[PATCH v5 2/2] gpio: add gpio-line-mux driver

[Jonas Jelonek <jelonek.jonas@gmail.com>]

Add a new driver which provides a 1-to-many mapping for a single real
GPIO using a multiplexer. Each virtual GPIO corresponds to a multiplexer
state which, if set for the multiplexer, connects the real GPIO to the
corresponding virtual GPIO.

This can help in various usecases. One practical case is the special
hardware design of the Realtek-based XS1930-10 switch from Zyxel. It
features two SFP+ ports/cages whose signals are wired directly to the
switch SoC. Although Realtek SoCs are short on GPIOs, there are usually
enough the fit the SFP signals without any hacks.

However, Zyxel did some weird design and connected RX_LOS, MOD_ABS and
TX_FAULT of one SFP cage onto a single GPIO line controlled by a
multiplexer (the same for the other SFP cage). The single multiplexer
controls the lines for both SFP and depending on the state, the
designated 'signal GPIO lines' are connected to one of the three SFP
signals.

Because the SFP core/driver doesn't support multiplexer but needs single
GPIOs for each of the signals, this driver fills the gap between both.
It registers a gpio_chip, provides multiple virtual GPIOs and sets the
backing multiplexer accordingly.

Due to several practical issues, this is input-only and doesn't support
IRQs.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
---
 MAINTAINERS                  |   6 ++
 drivers/gpio/Kconfig         |   9 +++
 drivers/gpio/Makefile        |   1 +
 drivers/gpio/gpio-line-mux.c | 125 +++++++++++++++++++++++++++++++++++
 4 files changed, 141 insertions(+)
 create mode 100644 drivers/gpio/gpio-line-mux.c

diff --git a/MAINTAINERS b/MAINTAINERS
index 3da2c26a796b..66f8706d9b4b 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -10653,6 +10653,12 @@ S:	Maintained
 F:	Documentation/devicetree/bindings/leds/irled/gpio-ir-tx.yaml
 F:	drivers/media/rc/gpio-ir-tx.c
 
+GPIO LINE MUX
+M:	Jonas Jelonek <jelonek.jonas@gmail.com>
+S:	Maintained
+F:	Documentation/devicetree/bindings/gpio/gpio-line-mux.yaml
+F:	drivers/gpio/gpio-line-mux.c
+
 GPIO MOCKUP DRIVER
 M:	Bamvor Jian Zhang <bamv2005@gmail.com>
 L:	linux-gpio@vger.kernel.org
diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index ce237398fa00..5f8082ae99cc 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -1986,6 +1986,15 @@ config GPIO_LATCH
 	  Say yes here to enable a driver for GPIO multiplexers based on latches
 	  connected to other GPIOs.
 
+config GPIO_LINE_MUX
+	tristate "GPIO line mux driver"
+	depends on OF_GPIO
+	select MULTIPLEXER
+	help
+	  Say Y here to support the GPIO line mux, which can provide virtual
+	  GPIOs backed by a shared real GPIO and a multiplexer in a 1-to-many
+	  fashion.
+
 config GPIO_MOCKUP
 	tristate "GPIO Testing Driver (DEPRECATED)"
 	select IRQ_SIM
diff --git a/drivers/gpio/Makefile b/drivers/gpio/Makefile
index ee260a0809d3..6caee52b0356 100644
--- a/drivers/gpio/Makefile
+++ b/drivers/gpio/Makefile
@@ -89,6 +89,7 @@ obj-$(CONFIG_GPIO_IXP4XX)		+= gpio-ixp4xx.o
 obj-$(CONFIG_GPIO_JANZ_TTL)		+= gpio-janz-ttl.o
 obj-$(CONFIG_GPIO_KEMPLD)		+= gpio-kempld.o
 obj-$(CONFIG_GPIO_LATCH)		+= gpio-latch.o
+obj-$(CONFIG_GPIO_LINE_MUX)		+= gpio-line-mux.o
 obj-$(CONFIG_GPIO_LJCA) 		+= gpio-ljca.o
 obj-$(CONFIG_GPIO_LOGICVC)		+= gpio-logicvc.o
 obj-$(CONFIG_GPIO_LOONGSON1)		+= gpio-loongson1.o
diff --git a/drivers/gpio/gpio-line-mux.c b/drivers/gpio/gpio-line-mux.c
new file mode 100644
index 000000000000..759f7880e000
--- /dev/null
+++ b/drivers/gpio/gpio-line-mux.c
@@ -0,0 +1,125 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * GPIO line mux which acts as virtual gpiochip and provides a 1-to-many
+ * mapping between virtual GPIOs and a real GPIO + multiplexer.
+ *
+ * Copyright (c) 2025 Jonas Jelonek <jelonek.jonas@gmail.com>
+ */
+
+#include <linux/gpio/consumer.h>
+#include <linux/gpio/driver.h>
+#include <linux/mod_devicetable.h>
+#include <linux/mutex.h>
+#include <linux/mux/consumer.h>
+#include <linux/platform_device.h>
+
+#define MUX_SELECT_DELAY_US	100
+
+struct gpio_lmux {
+	struct gpio_chip gc;
+	struct mux_control *mux;
+
+	struct gpio_desc *muxed_gpio;
+	/* dynamically sized, must be last */
+	unsigned int gpio_mux_states[];
+};
+
+static int gpio_lmux_gpio_get(struct gpio_chip *gc, unsigned int offset)
+{
+	struct gpio_lmux *glm = gpiochip_get_data(gc);
+	int ret;
+
+	if (offset > gc->ngpio)
+		return -EINVAL;
+
+	ret = mux_control_select_delay(glm->mux, glm->gpio_mux_states[offset],
+				       MUX_SELECT_DELAY_US);
+	if (ret < 0)
+		return ret;
+
+	ret = gpiod_get_raw_value_cansleep(glm->muxed_gpio);
+	mux_control_deselect(glm->mux);
+	return ret;
+}
+
+static int gpio_lmux_gpio_set(struct gpio_chip *gc, unsigned int offset,
+			      int value)
+{
+	return -EOPNOTSUPP;
+}
+
+static int gpio_lmux_gpio_get_direction(struct gpio_chip *gc,
+					unsigned int offset)
+{
+	return GPIO_LINE_DIRECTION_IN;
+}
+
+static int gpio_lmux_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	struct gpio_lmux *glm;
+	unsigned int ngpio;
+	size_t size;
+	int ret;
+
+	ngpio = device_property_count_u32(dev, "gpio-line-mux-states");
+	if (!ngpio)
+		return -EINVAL;
+
+	size = struct_size(glm, gpio_mux_states, ngpio);
+	glm = devm_kzalloc(dev, size, GFP_KERNEL);
+	if (!glm)
+		return -ENOMEM;
+
+	glm->gc.base = -1;
+	glm->gc.can_sleep = true;
+	glm->gc.fwnode = dev_fwnode(dev);
+	glm->gc.label = dev_name(dev);
+	glm->gc.ngpio = ngpio;
+	glm->gc.owner = THIS_MODULE;
+	glm->gc.parent = dev;
+
+	glm->gc.get = gpio_lmux_gpio_get;
+	glm->gc.set = gpio_lmux_gpio_set;
+	glm->gc.get_direction = gpio_lmux_gpio_get_direction;
+
+	glm->mux = devm_mux_control_get(dev, NULL);
+	if (IS_ERR(glm->mux))
+		return dev_err_probe(dev, PTR_ERR(glm->mux),
+				     "could not get mux controller\n");
+
+	glm->muxed_gpio = devm_gpiod_get(dev, "muxed", GPIOD_IN);
+	if (IS_ERR(glm->muxed_gpio))
+		return dev_err_probe(dev, PTR_ERR(glm->muxed_gpio),
+				     "could not get muxed-gpio\n");
+
+	ret = device_property_read_u32_array(dev, "gpio-line-mux-states",
+					     &glm->gpio_mux_states[0], ngpio);
+	if (ret)
+		return dev_err_probe(dev, ret, "could not get mux states\n");
+
+	ret = devm_gpiochip_add_data(dev, &glm->gc, glm);
+	if (ret)
+		return dev_err_probe(dev, ret, "failed to add gpiochip\n");
+
+	return 0;
+}
+
+static const struct of_device_id gpio_lmux_of_match[] = {
+	{ .compatible = "gpio-line-mux" },
+	{ }
+};
+MODULE_DEVICE_TABLE(of, gpio_lmux_of_match);
+
+static struct platform_driver gpio_lmux_driver = {
+	.driver = {
+		.name = "gpio-line-mux",
+		.of_match_table = gpio_lmux_of_match,
+	},
+	.probe = gpio_lmux_probe,
+};
+module_platform_driver(gpio_lmux_driver);
+
+MODULE_AUTHOR("Jonas Jelonek <jelonek.jonas@gmail.com>");
+MODULE_DESCRIPTION("GPIO line mux driver");
+MODULE_LICENSE("GPL");
-- 
2.48.1