linux-kernel - Re: [PATCH] i2c: imx: choose the better clock divider

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Date:	Thu, 4 Aug 2011 17:51:46 +0100
From:	Ben Dooks <ben-i2c@...ff.org>
To:	Eric Miao <eric.miao@...aro.org>
Cc:	linux-i2c@...r.kernel.org, Jean Delvare <khali@...ux-fr.org>,
	linux-kernel@...r.kernel.org,
	Richard Zhao <richard.zhao@...aro.org>
Subject: Re: [PATCH] i2c: imx: choose the better clock divider

On Tue, Jun 14, 2011 at 03:17:05PM +0800, Eric Miao 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@...aro.org>
> Signed-off-by: Eric Miao <eric.miao@...aro.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]);

hmm, what happens for the case of i being zero?

> +
> +	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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-- 
Ben Dooks, ben@...ff.org, http://www.fluff.org/ben/

Large Hadron Colada: A large Pina Colada that makes the universe disappear.

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