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

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Date:   Tue, 5 Apr 2022 11:30:06 +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 2/3] i2c: meson: Use 50% duty cycle for I2C clock

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@...ux.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