[PATCH] dm: i2c-gpio: add support for clock stretching

[Heiko Schocher]

Hello Michael,

Am 23.01.2020 um 23:30 schrieb Michael Auchter:
> This adds support for clock stretching to the i2c-gpio driver. This is
> accomplished by switching the GPIO used for the SCL line to an input
> when it should be driven high, and polling on the SCL line value until
> it goes high (indicating that the I2C slave is no longer pulling it
> low).
> 
> Signed-off-by: Michael Auchter <michael.auchter at ni.com>
> Cc: Heiko Schocher <hs at denx.de>
> ---
>   drivers/i2c/i2c-gpio.c | 15 +++++++++++----
>   1 file changed, 11 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/i2c/i2c-gpio.c b/drivers/i2c/i2c-gpio.c
> index 4e8fa21473..80ac26e583 100644
> --- a/drivers/i2c/i2c-gpio.c
> +++ b/drivers/i2c/i2c-gpio.c
> @@ -49,11 +49,18 @@ static void i2c_gpio_sda_set(struct gpio_desc *sda, int bit)
>   
>   static void i2c_gpio_scl_set(struct gpio_desc *scl, int bit)
>   {
> -	ulong flags = GPIOD_IS_OUT;
> +	int count = 0;
>   
> -	if (bit)
> -		flags |= GPIOD_IS_OUT_ACTIVE;
> -	dm_gpio_set_dir_flags(scl, flags);
> +	if (bit) {
> +		dm_gpio_set_dir_flags(scl, GPIOD_IS_IN);
> +		while (!dm_gpio_get_value(scl) && count++ < 100000)
> +			udelay(1);
> +
> +		if (!dm_gpio_get_value(scl))
> +			pr_err("timeout waiting on slave to release scl\n");
> +	} else {
> +		dm_gpio_set_dir_flags(scl, GPIOD_IS_OUT);
> +	}
>   }
>   
>   static void i2c_gpio_write_bit(struct gpio_desc *scl, struct gpio_desc *sda,
> 

In principle I would say Ok, but your patch changes behaviour for
all boards which use this driver currently ...

May they cannot reread the gpio ... we have no scl_get() yet!

So please, make this change dependend on linux property "i2c-gpio,scl-output-only"
see:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/i2c/i2c-gpio.yaml#n36

As an example in linux:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/i2c/busses/i2c-gpio.c#n317

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/i2c/algos/i2c-algo-bit.c#n75

So at least, we have the chance to switch back to old behaviour by adding
this (new for U-Boot) property.

Thanks!

bye,
Heiko
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