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 23:47:46 +0300
From:   Grygorii Strashko <grygorii.strashko@...com>
To:     Richard Cochran <richardcochran@...il.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>,
        John Stultz <john.stultz@...aro.org>
Subject: Re: [PATCH 7/9] net: ethernet: ti: cpts: calc mult and shift from
 refclk freq

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