[PATCH v2 2/2] mux: gpio: Add optional enable gpio

[Tapio Reijonen]

Analog multiplexers have an enable input that disconnects all channels
when deasserted, independent of the address inputs; on a 74HC4051 it is
the E input. Add it as an optional gpio.

The mux gpios are not updated atomically. gpiod_multi_set_value_cansleep()
groups them per gpio controller, and only controllers implementing
set_multiple() update theirs in a single write, so a mux with its address
inputs spread over two controllers passes through the intermediate
addresses on every change. Deassert the enable gpio for the duration of
the update and assert it once the address is settled.

The enable gpio is also what makes an idle state of MUX_IDLE_DISCONNECT
possible, which the mux core applies when the chip is registered and after
every deselect: it leaves the enable gpio deasserted and the address
inputs alone. Refuse that idle state without an enable gpio, because the
address inputs cannot disconnect anything on their own and mux_gpio_set()
would instead drive them to the bit pattern of MUX_IDLE_DISCONNECT.

Signed-off-by: Tapio Reijonen <tapio.reijonen@xxxxxxxxxxx>
---
 drivers/mux/gpio.c | 34 ++++++++++++++++++++++++++++------
 1 file changed, 28 insertions(+), 6 deletions(-)

diff --git a/drivers/mux/gpio.c b/drivers/mux/gpio.c
index f9c7863e51b82fb76ed03cf17308fae74716cf6d..df5d24214bd5fa8c3c1d733b25d32ca88c06c8c0 100644
--- a/drivers/mux/gpio.c
+++ b/drivers/mux/gpio.c
@@ -18,6 +18,7 @@
 
 struct mux_gpio {
 	struct gpio_descs *gpios;
+	struct gpio_desc *enable_gpio;
 };
 
 static int mux_gpio_set(struct mux_control *mux, int state)
@@ -26,10 +27,18 @@ static int mux_gpio_set(struct mux_control *mux, int state)
 	DECLARE_BITMAP(values, BITS_PER_TYPE(state));
 	u32 value = state;
 
+	/* The gpios are not updated atomically, disable the mux meanwhile. */
+	gpiod_set_value_cansleep(mux_gpio->enable_gpio, 0);
+
+	if (state == MUX_IDLE_DISCONNECT)
+		return 0;
+
 	bitmap_from_arr32(values, &value, BITS_PER_TYPE(value));
 
 	gpiod_multi_set_value_cansleep(mux_gpio->gpios, values);
 
+	gpiod_set_value_cansleep(mux_gpio->enable_gpio, 1);
+
 	return 0;
 }
 
@@ -70,14 +79,27 @@ 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_gpio = devm_gpiod_get_optional(dev, "enable", GPIOD_OUT_LOW);
+	if (IS_ERR(mux_gpio->enable_gpio))
+		return dev_err_probe(dev, PTR_ERR(mux_gpio->enable_gpio),
+				     "failed to get optional 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) {
-			dev_err(dev, "invalid idle-state %u\n", idle_state);
-			return -EINVAL;
+	if (ret >= 0) {
+		if (idle_state == MUX_IDLE_DISCONNECT) {
+			if (!mux_gpio->enable_gpio)
+				return dev_err_probe(dev, -EINVAL,
+						     "idle-state disconnect requires enable-gpios\n");
+
+			mux_chip->mux->idle_state = MUX_IDLE_DISCONNECT;
+		} else if (idle_state != MUX_IDLE_AS_IS) {
+			if (idle_state < 0 || idle_state >= mux_chip->mux->states) {
+				return dev_err_probe(dev, -EINVAL,
+						     "invalid idle-state %d\n",
+						     idle_state);
+			}
+			mux_chip->mux->idle_state = idle_state;
 		}
-
-		mux_chip->mux->idle_state = idle_state;
 	}
 
 	ret = devm_regulator_get_enable_optional(dev, "mux");

-- 
2.47.3