i2c: busses: i2c-mv64xxx: fix arb-loss i2c lock

From: Elad Nachman <enachman@marvell.com>

Some i2c slaves, mainly SFPs, might cause the bus to lose arbitration
while slave is in the middle of responding.

The solution is to change the I2C mode from mpps to gpios, and toggle
the i2c_scl gpio to emulate bus clock toggling, so slave will finish
its transmission, driven by the manual clock toggling, and will release
the i2c bus.

Signed-off-by: Elad Nachman <enachman@marvell.com>
---
 drivers/i2c/busses/i2c-mv64xxx.c | 81 ++++++++++++++++++++++++++++++++
 1 file changed, 81 insertions(+)

Hi Elad,

On Thu, Nov 30, 2023 at 06:25:22PM +0200, Elad Nachman wrote:
> From: Elad Nachman <enachman@marvell.com>
> 
> Some i2c slaves, mainly SFPs, might cause the bus to lose arbitration
> while slave is in the middle of responding.

Can you be more specific about how this is happening?

> The solution is to change the I2C mode from mpps to gpios, and toggle
> the i2c_scl gpio to emulate bus clock toggling, so slave will finish
> its transmission, driven by the manual clock toggling, and will release
> the i2c bus.
> 
> Signed-off-by: Elad Nachman <enachman@marvell.com>
> ---
>  drivers/i2c/busses/i2c-mv64xxx.c | 81 ++++++++++++++++++++++++++++++++
>  1 file changed, 81 insertions(+)
> 
> diff --git a/drivers/i2c/busses/i2c-mv64xxx.c b/drivers/i2c/busses/i2c-mv64xxx.c
> index dc160cbc3155..21715f31dc29 100644
> --- a/drivers/i2c/busses/i2c-mv64xxx.c
> +++ b/drivers/i2c/busses/i2c-mv64xxx.c
> @@ -26,6 +26,7 @@
>  #include <linux/clk.h>
>  #include <linux/err.h>
>  #include <linux/delay.h>
> +#include <linux/of_gpio.h>
>  
>  #define MV64XXX_I2C_ADDR_ADDR(val)			((val & 0x7f) << 1)
>  #define MV64XXX_I2C_BAUD_DIV_N(val)			(val & 0x7)
> @@ -104,6 +105,7 @@ enum {
>  	MV64XXX_I2C_ACTION_RCV_DATA,
>  	MV64XXX_I2C_ACTION_RCV_DATA_STOP,
>  	MV64XXX_I2C_ACTION_SEND_STOP,
> +	MV64XXX_I2C_ACTION_UNLOCK_BUS
>  };
>  
>  struct mv64xxx_i2c_regs {
> @@ -150,6 +152,11 @@ struct mv64xxx_i2c_data {
>  	bool			clk_n_base_0;
>  	struct i2c_bus_recovery_info	rinfo;
>  	bool			atomic;
> +	/* I2C mpp states & gpios needed for arbitration lost recovery */
> +	int			scl_gpio, sda_gpio;
> +	bool			arb_lost_recovery_ena;

mmhhh... this name here looks quite ugly, something like
"soft_reset" or "clock_stretch"?

> +	struct pinctrl_state *i2c_mpp_state;
> +	struct pinctrl_state *i2c_gpio_state;
>  };
>  
>  static struct mv64xxx_i2c_regs mv64xxx_i2c_regs_mv64xxx = {
> @@ -318,6 +325,11 @@ mv64xxx_i2c_fsm(struct mv64xxx_i2c_data *drv_data, u32 status)
>  		drv_data->state = MV64XXX_I2C_STATE_IDLE;
>  		break;
>  
> +	case MV64XXX_I2C_STATUS_MAST_LOST_ARB: /*0x38*/

Please, leave a space between the comments: /* 0x38 */

> +		drv_data->action = MV64XXX_I2C_ACTION_UNLOCK_BUS;
> +		drv_data->state = MV64XXX_I2C_STATE_IDLE;
> +		break;
> +
>  	case MV64XXX_I2C_STATUS_MAST_WR_ADDR_NO_ACK: /* 0x20 */
>  	case MV64XXX_I2C_STATUS_MAST_WR_NO_ACK: /* 30 */
>  	case MV64XXX_I2C_STATUS_MAST_RD_ADDR_NO_ACK: /* 48 */
> @@ -356,6 +368,9 @@ static void mv64xxx_i2c_send_start(struct mv64xxx_i2c_data *drv_data)
>  static void
>  mv64xxx_i2c_do_action(struct mv64xxx_i2c_data *drv_data)
>  {
> +	struct pinctrl *pc;
> +	int i, ret;
> +
>  	switch(drv_data->action) {
>  	case MV64XXX_I2C_ACTION_SEND_RESTART:
>  		/* We should only get here if we have further messages */
> @@ -409,6 +424,48 @@ mv64xxx_i2c_do_action(struct mv64xxx_i2c_data *drv_data)
>  			drv_data->reg_base + drv_data->reg_offsets.control);
>  		break;
>  
> +	case MV64XXX_I2C_ACTION_UNLOCK_BUS:
> +

for consistency, don't add a blank line here.

> +		if (!drv_data->arb_lost_recovery_ena)
> +			break;
> +
> +		pc = devm_pinctrl_get(drv_data->adapter.dev.parent);
> +		if (IS_ERR(pc))
> +			break;

I would add here some error message

> +
> +		/* Change i2c MPPs state to act as GPIOs: */
> +		if (pinctrl_select_state(pc, drv_data->i2c_gpio_state) >= 0) {
> +			ret = devm_gpio_request_one(drv_data->adapter.dev.parent,
> +					 drv_data->scl_gpio, GPIOF_DIR_OUT, NULL);
> +			ret |= devm_gpio_request_one(drv_data->adapter.dev.parent,
> +					 drv_data->sda_gpio, GPIOF_DIR_OUT, NULL);

mmhhh... these are requested everytime we do an UNLOCK_BUS and
freed only when the driver exits.

Why don't you request them in the probe()?

> +			if (!ret) {
> +				/* Toggle i2c scl (serial clock) 10 times.
> +				 * This allows the slave that occupies
> +				 * the bus to transmit its remaining data,
> +				 * so it can release the i2c bus:
> +				 */

The proper commenting style is:

	/*
	 * Toggle i2c scl (serial clock) 10 times.
	 * This allows the slave that occupies
	 * the bus to transmit its remaining data,
	 * so it can release the i2c bus:
	 */

Why 10 times? What is the requested time?

> +				for (i = 0; i < 10; i++) {
> +					gpio_set_value(drv_data->scl_gpio, 1);
> +					mdelay(1);

Please, no mdelay!

> +					gpio_set_value(drv_data->scl_gpio, 0);
> +				};
> +
> +				devm_gpiod_put(drv_data->adapter.dev.parent,
> +					gpio_to_desc(drv_data->scl_gpio));
> +				devm_gpiod_put(drv_data->adapter.dev.parent,
> +					gpio_to_desc(drv_data->sda_gpio));
> +			}
> +
> +			/* restore i2c pin state to MPPs: */
> +			pinctrl_select_state(pc, drv_data->i2c_mpp_state);
> +		}
> +
> +		/* Trigger controller soft reset: */
> +		writel(0x1, drv_data->reg_base + drv_data->reg_offsets.soft_reset);

0x1 stands for... ?

> +		mdelay(1);

Please, no mdelay and if there is a need to wait explain it in a
comment.

I need a very strong reason for using mdelay() at this point.

> +		fallthrough;

What is the rationale behind this fallthrough? Are we moving to
get a data stop later on?

> +
>  	case MV64XXX_I2C_ACTION_RCV_DATA_STOP:
>  		drv_data->msg->buf[drv_data->byte_posn++] =
>  			readl(drv_data->reg_base + drv_data->reg_offsets.data);
> @@ -985,6 +1042,7 @@ mv64xxx_i2c_probe(struct platform_device *pd)
>  {
>  	struct mv64xxx_i2c_data		*drv_data;
>  	struct mv64xxx_i2c_pdata	*pdata = dev_get_platdata(&pd->dev);
> +	struct pinctrl *pc;
>  	int	rc;
>  
>  	if ((!pdata && !pd->dev.of_node))
> @@ -1040,6 +1098,29 @@ mv64xxx_i2c_probe(struct platform_device *pd)
>  	if (rc == -EPROBE_DEFER)
>  		return rc;
>  
> +	drv_data->arb_lost_recovery_ena = false;
> +	pc = devm_pinctrl_get(&pd->dev);
> +	if (!IS_ERR(pc)) {

Is this optional? Please consider using
"i2c-scl-clk-low-timeout-us" in the devicetree.

Andi