Subject: [PATCH] i2c: rk3x: adjust the LOW divison based on characteristics of SCL

From: [email-masked] (addy ke)
Date: Wed, 24 Sep 2014 16:23:47 +0800

On 2014/9/24 12:10, Doug Anderson wrote:
> Addy,
> 
> On Tue, Sep 23, 2014 at 6:55 PM, Addy Ke <[email-masked]> wrote:
>> As show in I2C specification:
>> - Standard-mode:
>>   the minimum HIGH period of the scl clock is 4.0us
>>   the minimum LOW period of the scl clock is 4.7us
>> - Fast-mode:
>>   the minimum HIGH period of the scl clock is 0.6us
>>   the minimum LOW period of the scl clock is 1.3us
>> - Fast-mode plus:
>>   the minimum HIGH period of the scl clock is 0.26us
>>   the minimum LOW period of the scl clock is 0.5us
>> - HS-mode(<1.7MHz):
>>   the minimum HIGH period of the scl clock is 0.12us
>>   the minimum LOW period of the scl clock is 0.32us
>> - HS-mode(<3.4MHz):
>>   the minimum HIGH period of the scl clock is 0.06us
>>   the minimum LOW period of the scl clock is 0.16us
>>
>> I have measured i2c SCL waveforms in fast-mode by oscilloscope
>> on rk3288-pinky board. the LOW period of the scl clock is 1.3us.
>> It is so critical that we must adjust LOW division to increase
>> the LOW period of the scl clock.
>>
>> Thanks Doug for the suggestion about division formula.
>>
>> Signed-off-by: Addy Ke <[email-masked]>
>> ---
>>  drivers/i2c/busses/i2c-rk3x.c | 79 +++++++++++++++++++++++++++++++++++++++----
>>  1 file changed, 72 insertions(+), 7 deletions(-)
>>
>> diff --git a/drivers/i2c/busses/i2c-rk3x.c b/drivers/i2c/busses/i2c-rk3x.c
>> index 93cfc83..49d67b7 100644
>> --- a/drivers/i2c/busses/i2c-rk3x.c
>> +++ b/drivers/i2c/busses/i2c-rk3x.c
>> @@ -428,18 +428,83 @@ out:
>>         return IRQ_HANDLED;
>>  }
>>
>> +static void rk3x_i2c_get_ratios(unsigned long scl_rate,
>> +                               unsigned long *high_ratio,
>> +                               unsigned long *low_ratio)
>> +{
>> +       /* As show in I2C specification:
>> +        * - Standard-mode:
>> +        *   the minimum HIGH period of the scl clock is 4.0us
>> +        *   the minimum LOW period of the scl clock is 4.7us
>> +        * - Fast-mode:
>> +        *   the minimum HIGH period of the scl clock is 0.6us
>> +        *   the minimum LOW period of the scl clock is 1.3us
>> +        * - Fast-mode plus:
>> +        *   the minimum HIGH period of the scl clock is 0.26us
>> +        *   the minimum LOW period of the scl clock is 0.5us
>> +        * - HS-mode(<1.7MHz):
>> +        *   the minimum HIGH period of the scl clock is 0.12us
>> +        *   the minimum LOW period of the scl clock is 0.32us
>> +        * - HS-mode(<3.4MHz):
>> +        *   the minimum HIGH period of the scl clock is 0.06us
>> +        *   the minimum LOW period of the scl clock is 0.16us
> 
> Is the rest of the driver ready for Fast-mode plus or HS mode?  If not
> then maybe leave those off?  If nothing else the commit message should
> indicate that this is just being forward thinking.
> 
>> +        */
>> +       if (scl_rate <= 100000) {
>> +               *high_ratio = 40;
>> +               *low_ratio = 47;
>> +       } else if (scl_rate <= 400000) {
>> +               *high_ratio = 6;
>> +               *low_ratio = 13;
>> +       } else if (scl_rate <= 1000000) {
>> +               *high_ratio = 26;
>> +               *low_ratio = 50;
>> +       } else if (scl_rate <= 1700000) {
>> +               *high_ratio = 12;
>> +               *low_ratio = 32;
>> +       } else {
>> +               *high_ratio = 6;
>> +               *low_ratio = 16;
> 
> Since it's only the ratio of high to low that matters, you can combine
> the last two.  12 : 32 == 6 : 16
> 
>> +       }
>> +}
>> +
>> +static void rk3x_i2c_calc_divs(unsigned long i2c_rate, unsigned long scl_rate,
>> +                              unsigned long *divh, unsigned long *divl)
>> +{
>> +       unsigned long high_ratio, low_ratio;
>> +       unsigned long ratio_sum;
>> +
>> +       rk3x_i2c_get_ratios(scl_rate, &high_ratio, &low_ratio);
>> +       ratio_sum = high_ratio + low_ratio;
>> +
>> +       /* T_high = T_clk * (divh + 1) * 8
>> +        * T_low = T_clk * (divl + 1) * 8
>> +        * T_scl = T_high + T_low
>> +        * T_scl = 1 / scl_rate
>> +        * T_clk = 1 / i2c_rate
>> +        * T_high : T_low = high_ratio : low_ratio
>> +        * ratio_sum = high_ratio + low_ratio
>> +        *
>> +        * so:
>> +        * divh = (i2c_rate * high_ratio) / (scl_rate * ratio_sum * 8) - 1
>> +        * divl = (i2c_rate * low_ratio) / (scl_rate * ratio_sum * 8) - 1
>> +        */
>> +       *divh = DIV_ROUND_UP(i2c_rate * high_ratio, scl_rate * ratio_sum * 8);
>> +       if (*divh)
>> +               *divh = *divh - 1;
>> +
>> +       *divl = DIV_ROUND_UP(i2c_rate * low_ratio, scl_rate * ratio_sum * 8);
>> +       if (*divl)
>> +               *divl = *divl - 1;
> 
> When I sent you the sample formulas I purposely did it differently
> than this.  Any reason you changed from my formulas?
> 
>   div_low = DIV_ROUND_UP(clk_rate * low_ratio, scl_rate * 8 * ratio_sum)
>   div_high = DIV_ROUND_UP(clk_rate, scl_rate * 8) - div_low
> 
>   div_low -= 1
>   if div_high:
>     div_high -= 1
> 
> Why did I do it that way?
> 
> * Assuming i2c_rate and the ratio is non-zero then you can assume that
> DIV_ROUND_UP gives a value that is >= 1.  No need to test the result
> against 0.
> 
> * (I think) you'll get a more accurate clock rate by subtracting.
> 
> Try running your formula vs. my formula with a ratio of 13 : 6, an i2c
> rate of 12800000, and an scl rate of 400000
> 
> Mine will get:
>   Req = 400000, act = 400000, 1.88 us low, 0.62 us high, low/high = 3.00
> 
> Yours will get:
>   Req = 400000, act = 320000, 1.88 us low, 1.25 us high, low/high = 1.50
> 
yes, you are right. yours is closer to the scl clock what we want to set.

But if (clk_rate * low_ratio) can not be divisible by (scl_rate * 8 * ratio_sum),
div_low will be round up, and div _high will be round down.
The gap between div_low and div_high is increased.

so maybe we can set:
div_high = DIV_ROUND_UP(clk_rate * high_ratio, scl_rate * 8 * ratio_sum)
div_low = DIV_ROUND_UP(clk_rate, scl_rate * 8) - div_low

i2c rate is 128Mhz:
1) calculate div_high first:
div_high = DIV_ROUND_UP(clk_rate * high_ratio, scl_rate * 8 * ratio_sum)
div_low = DIV_ROUND_UP(clk_rate, scl_rate * 8) - div_low

req = 400000, act = 400000, div_high = 13, div_low = 27

2) calculate div_low first:
div_low = DIV_ROUND_UP(clk_rate * low_ratio, scl_rate * 8 * ratio_sum)
div_high = DIV_ROUND_UP(clk_rate, scl_rate * 8) - div_low

req = 400000, act = 400000, div_high = 12, div_high = 28

I think that the first is more appropriate.

> 
>> +}
>> +
>>  static void rk3x_i2c_set_scl_rate(struct rk3x_i2c *i2c, unsigned long scl_rate)
>>  {
>>         unsigned long i2c_rate = clk_get_rate(i2c->clk);
>> -       unsigned int div;
>> +       unsigned long divh, divl;
>>
>> -       /* set DIV = DIVH = DIVL
>> -        * SCL rate = (clk rate) / (8 * (DIVH + 1 + DIVL + 1))
>> -        *          = (clk rate) / (16 * (DIV + 1))
>> -        */
>> -       div = DIV_ROUND_UP(i2c_rate, scl_rate * 16) - 1;
>> +       rk3x_i2c_calc_divs(i2c_rate, scl_rate, &divh, &divl);
>>
>> -       i2c_writel(i2c, (div << 16) | (div & 0xffff), REG_CLKDIV);
>> +       i2c_writel(i2c, (divh << 16) | (divl & 0xffff), REG_CLKDIV);
>>  }
>>
>>  /**
>> --
>> 1.8.3.2
>>
>>
> 
> 
>