lkml.org 
[lkml]   [2011]   [Jun]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH] i2c: imx: choose the better clock divider
Date
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


\
 
 \ /
  Last update: 2011-06-14 09:19    [from the cache]
©2003-2011 Jasper Spaans