linux-kernel - Re: [PATCH 7/9] net: ethernet: ti: cpts: calc mult and shift from refclk freq

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Date:   Thu, 15 Sep 2016 13:58:15 +0200
From:   Richard Cochran <richardcochran@...il.com>
To:     Grygorii Strashko <grygorii.strashko@...com>
Cc:     "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Mugunthan V N <mugunthanvnm@...com>,
        Sekhar Nori <nsekhar@...com>, linux-kernel@...r.kernel.org,
        linux-omap@...r.kernel.org, WingMan Kwok <w-kwok2@...com>
Subject: Re: [PATCH 7/9] net: ethernet: ti: cpts: calc mult and shift from
 refclk freq

On Wed, Sep 14, 2016 at 10:26:19PM +0200, Richard Cochran wrote:
> On Wed, Sep 14, 2016 at 04:02:29PM +0300, Grygorii Strashko wrote:
> > +	clocks_calc_mult_shift(&mult, &shift, freq, NSEC_PER_SEC, maxsec);
> > +
> > +	cpts->cc_mult = mult;
> > +	cpts->cc.mult = mult;
> 
> In order to get good resolution on the frequency adjustment, we want
> to keep 'mult' as large as possible.  I don't see your code doing
> this.  We can rely on the watchdog reader (work queue) to prevent
> overflows.

I took a closer look, and assuming cc.mask = 2^32 - 1, then using
clocks_calc_mult_shift() produces good results for a reasonable range
of input frequencies.  Keeping 'maxsec' constant at 4 we have:

   | Freq. MHz |       mult | shift |
   |-----------+------------+-------|
   |       100 | 0xa0000000 |    28 |
   |       250 | 0x80000000 |    29 |
   |       500 | 0x80000000 |    30 |
   |      1000 | 0x80000000 |    31 |

Can the input clock be higher than 1 GHz?  If not, I suggest using
clocks_calc_mult_shift() with maxsec=4 and a setting the watchdog also
to 4*HZ.

Thanks,
Richard