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i2c: imx: choose the better clock divider

Message ID BANLkTi=WCC8NPadPZxtXtytPttiz8R5Ugw@mail.gmail.com
State New
Headers show

Commit Message

Eric Miao June 14, 2011, 7:20 a.m. UTC 
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(-)


--
1.7.4.1
diff mbox

Patch

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
 }