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

[Troy Kisky]

On 5/9/2013 10:34 PM, Dirk Behme wrote:
> Am 09.05.2013 20:00, schrieb Troy Kisky:
>> 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 at boundarydevices.com>
>>> Signed-off-by: Dirk Behme <dirk.behme at 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.
>
> Hmm, why is it not needed?
>
> If you remove the if, you *always* do at least the computation
>
> pre_div = DIV_ROUND_UP(clk_src, max_hz);
> post_div = fls(pre_div - 1);
> if (post_div > 4) {
>
> I would think that doing the initialization and the if is much cheaper 
> than always doing above computation, even if its not needed? I would 
> keep the if.
>
>>>       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);
>>> +        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);
>
> Well, where to do the -1 mainly depends on how we want to interpret 
> the output of
>
> debug("pre_div = %d, post_div=%d\n", pre_div, post_div);
I'd change to

debug("actual div = %d, pre_div = %d, post_div=%d\n", (pre_div + 1) << 
post_div, pre_div, post_div);

to eliminate confusion.


>
> There are two options how to understand this, at least the pre_div = 
> %d part:
>
> a) print the pre_div as the real divider used in a human readable 
> format. I.e. if we divide by /15 then print 15
>
> or
>
> b) print the pre_div value we write to the register. I.e. if we divide 
> by /15 then print 14
>
> Up to now we are doing (a) and calculate the register value after the 
> debug print. Your proposal would switch this to (b). Anyway, if it 
> makes the algorithm simpler I'd agree that it's fine to switch to (b).
>
> To summarize, this would become then:
>
> s32 pre_div = 1, post_div = 0, reg_ctrl, reg_config;
>
> ...

If you keep the "if", pre_div = 0
Also, I'd change s32 to unsigned or u32.

But  usually the if will be true, so why keep it?
It is just code bloat and prone to errors like yours above.

The only logic difference between keeping/killing the "if" is when
clk_src == 0.  Keeping will give a divide by 1. Killing will give an error.
I'd argue that an error is more appropriate.

>
> if (clk_src > max_hz) {
>     pre_div = (clk_src - 1) / max_hz;
>     /* fls(1) = 1, fls(0x80000000) = 32, fls(16) = 5 */
>     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;
>         }
>         pre_div >>= post_div;
>     } else {
>         post_div = 0;
>     }
> }
>
> ...
>
> MXC_CSPICTRL_PREDIV(pre_div);
>
> Is this correct?
>
> Best regards
>
> Dirk