On 09/14/2016 11:26 PM, Richard Cochran wrote:
> 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?
no. can drop it.
>
>> + else if (maxsec > 600 && cpts->cc.mask > UINT_MAX)
>> + maxsec = 600;
>
> What is this all about? cc.mask is always 2^32 - 1.
Oh. Not sure if we will update CPTS to support this, but on
KS2 E/L (66AK2E) CPTS counter can work in 64bit mode.
>
>> + 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.
As I understand (and tested), clocks_calc_mult_shift() will
return max possible mult which can be used without overflow.
if calculated results do not satisfy end user - the custom values can
be passed in DT.
--
regards,
-grygorii
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