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    SubjectRe: [PATCH] i2c: imx: choose the better clock divider
    Sorry forgot to include Ben.

    Ben, any ideas?

    On Tue, Jun 14, 2011 at 3:17 PM, Eric Miao <eric.miao@linaro.org> wrote:
    > The original algorithm doesn't perform very well in some cases, e.g.
    >
    >  When the source clock of the I2C controller is 66MHz, and the
    >  requested rate is 100KHz, it gives a divider of 768 instead of
    >  the better 640.
    >
    > Choose a better clock divider so the final clock rate is closer to
    > the requested one by comparing the rate distances calculated by
    > two adjacent dividers.
    >
    > Cc: Richard Zhao <richard.zhao@linaro.org>
    > Signed-off-by: Eric Miao <eric.miao@linaro.org>
    > ---
    >  drivers/i2c/busses/i2c-imx.c |   46 ++++++++++++++++++++++++++++-------------
    >  1 files changed, 31 insertions(+), 15 deletions(-)
    >
    > diff --git a/drivers/i2c/busses/i2c-imx.c b/drivers/i2c/busses/i2c-imx.c
    > index 4c2a62b..cd640ff 100644
    > --- a/drivers/i2c/busses/i2c-imx.c
    > +++ b/drivers/i2c/busses/i2c-imx.c
    > @@ -112,6 +112,8 @@ static u16 __initdata i2c_clk_div[50][2] = {
    >        { 3072, 0x1E }, { 3840, 0x1F }
    >  };
    >
    > +#define I2C_CLK_DIV_NUM                ARRAY_SIZE(i2c_clk_div)
    > +
    >  struct imx_i2c_struct {
    >        struct i2c_adapter      adapter;
    >        struct resource         *res;
    > @@ -240,22 +242,37 @@ static void i2c_imx_stop(struct imx_i2c_struct *i2c_imx)
    >        clk_disable(i2c_imx->clk);
    >  }
    >
    > -static void __init i2c_imx_set_clk(struct imx_i2c_struct *i2c_imx,
    > -                                                       unsigned int rate)
    > +/* find the index into i2c_clk_div[] array, compare with the two
    > + * dividers found, return the one with smaller error
    > + */
    > +static int find_div(unsigned long i2c_clk_rate, unsigned long rate)
    >  {
    > -       unsigned int i2c_clk_rate;
    > -       unsigned int div;
    > +       unsigned long div, delta_l, delta_h;
    >        int i;
    >
    > -       /* Divider value calculation */
    > -       i2c_clk_rate = clk_get_rate(i2c_imx->clk);
    >        div = (i2c_clk_rate + rate - 1) / rate;
    > +
    >        if (div < i2c_clk_div[0][0])
    > -               i = 0;
    > -       else if (div > i2c_clk_div[ARRAY_SIZE(i2c_clk_div) - 1][0])
    > -               i = ARRAY_SIZE(i2c_clk_div) - 1;
    > -       else
    > -               for (i = 0; i2c_clk_div[i][0] < div; i++);
    > +               return 0;
    > +
    > +       if (div > i2c_clk_div[I2C_CLK_DIV_NUM - 1][0])
    > +               return I2C_CLK_DIV_NUM;
    > +
    > +       for (i = 0; i < I2C_CLK_DIV_NUM; i++)
    > +               if (i2c_clk_div[i][0] > div)
    > +                       break;
    > +
    > +       delta_h = (i2c_clk_rate / i2c_clk_div[i - 1][0]) - rate;
    > +       delta_l = rate - (i2c_clk_rate / i2c_clk_div[i][0]);
    > +
    > +       return (delta_l < delta_h) ? i : i - 1;
    > +}
    > +
    > +static void __init i2c_imx_set_clk(struct imx_i2c_struct *i2c_imx,
    > +                                  unsigned int rate)
    > +{
    > +       unsigned long i2c_clk_rate = clk_get_rate(i2c_imx->clk);
    > +       int i = find_div(i2c_clk_rate, rate);
    >
    >        /* Store divider value */
    >        i2c_imx->ifdr = i2c_clk_div[i][1];
    > @@ -271,10 +288,9 @@ static void __init i2c_imx_set_clk(struct imx_i2c_struct *i2c_imx,
    >
    >        /* dev_dbg() can't be used, because adapter is not yet registered */
    >  #ifdef CONFIG_I2C_DEBUG_BUS
    > -       printk(KERN_DEBUG "I2C: <%s> I2C_CLK=%d, REQ DIV=%d\n",
    > -               __func__, i2c_clk_rate, div);
    > -       printk(KERN_DEBUG "I2C: <%s> IFDR[IC]=0x%x, REAL DIV=%d\n",
    > -               __func__, i2c_clk_div[i][1], i2c_clk_div[i][0]);
    > +       pr_debug("<%s> I2C_CLK=%ld, REQ RATE=%d, DIV=%d, IFDR[IC]=0x%x\n",
    > +               __func__, i2c_clk_rate, rate,
    > +               i2c_clk_div[i][0], i2c_clk_div[i][1]);
    >  #endif
    >  }
    >
    > --
    > 1.7.4.1
    >
    >
    --
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