Re: [PATCH v2 2/2] i2c: meson: Use 50% duty cycle for I2C clock

Neil Armstrong
2022-04-11T07:31:31Z

On 09/04/2022 18:43, 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 | 70 +++++++++++++++++++++++++++++-----
> 1 file changed, 60 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/i2c/busses/i2c-meson.c b/drivers/i2c/busses/i2c-meson.c
> index 4b4a5b2d77ab..50dab123380a 100644
> --- a/drivers/i2c/busses/i2c-meson.c
> +++ b/drivers/i2c/busses/i2c-meson.c
> @@ -65,10 +65,6 @@ enum {
> STATE_WRITE,
> };
>
> -struct meson_i2c_data {
> - unsigned char div_factor;
> -};
> -
> /**
> * struct meson_i2c - Meson I2C device private data
> *
> @@ -109,6 +105,10 @@ struct meson_i2c {
> const struct meson_i2c_data *data;
> };
>
> +struct meson_i2c_data {
> + void (*set_clk_div)(struct meson_i2c *i2c, unsigned int freq);
> +};
> +
> static void meson_i2c_set_mask(struct meson_i2c *i2c, int reg, u32 mask,
> u32 val)
> {
> @@ -137,14 +137,62 @@ static void meson_i2c_add_token(struct meson_i2c *i2c, int token)
> i2c->num_tokens++;
> }
>
> -static void meson_i2c_set_clk_div(struct meson_i2c *i2c, unsigned int freq)
> +static void meson_gxbb_axg_i2c_set_clk_div(struct meson_i2c *i2c, unsigned int freq)
> +{
> + unsigned long clk_rate = clk_get_rate(i2c->clk);
> + unsigned int div_h, div_l;
> +
> + /* 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
> + */
> + if (freq <= I2C_MAX_STANDARD_MODE_FREQ) {
> + 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 {
> + 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_h > GENMASK(11, 0)) {
> + dev_err(i2c->dev, "requested bus frequency too low\n");
> + 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_h & GENMASK(9, 0)));
> +
> + meson_i2c_set_mask(i2c, REG_CTRL, REG_CTRL_CLKDIVEXT_MASK,
> + 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,
> + FIELD_PREP(REG_SLV_SCL_LOW_MASK, div_l));
> +
> + /* Enable HIGH/LOW mode */
> + meson_i2c_set_mask(i2c, REG_SLAVE_ADDR, REG_SLV_SCL_LOW_EN, REG_SLV_SCL_LOW_EN);
> +
> + dev_dbg(i2c->dev, "%s: clk %lu, freq %u, divh %u, divl %u\n", __func__,
> + clk_rate, freq, div_h, div_l);
> +}
> +
> +static void meson6_i2c_set_clk_div(struct meson_i2c *i2c, unsigned int freq)
> {
> unsigned long clk_rate = clk_get_rate(i2c->clk);
> unsigned int div;
>
> div = DIV_ROUND_UP(clk_rate, freq);
> div -= FILTER_DELAY;
> - div = DIV_ROUND_UP(div, i2c->data->div_factor);
> + div = DIV_ROUND_UP(div, 4);
>
> /* clock divider has 12 bits */
> if (div > GENMASK(11, 0)) {
> @@ -472,7 +520,9 @@ static int meson_i2c_probe(struct platform_device *pdev)
> meson_i2c_set_mask(i2c, REG_SLAVE_ADDR,
> REG_SLV_SDA_FILTER_MASK | REG_SLV_SCL_FILTER_MASK, 0);
>
> - meson_i2c_set_clk_div(i2c, timings.bus_freq_hz);
> + if (!i2c->data->set_clk_div)
> + return -EINVAL;
> + i2c->data->set_clk_div(i2c, timings.bus_freq_hz);
>
> ret = i2c_add_adapter(&i2c->adap);
> if (ret < 0) {
> @@ -494,15 +544,15 @@ static int meson_i2c_remove(struct platform_device *pdev)
> }
>
> static const struct meson_i2c_data i2c_meson6_data = {
> - .div_factor = 4,
> + .set_clk_div = meson6_i2c_set_clk_div,
> };
>
> static const struct meson_i2c_data i2c_gxbb_data = {
> - .div_factor = 4,
> + .set_clk_div = meson_gxbb_axg_i2c_set_clk_div,
> };
>
> static const struct meson_i2c_data i2c_axg_data = {
> - .div_factor = 3,
> + .set_clk_div = meson_gxbb_axg_i2c_set_clk_div,
> };
>
> static const struct of_device_id meson_i2c_match[] = {

Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>