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

From: addy ke
Date: Wed Sep 24 2014 - 21:57:17 EST

Hi, Doug

On 2014/9/25 1:13, Doug Anderson wrote:
> Addy,
>
> On Wed, Sep 24, 2014 at 1:23 AM, addy ke <addy.ke@xxxxxxxxxxxxxx> wrote:
>>
>>
>> On 2014/9/24 12:10, Doug Anderson wrote:
>>> Addy,
>>>
>>> On Tue, Sep 23, 2014 at 6:55 PM, Addy Ke <addy.ke@xxxxxxxxxxxxxx> 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 <addy.ke@xxxxxxxxxxxxxx>
>>>> ---
>>>> 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)
>
>
>

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