From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Miao Subject: Re: [PATCH] i2c: imx: choose the better clock divider Date: Fri, 5 Aug 2011 17:22:12 +0100 Message-ID: References: <1308035825-22410-1-git-send-email-eric.miao@linaro.org> <20110804165146.GB19115@trinity.fluff.org> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <20110804165146.GB19115-SMNkleLxa3Z6Wcw2j4pizdi2O/JbrIOy@public.gmane.org> Sender: linux-i2c-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Ben Dooks 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 Thu, Aug 4, 2011 at 5:51 PM, Ben Dooks wrote: > 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. >> >> =C2=A0 When the source clock of the I2C controller is 66MHz, and the >> =C2=A0 requested rate is 100KHz, it gives a divider of 768 instead o= f >> =C2=A0 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 >> --- >> =C2=A0drivers/i2c/busses/i2c-imx.c | =C2=A0 46 +++++++++++++++++++++= +++++++------------- >> =C2=A01 files changed, 31 insertions(+), 15 deletions(-) >> >> diff --git a/drivers/i2c/busses/i2c-imx.c b/drivers/i2c/busses/i2c-i= mx.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] =3D { >> =C2=A0 =C2=A0 =C2=A0 { 3072, 0x1E }, { 3840, 0x1F } >> =C2=A0}; >> >> +#define I2C_CLK_DIV_NUM =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2= =A0ARRAY_SIZE(i2c_clk_div) >> + >> =C2=A0struct imx_i2c_struct { >> =C2=A0 =C2=A0 =C2=A0 struct i2c_adapter =C2=A0 =C2=A0 =C2=A0adapter; >> =C2=A0 =C2=A0 =C2=A0 struct resource =C2=A0 =C2=A0 =C2=A0 =C2=A0 *re= s; >> @@ -240,22 +242,37 @@ static void i2c_imx_stop(struct imx_i2c_struct= *i2c_imx) >> =C2=A0 =C2=A0 =C2=A0 clk_disable(i2c_imx->clk); >> =C2=A0} >> >> -static void __init i2c_imx_set_clk(struct imx_i2c_struct *i2c_imx, >> - =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2= =A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0= =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 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) >> =C2=A0{ >> - =C2=A0 =C2=A0 unsigned int i2c_clk_rate; >> - =C2=A0 =C2=A0 unsigned int div; >> + =C2=A0 =C2=A0 unsigned long div, delta_l, delta_h; >> =C2=A0 =C2=A0 =C2=A0 int i; >> >> - =C2=A0 =C2=A0 /* Divider value calculation */ >> - =C2=A0 =C2=A0 i2c_clk_rate =3D clk_get_rate(i2c_imx->clk); >> =C2=A0 =C2=A0 =C2=A0 div =3D (i2c_clk_rate + rate - 1) / rate; >> + >> =C2=A0 =C2=A0 =C2=A0 if (div < i2c_clk_div[0][0]) >> - =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 i =3D 0; >> - =C2=A0 =C2=A0 else if (div > i2c_clk_div[ARRAY_SIZE(i2c_clk_div) -= 1][0]) >> - =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 i =3D ARRAY_SIZE(i2c_clk= _div) - 1; >> - =C2=A0 =C2=A0 else >> - =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 for (i =3D 0; i2c_clk_di= v[i][0] < div; i++); >> + =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 return 0; >> + >> + =C2=A0 =C2=A0 if (div > i2c_clk_div[I2C_CLK_DIV_NUM - 1][0]) >> + =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 return I2C_CLK_DIV_NUM; >> + >> + =C2=A0 =C2=A0 for (i =3D 0; i < I2C_CLK_DIV_NUM; i++) >> + =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 if (i2c_clk_div[i][0] > = div) >> + =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2= =A0 break; >> + >> + =C2=A0 =C2=A0 delta_h =3D (i2c_clk_rate / i2c_clk_div[i - 1][0]) -= rate; >> + =C2=A0 =C2=A0 delta_l =3D rate - (i2c_clk_rate / i2c_clk_div[i][0]= ); > > hmm, what happens for the case of i being zero? Good catch, although it's never going to be zero as that case has alrea= dy been ruled out by the previous if() statement. However, to change the code to be more readable, I revised the patch a bit like below: i2c: imx: choose the better clock divider The original algorithm doesn't perform very well in some cases, e.g= =2E 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 diff --git a/drivers/i2c/busses/i2c-imx.c b/drivers/i2c/busses/i2c-imx.= c index 4c2a62b..2170a58 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] =3D { { 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 *i= 2c_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 =3D clk_get_rate(i2c_imx->clk); div =3D (i2c_clk_rate + rate - 1) / rate; - if (div < i2c_clk_div[0][0]) - i =3D 0; - else if (div > i2c_clk_div[ARRAY_SIZE(i2c_clk_div) - 1][0]) - i =3D ARRAY_SIZE(i2c_clk_div) - 1; - else - for (i =3D 0; i2c_clk_div[i][0] < div; i++); + + for (i =3D 0; i < I2C_CLK_DIV_NUM; i++) + if (i2c_clk_div[i][0] > div) + break; + + if (i =3D=3D 0) + return 0; + + if (i >=3D I2C_CLK_DIV_NUM) + return I2C_CLK_DIV_NUM - 1; + + delta_h =3D (i2c_clk_rate / i2c_clk_div[i - 1][0]) - rate; + delta_l =3D 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 =3D clk_get_rate(i2c_imx->clk); + int i =3D find_div(i2c_clk_rate, rate); /* Store divider value */ i2c_imx->ifdr =3D 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=3D%d, REQ DIV=3D%d\n", - __func__, i2c_clk_rate, div); - printk(KERN_DEBUG "I2C: <%s> IFDR[IC]=3D0x%x, REAL DIV=3D%d\n", - __func__, i2c_clk_div[i][1], i2c_clk_div[i][0]); + pr_debug("<%s> I2C_CLK=3D%ld, REQ RATE=3D%d, DIV=3D%d, IFDR[IC]=3D0x%= x\n", + __func__, i2c_clk_rate, rate, + i2c_clk_div[i][0], i2c_clk_div[i][1]); #endif }