Subject: [PATCH 1/1] ARM iMX28: incorrect frac/div calculation

From: [email-masked] (Matt Burtch)
Date: Fri, 23 Mar 2012 11:38:41 -0700

On 03/22/2012 07:52 PM, Shawn Guo wrote:
> On Thu, Mar 22, 2012 at 04:05:01PM -0700, Matt Burtch wrote:
>> The frac and div values in _CLK_SET_RATE are calculated incorrectly
>> for some clock rates ie. CPU clk of 454.73MHz, 392.72MHz, 261.81Mhz,
>> etc.
>>
>> Now the frac and div values which result in the closest actual clock
>> rate to that requested is selected.
> My understanding on clk_set_rate is we need to set the clk to a rate
> closest to the requested one but never exceeding it.  Does your change
> ensure that?
>
> Regards,
> Shawn

It currently doesn't check that the resultant clock is lower than the 
requested.  I can certainly make that change and resubmit.  I wasn't 
aware of this requirement.

So with the patch as it stands, if you request a CPU clock of 391Mhz, 
it'll give you 392.71MHz. Would it be correct for the clock to be set at 
375.65MHz (the next closest valid clock rate without going over)?

Thanks,
Matt

>> Also there are no limitations
>> on what frac values can be used, as long as they fall in the valid
>> (18<  frac<  35) range.
>>
>> Tested on custom iMX28 board.
>>
>> Signed-off-by: Matt Burtch<[email-masked]>
>> ---
>>   arch/arm/mach-mxs/clock-mx28.c |    8 ++------
>>   1 files changed, 2 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/arm/mach-mxs/clock-mx28.c b/arch/arm/mach-mxs/clock-mx28.c
>> index 5d68e41..a058b99 100644
>> --- a/arch/arm/mach-mxs/clock-mx28.c
>> +++ b/arch/arm/mach-mxs/clock-mx28.c
>> @@ -354,19 +354,15 @@ static int name##_set_rate(struct clk *clk, unsigned long rate)		\
>>   		}							\
>>   		for (d = 1; d<= div_max; d++) {			\
>>   			f = parent_rate * 18 / d / rate;		\
>> -			if ((parent_rate * 18 / d) % rate)		\
>> -				f++;					\
>>   			if (f<  18 || f>  35)				\
>>   				continue;				\
>>   									\
>>   			calc_rate = parent_rate * 18 / f / d;		\
>> -			if (calc_rate>  rate)				\
>> -				continue;				\
>>   									\
>> -			if (rate - calc_rate<  diff) {			\
>> +			if (abs(rate - calc_rate)<  diff) {		\
>>   				frac = f;				\
>>   				div = d;				\
>> -				diff = rate - calc_rate;		\
>> +				diff = abs(rate - calc_rate);		\
>>   			}						\
>>   									\
>>   			if (diff == 0)					\
>> -- 
>> 1.7.5.4
>>