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SubjectRe: [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@ophrys.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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