RE: [PATCH v6 09/11] pps: generators: Add PPS Generator TIO Driver

[D, Lakshmi Sowjanya]

> -----Original Message-----
> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Sent: Thursday, April 11, 2024 3:59 AM
> To: D, Lakshmi Sowjanya <lakshmi.sowjanya.d@xxxxxxxxx>;
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> Subramanian <subramanian.mohan@xxxxxxxxx>; T R, Thejesh Reddy
> <thejesh.reddy.t.r@xxxxxxxxx>; D, Lakshmi Sowjanya
> <lakshmi.sowjanya.d@xxxxxxxxx>
> Subject: Re: [PATCH v6 09/11] pps: generators: Add PPS Generator TIO Driver
> 
> On Wed, Apr 10 2024 at 17:18, lakshmi.sowjanya.d@xxxxxxxxx wrote:
> > +static bool pps_generate_next_pulse(struct pps_tio *tio, ktime_t
> > +expires) {
> > +	u64 art;
> > +
> > +	if (!ktime_real_to_base_clock(expires, CSID_X86_ART, &art)) {
> > +		pps_tio_disable(tio);
> > +		return false;
> > +	}
> > +
> > +	pps_compv_write(tio, art - ART_HW_DELAY_CYCLES);
> > +	return true;
> > +}
> > +
> > +static enum hrtimer_restart hrtimer_callback(struct hrtimer *timer) {
> > +	struct pps_tio *tio = container_of(timer, struct pps_tio, timer);
> > +	ktime_t expires, now;
> > +	u32 event_count;
> > +
> > +	guard(spinlock)(&tio->lock);
> > +
> > +	/* Check if any event is missed. If an event is missed, TIO will be
> disabled*/
> > +	event_count = pps_tio_read(tio, TIOEC);
> > +	if (!tio->prev_count && tio->prev_count == event_count)
> > +		goto err;
> > +	tio->prev_count = event_count;
> > +	expires = hrtimer_get_expires(timer);
> > +	now = ktime_get_real();
> > +
> > +	if (now - expires < SAFE_TIME_NS) {
> > +		if (!pps_generate_next_pulse(tio, expires + SAFE_TIME_NS))
> > +			goto err;
> > +	}
> 
> If the hrtimer callback is invoked late so that now - expires is >= SAFE_TIME_NS
> then this just forwards the timer and tries again.

Yes we will introduce a return HRTIMER_NORESTART if the time is expired.

> 
> This lacks any form of explanation why this is correct as obviously there will be a
> missing pulse, no?

We had added an event count check to detect the missed pulse(i.e if we had programmed an expired time). 
Timed I/O hardware has an event count register to log the number of pulses generated.

> 
> > +	hrtimer_forward(timer, now, NSEC_PER_SEC / 2);
> > +	return HRTIMER_RESTART;
> > +err:
> > +	dev_err(tio->dev, "Event missed, Disabling Timed I/O");
> > +	pps_tio_disable(tio);
> 
> Why does this disable it again? The failure path in
> pps_generate_next_pulse() does so already, no?

will remove disabling twice in the next version of patchset.

> 
> > +	return HRTIMER_NORESTART;
> > +}
> > +
> 
> Thanks,
> 
>         tglx