Re: [PATCH 1/5] ptp: Added a brand new class driver for ptp clocks.

[Richard Cochran <richardcochran@gmail.com>]

On Mon, Aug 23, 2010 at 01:21:39PM -0700, john stultz wrote:
> On Thu, 2010-08-19 at 17:38 +0200, Richard Cochran wrote:
> > On Thu, Aug 19, 2010 at 02:28:04PM +0200, Arnd Bergmann wrote:
> > > My point was that a syscall is better than an ioctl based interface here,
> > > which I definitely still think. Given that John knows much more about
> > > clocks than I do, we still need to get agreement on the question that
> > > he raised, which is whether we actually need to expose this clock to the
> > > user or not.
> > > 
> > > If we can find a way to sync system time accurate enough with PTP and
> > > PPS, user applications may not need to see two separate clocks at all.
> > 
> > At the very least, one user application (the PTPd) needs to see the
> > PTP clock.
> > 
> > > > SYSCALL_DEFINE3(clock_adjtime, const clockid_t, clkid,
> > > > 		int, ppb, struct timespec __user *, ts)
> > > > 
> > > > ppb - desired frequency adjustment in parts per billion
> > > > ts  - desired time step (or jump) in <sec,nsec> to correct
> > > >       a measured offset
> > > > 
> > > > Arguably, this syscall might be useful for other clocks, too.
> > > 
> > > This is a mix of adjtime and adjtimex with the addition of
> > > the clkid parameter, right?
> > 
> > Sort of, but not really. ADJTIME(3) takes an offset and slowly
> > corrects the clock using a servo in the kernel, over hours.
> > 
> > For this function, the offset passed in the 'ts' parameter will be
> > immediately corrected, by jumping to the new time. This reflects the
> > way that PTP works. After the first few samples, the PTPd has an
> > estimate of the offset to the master and the rate difference. The PTPd
> > can proceed in one of two ways.
> > 
> > 1. If resetting the clock is not desired, then the clock is set to the
> >    maximum adjustment (in the right direction) until the clock time is
> >    close to the master's time.
> > 
> > 2. The estimated offset is added to the current time, resulting in a
> >    jump in time.
> > 
> > We need clock_adjtime(id, 0, ts) for the second case.
> >
> > > Have you considered passing a struct timex instead of ppb and ts?
> > 
> > Yes, but the timex is not suitable, IMHO.
> 
> Could you expand on this?

We need to able to specify that the call is for a PTP clock. We could
add that to the modes flag, like this:

/*timex.h*/
#define ADJ_PTP_0 0x10000
#define ADJ_PTP_1 0x20000
#define ADJ_PTP_2 0x30000
#define ADJ_PTP_3 0x40000

I can live with this, if everyone else can, too.
 
> Could we not add a adjustment mode ADJ_SETOFFSET or something that would
> provide the instantaneous offset correction?

Yes, but we would also need to add a struct timespec to the struct
timex, in order to get nanosecond resolution. I think it would be
possible to do in the padding at the end?

> You're right that the timex is a little crufty. But its legacy that we
> will support indefinitely. So following the established interface helps
> maintainability.

We can use it for PTP, with the modifications suggested above. Or we
can just introduce the clock_adjtime method, instead.
 
> So if the clock_adjtime interface is needed, it would seem best for it
> to be generic enough to support not only PTP, but also the NTP kernel
> PLL.

For the proposed clock_adjime, what else is needed to support clock
adjustment in general?

I don't mind making the interface generic enough to support any
(realistic) conceivable clock adjustment scheme, but beyond the
present PTP hardware clocks, I don't know what else might be needed.

Richard