Re: [PATCH v6 0/8] ptp: IEEE 1588 hardware clock support

[Christoph Lameter]

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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