[v4] i2c: rk3x: adjust the LOW divison based on characteristics of SCL
diff mbox

Message ID 1412837235-4202-1-git-send-email-addy.ke@rock-chips.com
State New, archived
Headers show

Commit Message

addy ke Oct. 9, 2014, 6:47 a.m. UTC
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.

Reviewed-by: Doug Anderson <dianders@chromium.org>
Tested-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Addy Ke <addy.ke@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
Changes in V3:
- fix some wrong style
- WARN_ONCE if min_low_div > max_low_div

 drivers/i2c/busses/i2c-rk3x.c | 140 +++++++++++++++++++++++++++++++++++++++---
 1 file changed, 133 insertions(+), 7 deletions(-)

Comments

Heiko Stuebner Oct. 11, 2014, 3:49 p.m. UTC | #1
Am Donnerstag, 9. Oktober 2014, 14:47:15 schrieb Addy Ke:
> 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.
> 
> Reviewed-by: Doug Anderson <dianders@chromium.org>
> Tested-by: Doug Anderson <dianders@chromium.org>
> Signed-off-by: Addy Ke <addy.ke@rock-chips.com>

On rk3288, rk3188 and rk3066 (= all currently supported SoCs)

Tested-by: Heiko Stuebner <heiko@sntech.de>

My i2c knowledge sadly is not sufficient enough to provide a meaningful
review, but perhaps Max can provide some.


Heiko

> ---
> 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
> Changes in V3:
> - fix some wrong style
> - WARN_ONCE if min_low_div > max_low_div
> 
>  drivers/i2c/busses/i2c-rk3x.c | 140
> +++++++++++++++++++++++++++++++++++++++--- 1 file changed, 133
> insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/i2c/busses/i2c-rk3x.c b/drivers/i2c/busses/i2c-rk3x.c
> index b41d979..f17f7a2 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 +429,143 @@ 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. */
> +	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) {
> +		WARN_ONCE(true,
> +			  "Conflicting, min_low_div %lu, max_low_div %lu\n",
> +			  min_low_div, max_low_div);
> +		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).
> +		 */
> +		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.
> +		 */
> +		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);
>  }
> 
>  /**
Max Schwarz Oct. 12, 2014, 12:10 p.m. UTC | #2
Hi Addy,

sorry for the long delay, I finally had time to look at your work.

On Thursday 09 October 2014 at 14:47:15, Addy Ke 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.
> 
> Reviewed-by: Doug Anderson <dianders@chromium.org>
> Tested-by: Doug Anderson <dianders@chromium.org>
> Signed-off-by: Addy Ke <addy.ke@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
> Changes in V3:
> - fix some wrong style
> - WARN_ONCE if min_low_div > max_low_div
> 
>  drivers/i2c/busses/i2c-rk3x.c | 140
> +++++++++++++++++++++++++++++++++++++++--- 1 file changed, 133
> insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/i2c/busses/i2c-rk3x.c b/drivers/i2c/busses/i2c-rk3x.c
> index b41d979..f17f7a2 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 +429,143 @@ 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);

I'm not really comfortable with using DIV_ROUND_UP on the last line, since 
this may violate the user's set SCL rate. Rounding up 1.1kHz SCL rate to 2kHz 
is not good.

I suggest using scl_rate / 1000 and placing a

if(WARN_ON(scl_rate < 1000))
  scl_rate = 1000;

somewhere to prevent scl_rate_khz from becoming 0.

> +
> +	/*
> +	 * 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. */
> +	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) {
> +		WARN_ONCE(true,
> +			  "Conflicting, min_low_div %lu, max_low_div %lu\n",
> +			  min_low_div, max_low_div);
> +		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).
> +		 */
> +		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.
> +		 */
> +		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;

I know the old code didn't have it either, but can we add overflow protection 
here? I.e. check for *div_low > 0xFFFF || *div_high > 0xFFFF and return
-EINVAL from rk3x_i2c_calc_divs?

My WIP dynamic clock rate change patch also needs to detect this condition to 
reject too fast input clock rates, so I'm in favor of returning an error code 
instead of limiting silently or using WARN_ON.

> +}
> +
>  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);
>  }
> 
>  /**

Cheers,
  Max
Douglas Anderson Oct. 12, 2014, 9:24 p.m. UTC | #3
Max,

On Sun, Oct 12, 2014 at 5:10 AM, Max Schwarz <max.schwarz@online.de> wrote:
>> +     /* Adjust to avoid overflow */
>> +     i2c_rate_khz = DIV_ROUND_UP(i2c_rate, 1000);
>> +     scl_rate_khz = DIV_ROUND_UP(scl_rate, 1000);
>
> I'm not really comfortable with using DIV_ROUND_UP on the last line, since
> this may violate the user's set SCL rate. Rounding up 1.1kHz SCL rate to 2kHz
> is not good.

Ah, good point.  So we round up for i2c_rate and down for scl_rate:

  i2c_rate_khz = DIV_ROUND_UP(i2c_rate, 1000);
  scl_rate_khz = scl_rate / 1000;


> I suggest using scl_rate / 1000 and placing a
>
> if(WARN_ON(scl_rate < 1000))
>   scl_rate = 1000;
>
> somewhere to prevent scl_rate_khz from becoming 0.

Ah, makes sense.  Yeah, that should be at the top.

Patch
diff mbox

diff --git a/drivers/i2c/busses/i2c-rk3x.c b/drivers/i2c/busses/i2c-rk3x.c
index b41d979..f17f7a2 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 +429,143 @@  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. */
+	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) {
+		WARN_ONCE(true,
+			  "Conflicting, min_low_div %lu, max_low_div %lu\n",
+			  min_low_div, max_low_div);
+		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).
+		 */
+		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.
+		 */
+		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);
 }
 
 /**