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.
Thanks,
Richard
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