Re: [PATCH] i2c: exynos5: Calculate t_scl_l, t_scl_h according to i2c spec

[Marek Szyprowski <m.szyprowski@samsung.com>]

On 12.09.2022 10:59, Camel Guo wrote:
> Previously the duty cycle was divided equally into h_scl_l, t_scl_h.
> This makes the low period of the SCL clock in Fast Mode is only 1.25us
> which is way lower than the minimal value (1.3) specified in i2c
> specification. In order to make sure t_scl_l, t_scl_h always fullfill
> i2c specification, this commit calculates t_scl_l using this formula:
>
> t_scl_l = clk_cycle *
>      ((t_low_min + (scl_clock - t_low_min - t_high_min) / 2) / scl_clock)
>
> where:
> t_low_min is the minimal value of low period of the SCL clock in us;
> t_high_min is the minimal value of high period of the SCL clock in us;
> scl_clock is converted from SCL clock frequency into us.
>
> Signed-off-by: Camel Guo <camel.guo@axis.com>


This sounds reasonable and works fine on all Exynos-based test board I 
have. Feel free to add:

Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>

Reviewed-by: Marek Szyprowski <m.szyprowski@samsung.com>


> ---
>   drivers/i2c/busses/i2c-exynos5.c | 34 +++++++++++++++++++++++++++++---
>   1 file changed, 31 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/i2c/busses/i2c-exynos5.c b/drivers/i2c/busses/i2c-exynos5.c
> index 4a6260d04db2..72bc8adea8d5 100644
> --- a/drivers/i2c/busses/i2c-exynos5.c
> +++ b/drivers/i2c/busses/i2c-exynos5.c
> @@ -267,7 +267,7 @@ static void exynos5_i2c_clr_pend_irq(struct exynos5_i2c *i2c)
>    * exynos5_i2c_set_timing: updates the registers with appropriate
>    * timing values calculated
>    *
> - * Timing values for operation are calculated against either 100kHz
> + * Timing values for operation are calculated against 100kHz, 400kHz
>    * or 1MHz controller operating frequency.
>    *
>    * Returns 0 on success, -EINVAL if the cycle length cannot
> @@ -330,6 +330,23 @@ static int exynos5_i2c_set_timing(struct exynos5_i2c *i2c, bool hs_timings)
>   	 *
>   	 * Constraints: 4 <= temp, 0 <= CLK_DIV < 256, 2 <= clk_cycle <= 510
>   	 *
> +	 * To split SCL clock into low, high periods appropriately, one
> +	 * proportion factor for each I2C mode is used, which is calculated
> +	 * using this formula.
> +	 * ```
> +	 * ((t_low_min + (scl_clock - t_low_min - t_high_min) / 2) / scl_clock)
> +	 * ```
> +	 * where:
> +	 * t_low_min is the minimal value of low period of the SCL clock in us;
> +	 * t_high_min is the minimal value of high period of the SCL clock in us;
> +	 * scl_clock is converted from SCL clock frequency into us.
> +	 *
> +	 * Below are the proportion factors for these I2C modes:
> +	 *                t_low_min, t_high_min, scl_clock, proportion
> +	 * Standard Mode:     4.7us,      4.0us,      10us,      0.535
> +	 * Fast Mode:         1.3us,      0.6us,     2.5us,       0.64
> +	 * Fast-Plus Mode:    0.5us,     0.26us,       1us,       0.62
> +	 *
>   	 */
>   	t_ftl_cycle = (readl(i2c->regs + HSI2C_CONF) >> 16) & 0x7;
>   	temp = clkin / op_clk - 8 - t_ftl_cycle;
> @@ -343,8 +360,19 @@ static int exynos5_i2c_set_timing(struct exynos5_i2c *i2c, bool hs_timings)
>   		return -EINVAL;
>   	}
>   
> -	t_scl_l = clk_cycle / 2;
> -	t_scl_h = clk_cycle / 2;
> +	/*
> +	 * Scale clk_cycle to get t_scl_l using the proption factors for individual I2C modes.
> +	 */
> +	if (op_clk <= I2C_MAX_STANDARD_MODE_FREQ)
> +		t_scl_l = clk_cycle * 535 / 1000;
> +	else if (op_clk <= I2C_MAX_FAST_MODE_FREQ)
> +		t_scl_l = clk_cycle * 64 / 100;
> +	else
> +		t_scl_l = clk_cycle * 62 / 100;
> +
> +	if (t_scl_l > 0xFF)
> +		t_scl_l = 0xFF;
> +	t_scl_h = clk_cycle - t_scl_l;
>   	t_start_su = t_scl_l;
>   	t_start_hd = t_scl_l;
>   	t_stop_su = t_scl_l;
>
> base-commit: ce888220d5c7a805e0e155302a318d5d23e62950

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland