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SubjectRe: [PATCH v6 0/8] ptp: IEEE 1588 hardware clock support
On Thu, 23 Sep 2010, Richard Cochran wrote:

> Support for obtaining timestamps from a PHC already exists via the
> SO_TIMESTAMPING socket option, integrated in kernel version 2.6.30.
> This patch set completes the picture by allow user space programs to
> adjust the PHC and to control its ancillary features.

Is there a way to use the PHC as a system clock? I think the main benefit
of PTP is to have syncronized time on multiple machines in a cluster. That
may mean getting rid of ntp and using an in kernel PHC based way to sync time.

> So as far as the POSIX standard is concerned, offering a clock id
> to represent the PHC would be acceptable.

Sure but what would you do with it? HPET timer support has no such need.

> 3.2.1 Using the POSIX Clock API
> --------------------------------
> Looking at the mapping from PHC operation to the POSIX clock API,
> we see that two of the basic clock operations, marked with *, have
> no POSIX equivalent. The items marked NA are peculiar to PHCs and
> will be discussed separately, below.
> Clock Operation POSIX function
> -----------------------------+-----------------------------
> Set time clock_gettime
> Get time clock_settime
> Shift the clock *
> Adjust clock frequency *
> -----------------------------+-----------------------------
> Time stamp external events NA
> Enable PPS events NA
> Periodic output signals NA
> One shot or periodic alarms timer_create, timer_settime
> In contrast to the standard Linux system clock, a PHC is
> adjustable in hardware, for example using frequency compensation
> registers or a VCO. The ability to directly tune the PHC is
> essential to reap the benefit of hardware timestamping.

There is a reason for not being able to shift posix clocks: The system has
one time base. The various clocks are contributing to maintaining that
sytem wide time.

I do not understand why you want to maintain different clocks running at
different speeds. Certainly interesting for some uses I guess that I
do not have the energy to imagine right now. But can we get the PTP killer
feature of synchronized accurate system time first?

> 3.3 Synchronizing the Linux System Time
> ========================================
> One could offer a PHC as a combined clock source and clock event
> device. The advantage of this approach would be that it obviates
> the need for synchronization when the PHC is selected as the system
> timer. However, some PHCs, namely the PHY based clocks, cannot be
> used in this way.

Why not? Do PHY based clock not at least provide a counter that increments
in synchronized intervals throughout the network?

> Instead, the patch set provides a way to offer a Pulse Per Second
> (PPS) event from the PHC to the Linux PPS subsystem. A user space
> application can read the PPS events and tune the system clock, just
> like when using other external time sources like radio clocks or
> GPS.

User space is subject to various latencies created by the OS etc. I would
that in order to have fine grained (read microsecond) accurary we would
have to run the portions that are relevant to obtaining the desired
accuracy in the kernel.

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