Hello,

now that I understood the formula some more comments to the calculation.

On Tue, May 19, 2015 at 12:40:08AM +0800, Eddie Huang wrote:

+#define I2C_DEFAUT_SPEED		100000	/* hz */

DEFAULT?

+#define MAX_FS_MODE_SPEED		400000
+#define MAX_HS_MODE_SPEED		3400000
+#define MAX_SAMPLE_CNT_DIV		8
+#define MAX_STEP_CNT_DIV		64
+#define MAX_HS_STEP_CNT_DIV		8
[...]
+/* calculate i2c port speed */
+static int mtk_i2c_set_speed(struct mtk_i2c *i2c, unsigned int clk_src_in_hz)
+{

add a comment here, that clk_src_in_hz is the parent clock already
divided by clock-div.

+	unsigned int khz;
+	unsigned int step_cnt;
+	unsigned int sample_cnt;
+	unsigned int sclk;
+	unsigned int hclk;
+	unsigned int max_step_cnt;
+	unsigned int sample_div = MAX_SAMPLE_CNT_DIV;
+	unsigned int step_div;
+	unsigned int min_div;
+	unsigned int best_mul;
+	unsigned int cnt_mul;
+
+	if (i2c->speed_hz > MAX_HS_MODE_SPEED)
+		return -EINVAL;

According to the plan to tune for the highest possible rate <=
i2c->speed_hz, you should handle the case (i2c->speed_hz >
MAX_HS_MODE_SPEED) like i2c->speed_hz == MAX_HS_MODE_SPEED.
Well, you might want to prevent an overflow in the calculation below
however.

+	else if (i2c->speed_hz > MAX_FS_MODE_SPEED)
+		max_step_cnt = MAX_HS_STEP_CNT_DIV;
+	else
+		max_step_cnt = MAX_STEP_CNT_DIV;

So I assume this is the hardware limit on the step_cnt value. For
FS_MODE and below you have 6 bits and writing X corresponds to
step_cnt = X + 1. For HS_MODE there are only 3 bits. right?

+	step_div = max_step_cnt;
+	/* Find the best combination */
+	khz = i2c->speed_hz / 1000;
+	hclk = clk_src_in_hz / 1000;

Why are you dividing here? There shouldn't be an overflow problem and
you're loosing precision.

+	min_div = ((hclk >> 1) + khz - 1) / khz;

The shift accounts for the fixed divider 2 in

	i2c_bus_freq = parent_clk / (clock-div * 2 * sample_cnt * step_cnt

? Maybe better call this opt_div instead of min_div? So now we're
searching for the best pair (sample_cnt, step_cnt) with:

	* 0 < sample_cnt < MAX_SAMPLE_CNT_DIV
	* 0 < step_cnt < max_step_cnt
	* sample_cnt * step_cnt >= min_div
	* optimizing for sample_cnt * step_cnt being minimal

Right?

+	best_mul = MAX_SAMPLE_CNT_DIV * max_step_cnt;
+
+	for (sample_cnt = 1; sample_cnt <= MAX_SAMPLE_CNT_DIV; sample_cnt++) {
+		step_cnt = (min_div + sample_cnt - 1) / sample_cnt;

DIV_ROUND_UP

+		cnt_mul = step_cnt * sample_cnt;
+		if (step_cnt > max_step_cnt)
+			continue;

I think it can happen that you have step_cnt > max_step_cnt here, but
that (sample_cnt, max_step_cnt) still is a good pair to consider. So:

	step_cnt = DIV_ROUND_UP(min_div, sample_cnt);
	if (step_cnt > max_step_cnt)
		step_cnt = max_step_cnt;

	cnt_mul = step_cnt * sample_cnt;

+
+		if (cnt_mul < best_mul) {
+			best_mul = cnt_mul;
+			sample_div = sample_cnt;
+			step_div = step_cnt;

I'd call these best_sample_cnt and best_step_cnt instead of sample_div
and step_div.

+			if (best_mul == min_div)
+				break;
+		}
+	}
+
+	sample_cnt = sample_div;
+	step_cnt = step_div;
+	sclk = hclk / (2 * sample_cnt * step_cnt);
+	if (sclk > khz) {

Can this happen? A better name for "sclk" would be "bus_freq"?

+		dev_dbg(i2c->dev, "%s mode: unsupported speed (%ldkhz)\n",
+			(i2c->speed_hz > MAX_HS_MODE_SPEED) ? "HS" : "ST/FT",

What is ST/FR? I would have expected FS here.

+			(long int)khz);
+		return -EINVAL;
+	}
+
+	step_cnt--;
+	sample_cnt--;
+
+	if (i2c->speed_hz > MAX_FS_MODE_SPEED) {
+		/* Set the hign speed mode register */
+		i2c->timing_reg = I2C_FS_TIME_INIT_VALUE;
+		i2c->high_speed_reg = I2C_TIME_DEFAULT_VALUE |
+			(sample_cnt & I2C_TIMING_SAMPLE_COUNT_MASK) << 12 |
+			(step_cnt & I2C_TIMING_SAMPLE_COUNT_MASK) << 8;
+	} else {
+		i2c->timing_reg =
+			(sample_cnt & I2C_TIMING_SAMPLE_COUNT_MASK) << 8 |
+			(step_cnt & I2C_TIMING_STEP_DIV_MASK) << 0;
+		/* Disable the high speed transaction */
+		i2c->high_speed_reg = I2C_TIME_CLR_VALUE;
+	}

Would it be sensible to write these values directly into hardware here?

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-K?nig            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |