[PATCH v3] i2c: rk3x: adjust the LOW divison based on characteristics of SCL
Doug Anderson
dianders at chromium.org
Tue Oct 7 23:04:52 PDT 2014
Addy,
On Tue, Oct 7, 2014 at 8:50 PM, Addy Ke <addy.ke at rock-chips.com> 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
>
> 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 formulas.
>
> Signed-off-by: Addy Ke <addy.ke at rock-chips.com>
> ---
> Changes in v2:
> - remove Fast-mode plus and HS-mode
> - use new formulas suggested by Doug
> Changes in V3:
> - use new formulas (sep 30) suggested by Doug
>
> drivers/i2c/busses/i2c-rk3x.c | 135 +++++++++++++++++++++++++++++++++++++++---
> 1 file changed, 128 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/i2c/busses/i2c-rk3x.c b/drivers/i2c/busses/i2c-rk3x.c
> index b41d979..9612eba 100644
> --- a/drivers/i2c/busses/i2c-rk3x.c
> +++ b/drivers/i2c/busses/i2c-rk3x.c
> @@ -24,6 +24,7 @@
> #include <linux/wait.h>
> #include <linux/mfd/syscon.h>
> #include <linux/regmap.h>
> +#include <linux/math64.h>
>
>
> /* Register Map */
> @@ -428,18 +429,138 @@ out:
> return IRQ_HANDLED;
> }
>
> +static void rk3x_i2c_calc_divs(unsigned long i2c_rate, unsigned long scl_rate,
> + unsigned long *div_low, unsigned long *div_high)
> +{
> + unsigned long min_low_ns, min_high_ns;
> + unsigned long max_data_hold_ns;
> + unsigned long data_hold_buffer_ns;
> + unsigned long max_low_ns, min_total_ns;
> +
> + unsigned long i2c_rate_khz, scl_rate_khz;
> +
> + unsigned long min_low_div, min_high_div;
> + unsigned long max_low_div;
> +
> + unsigned long min_div_for_hold, min_total_div;
> + unsigned long extra_div, extra_low_div, ideal_low_div;
> +
> + /* Only support standard-mode and fast-mode */
> + WARN_ON(scl_rate > 400000);
> +
> + /*
> + * min_low_ns: The minimum number of ns we need to hold low
> + * to meet i2c spec
> + * min_high_ns: The minimum number of ns we need to hold high
> + * to meet i2c spec
> + * max_low_ns: The maximum number of ns we can hold low
> + * to meet i2c spec
> + *
> + * Note: max_low_ns should be (max data hold time * 2 - buffer)
> + * This is because the i2c host on Rockchip holds the data line
> + * for half the low time.
> + */
> + if (scl_rate <= 100000) {
> + min_low_ns = 4700;
> + min_high_ns = 4000;
> + max_data_hold_ns = 3450;
> + data_hold_buffer_ns = 50;
> + } else {
> + min_low_ns = 1300;
> + min_high_ns = 600;
> + max_data_hold_ns = 900;
> + data_hold_buffer_ns = 50;
> + }
> + max_low_ns = max_data_hold_ns * 2 - data_hold_buffer_ns;
> + min_total_ns = min_low_ns + min_high_ns;
> +
> + /* Adjust to avoid overflow */
> + i2c_rate_khz = DIV_ROUND_UP(i2c_rate, 1000);
> + scl_rate_khz = DIV_ROUND_UP(scl_rate, 1000);
> +
> + /*
> + * We need the total div to be >= this number
> + * so we don't clock too fast.
> + */
> + min_total_div = DIV_ROUND_UP(i2c_rate_khz, scl_rate_khz * 8);
> +
> + /* These are the min dividers needed for min hold times.*/
nit: a space before the "*" in the comment.
> + min_low_div = DIV_ROUND_UP(i2c_rate_khz * min_low_ns, 8 * 1000000);
> + min_high_div = DIV_ROUND_UP(i2c_rate_khz * min_high_ns, 8 * 1000000);
> + min_div_for_hold = (min_low_div + min_high_div);
> +
> + /*
> + * This is the maximum divider so we don't go over the max.
> + * We don't round up here (we round down) since this is a max.
> + */
> + max_low_div = i2c_rate_khz * max_low_ns / (8 * 1000000);
> +
> + if (min_low_div > max_low_div)
It feels like we should print some sort of warning here. Maybe WARN_ONCE?
> + max_low_div = min_low_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).
> + */
> +
nit: The comment above should probably be next to the line below, not
to the line above it.
> + ideal_low_div = DIV_ROUND_UP(i2c_rate_khz * min_low_ns,
> + scl_rate_khz * 8 * min_total_ns);
> +
> + /* Don't allow it to go over the max */
> + if (ideal_low_div > max_low_div)
> + ideal_low_div = max_low_div;
> +
> + /* Handle when the ideal low div is going to take up
> + * more than we have.
> + */
nit: above comment had the wrong style still
> + if (ideal_low_div > min_low_div + extra_div)
> + ideal_low_div = min_low_div + extra_div;
> +
> + /* Give low the "ideal" and give high whatever extra is left */
> + extra_low_div = ideal_low_div - min_low_div;
> + *div_low = ideal_low_div;
> + *div_high = min_high_div + (extra_div - extra_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 = *div_low - 1;
> + *div_high = *div_high - 1;
> +}
> +
> 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 div_low, div_high;
> + u64 t_low_ns, t_high_ns;
>
> - /* 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, &div_low, &div_high);
> +
> + i2c_writel(i2c, (div_high << 16) | (div_low & 0xffff), REG_CLKDIV);
>
> - i2c_writel(i2c, (div << 16) | (div & 0xffff), REG_CLKDIV);
> + t_low_ns = div_u64(((u64)div_low + 1) * 8 * 1000000000, i2c_rate);
> + t_high_ns = div_u64(((u64)div_high + 1) * 8 * 1000000000, i2c_rate);
> + dev_dbg(i2c->dev,
> + "CLK %lukhz, Req %luns, Act low %lluns high %lluns\n",
> + i2c_rate / 1000,
> + 1000000000 / scl_rate,
> + t_low_ns, t_high_ns);
> }
>
> /**
All of my comments are just nits and this appears to match the python
script I sent out exactly. I guess that means you're happy with these
formulas now. :) Hopefully others (Max, Wolfram?) can take a look at
this now and see what they think. It's definitely long but I think it
should give us the best numbers we can get (and most of the length is
all the comments).
Maybe you can send up a v4 that addresses nits? After nits:
Reviewed-by: Doug Anderson <dianders at chromium.org>
I've tested this on my rk3288 board and i2c still seems to work fine
at 400kHz. I haven't scoped it myself, but I trust that you have.
Tested-by: Doug Anderson <dianders at chromium.org>
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