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

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Date:   Wed, 14 Sep 2016 22:26:19 +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 04:02:29PM +0300, Grygorii Strashko wrote:
> +static void cpts_calc_mult_shift(struct cpts *cpts)
> +{
> +	u64 maxsec;
> +	u32 freq;
> +	u32 mult;
> +	u32 shift;
> +	u64 ns;
> +	u64 frac;
> +
> +	if (cpts->cc_mult || cpts->cc.shift)
> +		return;
> +
> +	freq = clk_get_rate(cpts->refclk);
> +
> +	/* Calc the maximum number of seconds which we can run before
> +	 * wrapping around.
> +	 */
> +	maxsec = cpts->cc.mask;
> +	do_div(maxsec, freq);
> +	if (!maxsec)
> +		maxsec = 1;

This doesn't pass the smell test.  If the max counter value is M, you
are figuring M*1/F which is the time in seconds corresponding to M.
We set M=2^32-1, and so 'freq' would have to be greater than 4 GHz in
order for 'maxsec' to be zero.  Do you really expect such high
frequency input clocks?

> +	else if (maxsec > 600 && cpts->cc.mask > UINT_MAX)
> +		maxsec = 600;

What is this all about?  cc.mask is always 2^32 - 1.

> +	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.

Thanks,
Richard

> +	cpts->cc.shift = shift;
> +	/* Check calculations and inform if not precise */
> +	frac = 0;
> +	ns = cyclecounter_cyc2ns(&cpts->cc, freq, cpts->cc.mask, &frac);
> +
> +	dev_info(cpts->dev,
> +		 "CPTS: ref_clk_freq:%u calc_mult:%u calc_shift:%u error:%lld nsec/sec\n",
> +		 freq, cpts->cc_mult, cpts->cc.shift, (ns - NSEC_PER_SEC));
> +}