Re: [PATCH v3 3/4] Add support for driver cross-timestamp to PTP_SYS_OFFSET ioctl

[Thomas Gleixner <tglx@linutronix.de>]

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