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

From: [email-masked] (addy ke)
Date: Thu, 25 Sep 2014 09:56:43 +0800

Hi, Doug

On 2014/9/25 1:13, Doug Anderson wrote:
> Addy,
> 
> On Wed, Sep 24, 2014 at 1:23 AM, addy ke <[email-masked]> wrote:
>>
>>
>> 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.
> 
> Is that important?  As far as I can tell as long as we are meeting the
> minimum requirements for low and high hold times then we're OK.
> 
> 
In my measurement,all paramter but "Data hold time" are match the characteristics of SCL bus line.
the measured value is 0.928us("data hold time on RK3X"  ~=  "the low period / 2")
but the maximum value described in table is 0.9us

About "Data hold time", there are described in I2C specification:
- for CBUS compatible masters for I2C-bus deivices
- the maximum data hold time has only be met if the device does not stretch the LOW period of the SCL signal.

I have tested on RK3288-Pinky board, there are no error.
But I don't known whether this paramter will affect i2c communications.

>> 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.
> 
> I guess I was biasing towards making low a little longer instead of
> biasing towards keeping high and low the same.  I'm not sure the bias
> is terribly critical.
> 
> In any case, I spent more time testing and I realized that my old
> formulas could potentially violate hold times because I wasn't careful
> enough.  I've crafted some new code that is more careful to ensure
> _both_ minimum hold times and minimum clock rate times.
> 
> Do you want to spin your patch with these new formulas, or do you
> think I should?
> 
I think the new formulas is reasonable, so I will send patch v2 with it today.
Thank you very much.
> 
> (anyone else on the thread listening, feel free to comment).
> 
> ---
> 
> def test_it(min_low_ns, min_high_ns, clk_rate, scl_rate):
>   min_total_ns = min_low_ns + min_high_ns
> 
>   # We need the total div to be >= this number so we don't clock too fast.
>   min_total_div = DIV_ROUND_UP(clk_rate, scl_rate * 8);
> 
>   # These are the min dividers needed for hold times.
>   min_low_div = DIV_ROUND_UP(clk_rate * min_low_ns, 8 * 1000000000)
>   min_high_div = DIV_ROUND_UP(clk_rate * min_high_ns, 8 * 1000000000)
>   min_div_for_hold = (min_low_div + min_high_div)
> 
>   if min_div_for_hold > min_total_div:
>     # Time needed to meet hold requirements is important.  Just use that
>     div_low = min_low_div
>     div_high = min_high_div
>   else:
>     # We've got to distribute some time among the low and high so we
>     # don't run too fast.
>     extra_div = min_total_div - min_div_for_hold
> 
>     # We'll try to split things up perfectly evenly, biasing slightly
>     # towards having a higher div for low (spend more time low).
>     ideal_low_div = DIV_ROUND_UP(clk_rate * min_low_ns,
>                                  scl_rate * 8 * min_total_ns)
> 
>     # Handle when the ideal low div is going to take up more than we have
>     if ideal_low_div > min_low_div + extra_div:
>       assert ideal_low_div == min_low_div + extra_div + 1
>       ideal_low_div = min_low_div + extra_div
> 
>     # Give low the "ideal" and give high whatever extra is left.
>     div_low = ideal_low_div
>     div_high = min_high_div + (extra_div - (ideal_low_div - min_low_div))
> 
>   # Adjust to the fact that the hardware has an implicit "+1".
>   # NOTE: Above calculations always produce div_low > 0 and  div_high > 0.
>   div_low -= 1
>   div_high -= 1
> 
>   T_pclk_us = 1000000. / clk_rate
>   T_sclk_us = 1000000. / scl_rate
> 
>   T_low_us = T_pclk_us * (div_low + 1) * 8
>   T_high_us = T_pclk_us * (div_high + 1) * 8
> 
>   T_tot_us = (T_high_us + T_low_us)
>   freq = 1000000. / T_tot_us
> 
>   if T_low_us * 1000 < min_low_ns:
>     print "ERROR: not low long enough"
>   if T_high_us * 1000 < min_high_ns:
>     print "ERROR: not high long enough"
> 
>   print "CLK = %d, Req = %d, act = %.2f, %.2f us low, " \
>         "%.2f us high, low/high = %.2f" % (
>         clk_rate, scl_rate, freq, T_low_us,
>         T_high_us, T_low_us / T_high_us)
> 
>   return (clk_rate, scl_rate, freq, T_low_us, T_high_us)
> 
> test_it(4700, 4000, 1484000, 100000)
> test_it(4700, 4000, 2000001, 100000)
> test_it(4700, 4000, 74250000, 99799)
> test_it(4700, 4000, 74250000, 99798)
> test_it(4700, 4000, 74250000, 99797)
> test_it(4700, 4000, 74250000, 100000)
> 
> test_it(1300, 600,   5000000, 400000)
> test_it(1300, 600,   9400000, 400000)
> 
> test_it(1300,  600, 74250000, 400000)
> test_it(1300,  600, 12800000, 400000)
> test_it(1300,  600,  6400000, 400000)
> test_it(1300,  600,  3200000, 400000)
> test_it(1300,  600,  1600000, 400000)
> test_it(1300,  600,   800000, 400000)
> 
> for i in xrange(800000, 74250000, 100):
>   test_it(4700, 4000, i, 100000)
> 
> for i in xrange(800000, 74250000, 100):
>   test_it(1300, 600, i, 400000)
> 
> 
>