[RFC PATCH net-next v3 1/2] macb: Add 1588 support in Cadence GEM.

[Rafal Ozieblo]

-----Original Message-----
> From: Andrei.Pistirica at microchip.com [mailto:Andrei.Pistirica at microchip.com]
> Sent: 8 grudnia 2016 15:42
> To: richardcochran at gmail.com
> Cc: netdev at vger.kernel.org; linux-kernel at vger.kernel.org; linux-arm-kernel at lists.infradead.org; davem at davemloft.net; nicolas.ferre at atmel.com; harinikatakamlinux at gmail.com; harini.katakam at xilinx.com; punnaia at xilinx.com; michals at xilinx.com; anirudh at xilinx.com; boris.brezillon at free-electrons.com; alexandre.belloni at free-electrons.com; tbultel at pixelsurmer.com; Rafal Ozieblo
> Subject: RE: [RFC PATCH net-next v3 1/2] macb: Add 1588 support in Cadence GEM.
>
>
>
> > -----Original Message-----
> > From: Richard Cochran [mailto:richardcochran at gmail.com]
> > Sent: Wednesday, December 07, 2016 11:04 PM
> > To: Andrei Pistirica - M16132
> > Cc: netdev at vger.kernel.org; linux-kernel at vger.kernel.org; linux-arm-
> > kernel at lists.infradead.org; davem at davemloft.net;
> > nicolas.ferre at atmel.com; harinikatakamlinux at gmail.com;
> > harini.katakam at xilinx.com; punnaia at xilinx.com; michals at xilinx.com;
> > anirudh at xilinx.com; boris.brezillon at free-electrons.com;
> > alexandre.belloni at free-electrons.com; tbultel at pixelsurmer.com;
> > rafalo at cadence.com
> > Subject: Re: [RFC PATCH net-next v3 1/2] macb: Add 1588 support in
> > Cadence GEM.
> >
> > On Wed, Dec 07, 2016 at 08:39:09PM +0100, Richard Cochran wrote:
> > > > +static s32 gem_ptp_max_adj(unsigned int f_nom) {
> > > > +       u64 adj;
> > > > +
> > > > +       /* The 48 bits of seconds for the GEM overflows every:
> > > > +        * 2^48/(365.25 * 24 * 60 *60) =~ 8 925 512 years (~= 9 mil years),
> > > > +        * thus the maximum adjust frequency must not overflow CNS
> > register:
> > > > +        *
> > > > +        * addend  = 10^9/nominal_freq
> > > > +        * adj_max = +/- addend*ppb_max/10^9
> > > > +        * max_ppb = (2^8-1)*nominal_freq-10^9
> > > > +        */
> > > > +       adj = f_nom;
> > > > +       adj *= 0xffff;
> > > > +       adj -= 1000000000ULL;
> > >
> > > What is this computation, and how does it relate to the comment?
>
> I considered the following simple equation: increment value at nominal frequency (which is 10^9/nominal frequency nsecs) + the maximum drift value (nsecs) <= maximum increment value at nominal frequency (which is 8bit:0xffff).
> If maximum drift is written as function of nominal frequency and maximum ppb, then the equation above yields that the maximum ppb is: (2^8 - 1) *nominal_frequency - 10^9. The equation is also simplified by the fact that the drift is written as ppm + 16bit_fractions and the increment value is written as nsec + 16bit_fractions.
>
> Rafal said that this value is hardcoded: 0x64E6, while Harini said: 250000000.

To clarify a little bit. In my reference code this value (0x64E6) was taken from our legacy code. It was used for testing only. I know it should be change to something more accurate. This is the reason why I asked how did you count it (250000000). According to our calculations this value depends on actual set period (incr_ns and incr_sub_ns) and min and max value we can set. The calculation were a little bit intricate, so we decided to leave it as it was.

>
> I need to dig into this...
>
> >
> > I am not sure what you meant, but it sounds like you are on the wrong track.
> > Let me explain...
>
> Thanks.
>
> >
> > The max_adj has nothing at all to do with the width of the time register.
> > Rather, it should reflect the maximum possible change in the tuning word.
> >
> > For example, with a nominal 8 ns period, the tuning word is 0x80000.
> > Looking at running the clock more slowly, the slowest possible word is
> > 0x00001, meaning a difference of 0x7FFFF.  This implies an adjustment
> > of
> > 0x7FFFF/0x80000 or 999998092 ppb.  Running more quickly, we can
> > already have 0x100000, twice as fast, or just under 2 billion ppb.
> >
> > You should consider the extreme cases to determine the most limited
> > (smallest) max_adj value:
> >
> > Case 1 - high frequency
> > ~~~~~~~~~~~~~~~~~~~~~~~
> >
> > With a nominal 1 ns period, we have the nominal tuning word 0x10000.
> > The smallest is 0x1 for a difference of 0xFFFF.  This corresponds to
> > an adjustment of 0xFFFF/0x10000 = .9999847412109375 or 999984741 ppb.
> >
> > Case 2 - low frequency
> > ~~~~~~~~~~~~~~~~~~~~~~
> >
> > With a nominal 255 ns period, the nominal word is 0xFF0000, the
> > largest 0xFFFFFF, and the difference is 0xFFFF.  This corresponds to
> > and adjustment of 0xFFFF/0xFF0000 = .0039215087890625 or 3921508 ppb.
> >
> > Since 3921508 ppb is a huge adjustment, you can simply use that as a
> > safe maximum, ignoring the actual input clock.
> >
> > Thanks,
> > Richard
> >
> >
>
> Regards,
> Andrei
>

Best regards,
Rafal Ozieblo   |   Firmware System Engineer, 
phone nbr.: +48 32 5085469
www.cadence.com