[PATCH v5 2/2] mux: gpio-mux: add support for enable GPIO

[Antoniu Miclaus <antoniu.miclaus@analog.com>]

Add support for an optional enable GPIO to the gpio-mux driver. This
allows the mux to be disabled before changing address lines and
re-enabled after, preventing glitches that could briefly activate
unintended channels during transitions.

The enable GPIO is optional and the driver maintains backward
compatibility with existing gpio-mux users.

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
---
Changes in v5:
 - Enhance code comment to explain high-impedance (high-Z) state when
   mux is disabled
 - Add context about downstream capacitance maintaining signal level
   for analog multiplexers
---
 drivers/mux/gpio.c | 35 ++++++++++++++++++++++++++++++++++-
 1 file changed, 34 insertions(+), 1 deletion(-)

diff --git a/drivers/mux/gpio.c b/drivers/mux/gpio.c
index 4cc3202c58f3..b95d2c7a53ca 100644
--- a/drivers/mux/gpio.c
+++ b/drivers/mux/gpio.c
@@ -19,6 +19,7 @@
 
 struct mux_gpio {
 	struct gpio_descs *gpios;
+	struct gpio_desc *enable;
 };
 
 static int mux_gpio_set(struct mux_control *mux, int state)
@@ -27,10 +28,31 @@ static int mux_gpio_set(struct mux_control *mux, int state)
 	DECLARE_BITMAP(values, BITS_PER_TYPE(state));
 	u32 value = state;
 
+	if (state == MUX_IDLE_DISCONNECT) {
+		if (mux_gpio->enable)
+			gpiod_set_value_cansleep(mux_gpio->enable, 0);
+		return 0;
+	}
+
+	if (mux_gpio->enable) {
+		/*
+		 * Disable the mux before changing address lines to prevent
+		 * glitches where an unintended channel could be briefly
+		 * activated during the transition. When disabled, all mux
+		 * outputs enter high-impedance (high-Z) state. For analog
+		 * signals, downstream capacitance typically maintains the
+		 * signal level during this brief disconnection.
+		 */
+		gpiod_set_value_cansleep(mux_gpio->enable, 0);
+	}
+
 	bitmap_from_arr32(values, &value, BITS_PER_TYPE(value));
 
 	gpiod_multi_set_value_cansleep(mux_gpio->gpios, values);
 
+	if (mux_gpio->enable)
+		gpiod_set_value_cansleep(mux_gpio->enable, 1);
+
 	return 0;
 }
 
@@ -71,9 +93,20 @@ static int mux_gpio_probe(struct platform_device *pdev)
 	WARN_ON(pins != mux_gpio->gpios->ndescs);
 	mux_chip->mux->states = BIT(pins);
 
+	mux_gpio->enable = devm_gpiod_get_optional(dev, "enable", GPIOD_OUT_LOW);
+	if (IS_ERR(mux_gpio->enable))
+		return dev_err_probe(dev, PTR_ERR(mux_gpio->enable),
+				     "failed to get enable gpio\n");
+
 	ret = device_property_read_u32(dev, "idle-state", (u32 *)&idle_state);
 	if (ret >= 0 && idle_state != MUX_IDLE_AS_IS) {
-		if (idle_state < 0 || idle_state >= mux_chip->mux->states) {
+		if (idle_state == MUX_IDLE_DISCONNECT) {
+			if (!mux_gpio->enable) {
+				dev_err(dev,
+					"invalid idle-state (MUX_IDLE_DISCONNECT requires enable-gpios)\n");
+				return -EINVAL;
+			}
+		} else if (idle_state < 0 || idle_state >= mux_chip->mux->states) {
 			dev_err(dev, "invalid idle-state %u\n", idle_state);
 			return -EINVAL;
 		}
-- 
2.43.0