linux-kernel - Re: [PATCH 4/4] net: dsa: mv88e6xxx: add PTP support for MV88E6250 family

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Date:   Tue, 30 Jul 2019 23:46:51 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Richard Cochran <richardcochran@...il.com>
Cc:     Hubert Feurstein <h.feurstein@...il.com>,
        netdev <netdev@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Subject: Re: [PATCH 4/4] net: dsa: mv88e6xxx: add PTP support for MV88E6250 family

Hi Hubert, Richard,

On Tue, 30 Jul 2019 at 19:44, Richard Cochran <richardcochran@...il.com> wrote:
>
> On Tue, Jul 30, 2019 at 12:04:29PM +0200, Hubert Feurstein wrote:
> > diff --git a/drivers/net/dsa/mv88e6xxx/ptp.c b/drivers/net/dsa/mv88e6xxx/ptp.c
> > index 768d256f7c9f..51cdf4712517 100644
> > --- a/drivers/net/dsa/mv88e6xxx/ptp.c
> > +++ b/drivers/net/dsa/mv88e6xxx/ptp.c
> > @@ -15,11 +15,38 @@
> >  #include "hwtstamp.h"
> >  #include "ptp.h"
> >
> > -/* Raw timestamps are in units of 8-ns clock periods. */
> > -#define CC_SHIFT     28
> > -#define CC_MULT              (8 << CC_SHIFT)
> > -#define CC_MULT_NUM  (1 << 9)
> > -#define CC_MULT_DEM  15625ULL
> > +/* The adjfine API clamps ppb between [-32,768,000, 32,768,000], and
>
> That is not true.
>

I was referring to this:
https://github.com/richardcochran/linuxptp/blob/master/phc.c#L38

/*
* On 32 bit platforms, the PHC driver's maximum adjustment (type
* 'int' in units of ppb) can overflow the timex.freq field (type
* 'long'). So in this case we clamp the maximum to the largest
* possible adjustment that fits into a 32 bit long.
*/
#define BITS_PER_LONG (sizeof(long)*8)
#define MAX_PPB_32 32767999 /* 2^31 - 1 / 65.536 */

Technically it is not "not true".

[snip]

On Tue, 30 Jul 2019 at 21:09, Richard Cochran <richardcochran@...il.com> wrote:
>
> On Tue, Jul 30, 2019 at 06:20:00PM +0200, Hubert Feurstein wrote:
> > > Please don't re-write this logic.  It is written like that for a reason.
> > I used the sja1105_ptp.c as a reference. So it is also wrong there.
>
> I'll let that driver's author worry about that.
>
> Thanks,
> Richard
>

And what is the reason for the neg_adj thing? Can you give an example
of when does the "normal way" of doing signed arithmetics not work?

Thanks,
-Vladimir