linux-kernel - Re: [PATCH] clk:mxs: Fix bug on frequency divider

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Date:	Wed, 19 Aug 2015 12:29:58 +0200
From:	victorien.vedrine@...rys.net
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	linux-kernel@...r.kernel.org, linux-clk@...r.kernel.org,
	mturquette@...libre.com, Shawn Guo <shawn.guo@...aro.org>
Subject: Re: [PATCH] clk:mxs: Fix bug on frequency divider

I'll modify the cast and propose a new version of patch.
This patch is specific for imx28 processor and for this one there is no 
mult_frac()
function. I checked the other functions of imx28 clock part and I didn't 
see a
similar problem.


Le 2015-07-31 19:34, Stephen Boyd a écrit :
> +Shawn
> 
> On 07/31/2015 09:24 AM, Victorien Vedrine wrote:
>> On drivers/clk/mxs/clk-frac.c, the function clk_frac_round_rate 
>> returned a bad
>> result. The division before multiplication computes a wrong value ; 
>> the
>> calculation is inverted to fix the problem. The second issue is that 
>> the exact
>> rate have decimals and they are truncate. The consequence is that the 
>> function
>> clk_frac_set_rate (which use the result of clk_frac_round_rate) 
>> computes a
>> wrong value for the register (the rate generated can be closer to the 
>> desired
>> rate). The correction is : if there is decimal to the result, it is 
>> rounded to
>> the next larger integer.
>> 
>> On drivers/clk/mxs/clk-frac.c, the function clk_frac_recalc_rate 
>> returned
>> a bad result. The multiplication is made before the division to 
>> compute a
>> correct value.
>> 
>> The issue is reproducible by this way : Set the SAIF frequency to 
>> 22579200Hz
>> (it's the appropriate frequency for 44100hz sample rate for SGTL5000 
>> codec).
>> With the divider register (HW_CLKCTRL_SAIF0), the closest lower value 
>> is
>> 22573242.1875Hz (0xC0A on register). The original clk-frac functions 
>> give 0xC09
>> on the divider register.
>> 
>> Signed-off-by: Victorien Vedrine <victorien.vedrine@...rys.net>
>> ---
>>   drivers/clk/mxs/clk-frac.c | 13 ++++++++++---
>>   1 file changed, 10 insertions(+), 3 deletions(-)
>> 
>> diff --git a/drivers/clk/mxs/clk-frac.c b/drivers/clk/mxs/clk-frac.c
>> index e6aa6b5..d51cf03 100644
>> --- a/drivers/clk/mxs/clk-frac.c
>> +++ b/drivers/clk/mxs/clk-frac.c
>> @@ -42,11 +42,13 @@ static unsigned long clk_frac_recalc_rate(struct 
>> clk_hw *hw,
>>   {
>>   	struct clk_frac *frac = to_clk_frac(hw);
>>   	u32 div;
>> +	u64 tmp_rate;
>>     	div = readl_relaxed(frac->reg) >> frac->shift;
>>   	div &= (1 << frac->width) - 1;
>>   -	return (parent_rate >> frac->width) * div;
>> +	tmp_rate = (u64)parent_rate * (u64)div;
> 
> We shouldn't need to cast both to 64 bits...
> 
>> +	return (unsigned long)(tmp_rate >> frac->width);
> 
> Isn't the cast implicit by means of the return type of this function?
> 
>>   }
>>     static long clk_frac_round_rate(struct clk_hw *hw, unsigned long 
>> rate,
>> @@ -55,7 +57,7 @@ static long clk_frac_round_rate(struct clk_hw *hw, 
>> unsigned long rate,
>>   	struct clk_frac *frac = to_clk_frac(hw);
>>   	unsigned long parent_rate = *prate;
>>   	u32 div;
>> -	u64 tmp;
>> +	u64 tmp, tmp_rate, result;
>>     	if (rate > parent_rate)
>>   		return -EINVAL;
>> @@ -68,7 +70,12 @@ static long clk_frac_round_rate(struct clk_hw *hw, 
>> unsigned long rate,
>>   	if (!div)
>>   		return -EINVAL;
>>   -	return (parent_rate >> frac->width) * div;
>> +	tmp_rate = (u64)parent_rate * (u64)div;
>> +	result = (u64)(tmp_rate >> frac->width);
> 
> Cast looks useless here too.
> 
>> +	if ((result << frac->width) < tmp_rate)
>> +		return result + 1;
>> +	else
>> +		return result;
> 
> The else could be dropped and just be return result. Also, have you
> see mult_frac()? I suppose we're doing shifts with width because it's
> a power-of-2 denominator?

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