From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ben Dooks Subject: Re: [PATCH] i2c: imx: choose the better clock divider Date: Thu, 4 Aug 2011 17:51:46 +0100 Message-ID: <20110804165146.GB19115@trinity.fluff.org> References: <1308035825-22410-1-git-send-email-eric.miao@linaro.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <1308035825-22410-1-git-send-email-eric.miao-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org> Sender: linux-i2c-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Eric Miao Cc: linux-i2c-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Jean Delvare , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Richard Zhao List-Id: linux-i2c@vger.kernel.org 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 > Signed-off-by: Eric Miao > --- > 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 > > -- > To unsubscribe from this list: send the line "unsubscribe linux-i2c" in > the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- Ben Dooks, ben-elnMNo+KYs3YtjvyW6yDsg@public.gmane.org, http://www.fluff.org/ben/ Large Hadron Colada: A large Pina Colada that makes the universe disappear.