linux-kernel - Aw: Re: [PATCH v2] clk: si5351: fix .round

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Date:	Wed, 8 Apr 2015 16:00:08 +0200
From:	"S. S." <ce3a@....de>
To:	"Sebastian Hesselbarth" <sebastian.hesselbarth@...il.com>,
	mturquette@...aro.org, sboyd@...eaurora.org
Cc:	linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Aw: Re: [PATCH v2] clk: si5351: fix .round_rate for multisynth 6-7

>> @@ -645,7 +646,7 @@ static long si5351_msynth_round_rate(struct clk_hw *hw, unsigned long rate,
>> struct si5351_hw_data *hwdata =
>> container_of(hw, struct si5351_hw_data, hw);
>> unsigned long long lltmp;
>> - unsigned long a, b, c;
>> + unsigned long a, b = 0, c = 1;
>
> Actually, moving b,c initialization up here is neither related
> to the patch itself nor is it mentioned in the commit log.
>
> Please do not mix different patches.

I agree. I just thought we could avoid repeating the b,c initialization
in the code, but You are right, that would be a different patch, if any 
at all.

>> *parent_rate = a * rate;
>
>> + if (rfrac)
>> + rational_best_approximation(rfrac, denom,
>> + SI5351_MULTISYNTH_B_MAX,
>> + SI5351_MULTISYNTH_C_MAX,
>> + &b, &c);
>> + }
>
> I am still not convinced that this is the right way to calculate the
> _best_ integer divider for ms6,7.
>
> The code above is written to (a) find the _largest_ integer "a" that
> will match
>
> a * rate <= parent_rate
>
> and then (b) determines the fractional part of the divider.
>
> As you correctly stated, ms6,7 do not support (b) but if we use (a) for
> those msynths, we will determine an "a" so that "a * rate" is always
> smaller than parent_rate.
> 
> What we actually want is the smallest error between generated and
> requested rate, so we have to find the closest integer with
> 
> a = DIV_ROUND_CLOSEST(parent_rate, rate)

Agreed, that's probably better.

> IMHO, the special divider calculation for ms6,7 is best dealt with
> in an extra else-if branch after the check for CLK_SET_RATE_PARENT
> flag.

Agreed.

> hwdata->params.p3 = 0;
> hwdata->params.p2 = 0;

Agreed.

> Out of curiosity, do you actually have a device that uses ms6,7 and can
> you measure the generated frequency - or did you just read the code as
> a bedtime story? ;)

:) I do have a device and have to use ms6,7, those are connected to pllb.
I have tested some integer pll dividers in a range from 30 to 130.
Measured frequencies and the frequencies shown in <debugfs>/clk/clk_summery
are the same.

I am going to send a patch v3.
Thanks for your feedback.

Sergej
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