[U-Boot] [PATCH 2/2] spi: mxc_spi: Update pre and post divider algorithm

[Troy Kisky <troy.kisky@boundarydevices.com>]

On 5/9/2013 11:00 AM, Troy Kisky wrote:
> On 5/8/2013 10:19 PM, Dirk Behme wrote:
>> The spi clock divisor is of the form x * (2**y),  or  x  << y, where 
>> x is
>> 1 to 16, and y is 0 to 15. Note the similarity with floating point 
>> numbers.
>> Convert the desired divisor to the smallest number which is >= 
>> desired divisor,
>> and can be represented in this form. The previous algorithm chose a 
>> divisor
>> which could be almost twice as large as needed.
>>
>> Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
>> Signed-off-by: Dirk Behme <dirk.behme@gmail.com>
>> ---
>>   drivers/spi/mxc_spi.c |   27 ++++++++++++---------------
>>   1 file changed, 12 insertions(+), 15 deletions(-)
>>
>> diff --git a/drivers/spi/mxc_spi.c b/drivers/spi/mxc_spi.c
>> index 3e903b3..66c2ad8 100644
>> --- a/drivers/spi/mxc_spi.c
>> +++ b/drivers/spi/mxc_spi.c
>> @@ -128,7 +128,7 @@ static s32 spi_cfg_mxc(struct mxc_spi_slave 
>> *mxcs, unsigned int cs,
>>           unsigned int max_hz, unsigned int mode)
>>   {
>>       u32 clk_src = mxc_get_clock(MXC_CSPI_CLK);
>> -    s32 pre_div = 1, post_div = 0, i, reg_ctrl, reg_config;
>> +    s32 pre_div = 1, post_div = 0, reg_ctrl, reg_config;
>
> First, I'm totally fine with the patch as it is. I'm just going to 
> point out things you may want to change, or
> send a follow-up patch.
>
> Here, no need to initialize pre_div, post_div, if you delete the if 
> (clk_src > max_hz) below which is not needed.
>
>>       u32 ss_pol = 0, sclkpol = 0, sclkpha = 0;
>>       struct cspi_regs *regs = (struct cspi_regs *)mxcs->base;
>>   @@ -147,27 +147,24 @@ static s32 spi_cfg_mxc(struct mxc_spi_slave 
>> *mxcs, unsigned int cs,
>>       reg_ctrl |=  MXC_CSPICTRL_EN;
>>       reg_write(&regs->ctrl, reg_ctrl);
>>   -    /*
>> -     * The following computation is taken directly from Freescale's 
>> code.
>> -     */
>>       if (clk_src > max_hz) {
> This "if" can be removed.
>
>>           pre_div = DIV_ROUND_UP(clk_src, max_hz);
> If you subtract -1 here instead of when you set the divisor register, 
> the logic becomes simpler
>
>
> pre_div = DIV_ROUND_UP(clk_src, max_hz) - 1;
>
> or just
>
> pre_div = (clk_src - 1) / max_hz;
>
>> -        if (pre_div > 16) {
>> -            post_div = pre_div / 16;
>> -            pre_div = 16;
>> -        }
>> -        if (post_div != 0) {
>> -            for (i = 0; i < 16; i++) {
>> -                if ((1 << i) >= post_div)
>> -                    break;
>> -            }
>> -            if (i == 16) {
>> +        /* fls(1) = 1, fls(0x80000000) = 32, fls(16) = 5 */
>> +        post_div = fls(pre_div - 1);

oops, this would also become
post_div = fls(pre_div);

>> +        if (post_div > 4) {
>> +            post_div -= 4;
>> +
>> +            if (post_div >= 16) {
>>                   printf("Error: no divider for the freq: %d\n",
>>                       max_hz);
>>                   return -1;
>>               }
>> -            post_div = i;
>> +            pre_div = (pre_div + (1 << post_div) - 1) >> post_div;
>
> This would become
> pre_div >>= post_div;
>> +
>> +        } else {
>> +            post_div = 0;
>>           }
>> +
>>       }
>>         debug("pre_div = %d, post_div=%d\n", pre_div, post_div);
>
> And
>
> MXC_CSPICTRL_PREDIV(pre_div - 1);
>
> would return to
>
> MXC_CSPICTRL_PREDIV(pre_div);
>
>
> Troy
>