netdev - Re: [PATCH net-next 7/9] mlxsw: spectrum_ptp: Add implementation for physical hardware clock operations

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Date:   Wed, 5 Jun 2019 10:40:25 -0700
From:   Richard Cochran <richardcochran@...il.com>
To:     Shalom Toledo <shalomt@...lanox.com>
Cc:     Ido Schimmel <idosch@...sch.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        Jiri Pirko <jiri@...lanox.com>,
        Petr Machata <petrm@...lanox.com>, mlxsw <mlxsw@...lanox.com>,
        Ido Schimmel <idosch@...lanox.com>
Subject: Re: [PATCH net-next 7/9] mlxsw: spectrum_ptp: Add implementation for
 physical hardware clock operations

On Wed, Jun 05, 2019 at 11:44:21AM +0000, Shalom Toledo wrote:
> On 04/06/2019 17:28, Richard Cochran wrote:
> > Now I see why you did this.  Nice try.
> 
> I didn't try anything.
> 
> The reason is that the hardware units is in ppb and not in scaled_ppm(or ppm),
> so I just converted to ppb in order to set the hardware.

Oh, I thought you were adapting code for the deprecated .adjfreq method.
 
> But I got your point, I will change my calculation to use scaled_ppm (to get a
> more finer resolution) and not ppb, and convert to ppb just before setting the
> hardware. Is that make sense?

So the HW actually accepts ppb adjustment values?  Fine.

But I don't understand this:

> >> +	if (ppb < 0) {
> >> +		neg_adj = 1;
> >> +		ppb = -ppb;
> >> +	}
> >> +
> >> +	adj = clock->nominal_c_mult;
> >> +	adj *= ppb;
> >> +	diff = div_u64(adj, NSEC_PER_SEC);
> >> +
> >> +	spin_lock(&clock->lock);
> >> +	timecounter_read(&clock->tc);
> >> +	clock->cycles.mult = neg_adj ? clock->nominal_c_mult - diff :
> >> +				       clock->nominal_c_mult + diff;
> >> +	spin_unlock(&clock->lock);

You have a SW time counter here

> >> +	return mlxsw_sp1_ptp_update_phc_adjfreq(clock, neg_adj ? -ppb : ppb);

and a hardware method here?

Why not choose one or the other?

Thanks,
Richard