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

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Message-ID: <d6fabb26-8a29-fd5a-c03e-c059b28c9407@baylibre.com>
Date:   Mon, 11 Apr 2022 09:29:09 +0200
From:   Neil Armstrong <narmstrong@...libre.com>
To:     Lucas Tanure <tanure@...ux.com>,
        Kevin Hilman <khilman@...libre.com>,
        Jerome Brunet <jbrunet@...libre.com>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Cc:     linux-i2c@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-amlogic@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] i2c: meson: Use 50% duty cycle for I2C clock

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@...ux.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@...libre.com>