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));
> +}
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