On Sat, 22 Aug 2015, Richard Cochran wrote:
> On Sat, Aug 22, 2015 at 10:33:48PM +0200, Thomas Gleixner wrote:
> > > @@ -196,19 +197,31 @@ long ptp_ioctl(struct posix_clock *pc, unsigned int
> > > cmd, unsigned long arg)
> > > break;
> > > }
> > > pct = &sysoff->ts[0];
> > > - for (i = 0; i < sysoff->n_samples; i++) {
> > > - getnstimeofday64(&ts);
> > > + if (ptp->info->getsynctime64 && sysoff->n_samples == 1 &&
> >
> > The number of samples should be irrelevant for this sampling method.
>
> Chris had send me a preview of this before he posted, so I can explain
> that test for one sample.
>
> User space requests N (1 to 25) samples of the two clocks. The kernel
> is supposed to deliver that many samples. This has always been the
> documented behavior. From ptp_clock.h:
>
> struct ptp_sys_offset {
> unsigned int n_samples; /* Desired number of measurements. */
> unsigned int rsv[3]; /* Reserved for future use. */
> /*
> * Array of interleaved system/phc time stamps. The kernel
> * will provide 2*n_samples + 1 time stamps, with the last
> * one as a system time stamp.
> */
> struct ptp_clock_time ts[2 * PTP_MAX_SAMPLES + 1];
> };
>
> So the kernel cannot simply change n_samples to 1.
>
> I would prefer to have a new system call that compares any two posix
> clock_t, but that is of course more work.
>
> Allowing n_samples=1 as a special case is a kind of overloading of the
> ioctl to support the new capability. At least it preserves the
> behavior of the interface from the user's perspective.
So why can't you take N samples from the synced hardware? It does not
make any sense to me to switch to the imprecise mode if nsamples > 1.
You can also provide a new IOCTL PTP_SYS_OFFSET_PRECISE which returns
-ENOSYS if hardware timestamping is not available and avoid the whole
nsamples dance for the case where we can get precise timestamps.
Thanks,
tglx
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