Subject: Re: [PATCH net-next v4 1/2] qed: Add infrastructure for PTP support.

From: Richard Cochran <[email-masked]>
Date: Sun, 12 Feb 2017 15:02:02 +0100

On Sun, Feb 12, 2017 at 11:27:16AM +0000, Mintz, Yuval wrote:
> Richard, there are quite a bit of inaccuracies in the calculation here.

Where?

If you compare this algorithm with yours, you will discover that it
produces significantly lower error for ppm < 60.

> Your suggestion seems to:
>   a. Assume that the required period should be in ns, not in
>       16*ns units.
>   b. mishandles the +8/-8 in the calculation.
>   c. Doesn't seem to consider the upper bound on period.

Duh, you would have to convert the result into the proper form for the
HW register and add bounds checking.  I mean, that goes without saying.
The important fact is that your algorithm it not optimal for ppm < 60.

(I assumed that the -8 thing was a typical HW programming effect,
where you dial N-1 to get N.  The fact that you add 8 back in to
calculate the effective ppb confirms that assumption.  If this isn't
the case, then maybe you can see a way to adapt what I wrote.)

> One thing I still don't get is *why* we're trying to optimize this
> area of the code -

So you prefer using 21 64-bit divisions when using 8 produces better
results?

*You* need to explain the "why"...

Thanks,
Richard