Re: [Intel PMC TGPIO Driver 0/5] Add support for Intel PMC Time GPIO Driver with PHC interface changes to support additional H/W Features

Christopher S. Hall
2020-03-03T02:02:05Z

Hi Richard,

On Tue, Feb 25, 2020 at 06:47:07PM -0800, Richard Cochran wrote:
> On Tue, Feb 25, 2020 at 03:37:07PM -0800, Christopher S. Hall wrote:
> > On Sun, Feb 02, 2020 at 08:08:38PM -0800, Richard Cochran wrote:
> > > The TGPIO input clock, the ART, is a free running counter, but you
> > > want to support frequency adjustments. Use a timecounter cyclecounter
> > > pair.
> >
> > I'm concerned about the complexity that the timecounter adds to
> > the driver. Specifically, the complexity of dealing with any rate mismatches
> > between the timecounter and the periodic output signal. The phase
> > error between the output and timecounter needs to be zero.
>
> If I understood correctly, the device's outputs are generated from a
> non-adjustable counter. So, no matter what, you will have the problem
> of changing the pulse period in concert with the user changing the
> desired frequency.
>

> > This leaves the PHC API behavior as it is currently and uses the frequency
> > adjust API to adjust the output rate.
> >
> > > Let the user dial a periodic output signal in the normal way.
> > >
> > > Let the user change the frequency in the normal way, and during this
> > > call, adjust the counter values accordingly.
> >
> > Yes to both of the above.
>
> So, why then do you need this?
>
> +#define PTP_EVENT_COUNT_TSTAMP2 \
> + _IOWR(PTP_CLK_MAGIC, 19, struct ptp_event_count_tstamp)
>
> If you can make the device work with the existing user space API,
>
> ioctl(fd, PTP_PEROUT_REQUEST2, ...);
> while (1) {
> clock_adjtimex(FD_TO_CLOCKID(fd), ...);
> }
>
> that would be ideal. But I will push back on anything like the
> following.
>
> ioctl(fd, PTP_PEROUT_REQUEST2, ...);
> while (1) {
> clock_adjtimex(FD_TO_CLOCKID(fd), ...);
> ioctl(fd, PTP_EVENT_COUNT_TSTAMP, ...);
> }
>
> But maybe I misunderstood?

Thank you for the feedback, but Thomas wants to see this as
an extension of GPIO. I'll work on an RFC patch for that instead.

> Thanks,
> Richard

Thanks,
Christopher