Re: [PATCH 2/3] i2c: meson: Use 50% duty cycle for I2C clock

Neil Armstrong
2022-04-05T09:30:28Z

On 26/03/2022 11:22, Lucas Tanure wrote:
> The duty cycle of 33% is less than the required
> by the I2C specs for the LOW period of the SCL
> clock.
>
> Move the duty cyle to 50% for 100Khz or lower
> clocks, and (40% High SCL / 60% Low SCL) duty
> cycle for clocks above 100Khz.
>
> Signed-off-by: Lucas Tanure <tanure@linux.com>
> ---
> drivers/i2c/busses/i2c-meson.c | 45 +++++++++++++++++++++++++---------
> 1 file changed, 33 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/i2c/busses/i2c-meson.c b/drivers/i2c/busses/i2c-meson.c
> index 4b4a5b2d77ab..b913ba20f06e 100644
> --- a/drivers/i2c/busses/i2c-meson.c
> +++ b/drivers/i2c/busses/i2c-meson.c
> @@ -140,29 +140,50 @@ static void meson_i2c_add_token(struct meson_i2c *i2c, int token)
> static void meson_i2c_set_clk_div(struct meson_i2c *i2c, unsigned int freq)
> {
> unsigned long clk_rate = clk_get_rate(i2c->clk);
> - unsigned int div;
> + unsigned int div_h, div_l;
>
> - div = DIV_ROUND_UP(clk_rate, freq);
> - div -= FILTER_DELAY;
> - div = DIV_ROUND_UP(div, i2c->data->div_factor);
> + if (freq <= 100000) {
> + div_h = DIV_ROUND_UP(clk_rate, freq);
> + div_l = DIV_ROUND_UP(div_h, 4);
> + div_h = DIV_ROUND_UP(div_h, 2) - FILTER_DELAY;
> + } else {
> + /* According to I2C-BUS Spec 2.1, in FAST-MODE, the minimum LOW period is 1.3uS, and
> + * minimum HIGH is least 0.6us.
> + * For 400000 freq, the period is 2.5us. To keep within the specs, give 40% of period to
> + * HIGH and 60% to LOW. This means HIGH at 1.0us and LOW 1.5us.
> + * The same applies for Fast-mode plus, where LOW is 0.5us and HIGH is 0.26us.
> + * Duty = H/(H + L) = 2/5
> + */
> + div_h = DIV_ROUND_UP(clk_rate * 2, freq * 5) - FILTER_DELAY;
> + div_l = DIV_ROUND_UP(clk_rate * 3, freq * 5 * 2);
> + }
>
> /* clock divider has 12 bits */
> - if (div > GENMASK(11, 0)) {
> + if (div_h > GENMASK(11, 0)) {
> dev_err(i2c->dev, "requested bus frequency too low\n");
> - div = GENMASK(11, 0);
> + div_h = GENMASK(11, 0);
> + }
> + if (div_l > GENMASK(11, 0)) {
> + dev_err(i2c->dev, "requested bus frequency too low\n");
> + div_l = GENMASK(11, 0);
> }
>
> meson_i2c_set_mask(i2c, REG_CTRL, REG_CTRL_CLKDIV_MASK,
> - FIELD_PREP(REG_CTRL_CLKDIV_MASK, div & GENMASK(9, 0)));
> + FIELD_PREP(REG_CTRL_CLKDIV_MASK, div_h & GENMASK(9, 0)));
>
> meson_i2c_set_mask(i2c, REG_CTRL, REG_CTRL_CLKDIVEXT_MASK,
> - FIELD_PREP(REG_CTRL_CLKDIVEXT_MASK, div >> 10));
> + FIELD_PREP(REG_CTRL_CLKDIVEXT_MASK, div_h >> 10));
> +
> +
> + /* set SCL low delay */
> + meson_i2c_set_mask(i2c, REG_SLAVE_ADDR, REG_SLV_SCL_LOW_MASK,
> + (div_l << REG_SLV_SCL_LOW_SHIFT) & REG_SLV_SCL_LOW_MASK);
>
> - /* Disable HIGH/LOW mode */
> - meson_i2c_set_mask(i2c, REG_SLAVE_ADDR, REG_SLV_SCL_LOW_EN, 0);
> + /* Enable HIGH/LOW mode */
> + meson_i2c_set_mask(i2c, REG_SLAVE_ADDR, REG_SLV_SCL_LOW_EN, REG_SLV_SCL_LOW_EN);

While reading registers documentation, it's written:
```
SCL Low delay. This is a new feature in M8baby. In the previous M8baby design, the SCL low time was controlled
by bits[21:12] of the register above. In this design, the SCL delay is controlled independently by these bits.
```

Could you keep the previous calculation for Meson6, with DIV_FACTOR fixed to 4, and use the new calculation only for GXBB & AXG ?

To ease this, replace the div_factor with a clock calculation function pointer in meson_i2c_data, and pass the old
one for Meson6 and the new one for GXBB & AXG.
This only slightly changes patch 2 and changes patch 3 from a removal to a variable replace.

>
> - dev_dbg(i2c->dev, "%s: clk %lu, freq %u, div %u\n", __func__,
> - clk_rate, freq, div);
> + dev_dbg(i2c->dev, "%s: clk %lu, freq %u, divh %u, divl %u\n", __func__,
> + clk_rate, freq, div_h, div_l);
> }
>
> static void meson_i2c_get_data(struct meson_i2c *i2c, char *buf, int len)

Thanks,
Neil