linux-kernel - 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

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Date:   Mon, 2 Mar 2020 18:01:48 -0800
From:   "Christopher S. Hall" <christopher.s.hall@...el.com>
To:     Richard Cochran <richardcochran@...il.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, hpa@...or.com, mingo@...hat.com,
        x86@...nel.org, jacob.e.keller@...el.com, davem@...emloft.net,
        sean.v.kelley@...el.com
Subject: 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

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