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-König |
Industrial Linux Solutions | http://www.pengutronix.de/ |
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