If the channel selection or deselection times out, it indicates
a failure in the mux's I2C subsystem. Without sending a reset pulse,
a power-on-reset of the entire device would be required to restore
communication.

The datasheet specifies a minimum hold time of 4 ns for the reset
pulse, but due to the path's capacitance and themux having its own
clock, it is recommended to extend this to approximately 1 us.

This option can be enabled using the i2c-mux-timeout-reset property
in the device tree and should only be used if the reset line is not
shared with other devices.

Signed-off-by: Wojciech Siudy <wojciech.siudy@nokia.com>
---
Changelog:
v2:
  * Removed mail header from the commit log
  * Decreased reset pulse hold time from 10 to 1 ms
v3:
  * Make this functionality enabled by appropriate property in
    devicetree
v4:
  * Fix missing condition check from devicetree
---
 drivers/i2c/muxes/i2c-mux-pca954x.c | 51 ++++++++++++++++++++++-------
 1 file changed, 40 insertions(+), 11 deletions(-)

diff --git a/drivers/i2c/muxes/i2c-mux-pca954x.c b/drivers/i2c/muxes/i2c-mux-pca954x.c
index 6f84018258c4..316048b0011d 100644
--- a/drivers/i2c/muxes/i2c-mux-pca954x.c
+++ b/drivers/i2c/muxes/i2c-mux-pca954x.c
@@ -110,6 +110,7 @@ struct pca954x {
 	u8 last_chan;		/* last register value */
 	/* MUX_IDLE_AS_IS, MUX_IDLE_DISCONNECT or >= 0 for channel */
 	s32 idle_state;
+	u8 timeout_reset;
 
 	struct i2c_client *client;
 
@@ -316,6 +317,30 @@ static u8 pca954x_regval(struct pca954x *data, u8 chan)
 		return 1 << chan;
 }
 
+static void pca954x_reset_deassert(struct pca954x *data)
+{
+	if (data->reset_cont)
+		reset_control_deassert(data->reset_cont);
+	else
+		gpiod_set_value_cansleep(data->reset_gpio, 0);
+}
+
+static void pca954x_reset_assert(struct pca954x *data)
+{
+	if (data->reset_cont)
+		reset_control_assert(data->reset_cont);
+	else
+		gpiod_set_value_cansleep(data->reset_gpio, 1);
+}
+
+static void pca954x_reset_mux(struct pca954x *data)
+{
+	dev_warn(&data->client->dev, "resetting the device\n");
+	pca954x_reset_assert(data);
+	udelay(1);
+	pca954x_reset_deassert(data);
+}
+
 static int pca954x_select_chan(struct i2c_mux_core *muxc, u32 chan)
 {
 	struct pca954x *data = i2c_mux_priv(muxc);
@@ -329,6 +354,9 @@ static int pca954x_select_chan(struct i2c_mux_core *muxc, u32 chan)
 		ret = pca954x_reg_write(muxc->parent, client, regval);
 		data->last_chan = ret < 0 ? 0 : regval;
 	}
+	if (ret == -ETIMEDOUT && (data->reset_cont || data->reset_gpio) &&
+	    data->timeout_reset)
+		pca954x_reset_mux(data);
 
 	return ret;
 }
@@ -338,6 +366,7 @@ static int pca954x_deselect_mux(struct i2c_mux_core *muxc, u32 chan)
 	struct pca954x *data = i2c_mux_priv(muxc);
 	struct i2c_client *client = data->client;
 	s32 idle_state;
+	int ret = 0;
 
 	idle_state = READ_ONCE(data->idle_state);
 	if (idle_state >= 0)
@@ -347,13 +376,16 @@ static int pca954x_deselect_mux(struct i2c_mux_core *muxc, u32 chan)
 	if (idle_state == MUX_IDLE_DISCONNECT) {
 		/* Deselect active channel */
 		data->last_chan = 0;
-		return pca954x_reg_write(muxc->parent, client,
-					 data->last_chan);
+		ret = pca954x_reg_write(muxc->parent, client, data->last_chan);
+		if (ret == -ETIMEDOUT &&
+		    (data->reset_cont || data->reset_gpio) &&
+		    data->timeout_reset)
+			pca954x_reset_mux(data);
 	}
 
 	/* otherwise leave as-is */
 
-	return 0;
+	return ret;
 }
 
 static ssize_t idle_state_show(struct device *dev,
@@ -543,14 +575,6 @@ static int pca954x_get_reset(struct device *dev, struct pca954x *data)
 	return 0;
 }
 
-static void pca954x_reset_deassert(struct pca954x *data)
-{
-	if (data->reset_cont)
-		reset_control_deassert(data->reset_cont);
-	else
-		gpiod_set_value_cansleep(data->reset_gpio, 0);
-}
-
 /*
  * I2C init/probing/exit functions
  */
@@ -625,6 +649,11 @@ static int pca954x_probe(struct i2c_client *client)
 			data->idle_state = MUX_IDLE_DISCONNECT;
 	}
 
+	if (device_property_read_bool(dev, "i2c-mux-timeout-reset"))
+		data->timeout_reset = 1;
+	else
+		data->timeout_reset = 0;
+
 	/*
 	 * Write the mux register at addr to verify
 	 * that the mux is in fact present. This also
-- 
2.34.1

Hi Wojciech,

> +	if (device_property_read_bool(dev, "i2c-mux-timeout-reset"))
> +		data->timeout_reset = 1;
> +	else
> +		data->timeout_reset = 0;
> +

For as much as I would like this patch to be in, I can't take it
if we don't sort it out in the binding.

I agree with the suggestion that Krzysztof has provided there and
I'm aligned with his comments.

Thanks,
Andi