cwlk3j@i7jm62zcsl67/
[3] https://gitlab.com/chrony/chrony/-/blob/master/sys_linux.c

> There is some extra stuff we want to do for "Precision RTCs" or
> whatever we're going to call them. They might actually have a known TAI
> offset, they might convey leap second indications, we might want to set
> the kernel's CLOCK_REALTIME from them at boot. And in the case of
> VMClock, I'm working on being able to clamp the kernel's timekeeping to
> it directly².
>
> So maybe what we want is linux/drivers/phc, to host those read-only
> devices which know real time. They can provide a simplified
> implementation; maybe *only* a function like vmclock_get_crosststamp(),
> which is just called in various different permutations by the various
> different PTP methods.
>
> The core linux/drivers/phc code would then handle the interface to the
> kernel's core timekeeping *and* wrap them to register a PTP device that
> existing userspace can understand. And deal with the kvmclock/TSC
> awfulness where needed.
>
> How does that sound?

I think a dedicated phc core would make the classification of read-only
clocks clearer, reducing ambiguity around where they belong. I assume
direct timekeeping integration would be optional, drivers providing
only a snapshot-based crosststamp would use /dev/ptpX alone, while the
timekeeping path would require additional capability (as vmclock provides)?

> ² https://lore.kernel.org/all/20260520135207.37826-9-dwmw2@xxxxxxxxxxxxx/