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

[Dirk Behme]

Am 10.05.2013 20:44, schrieb 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

Ah, yes, thanks.

> Also, I'd change s32 to unsigned or u32.
>
> But  usually the if will be true, so why keep it?

For cases where clk_src <= max_hz. Then don't do the calculation 
because it's unnecessary.

> 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
>
>