[PATCH v5 0/2] add gpio-line-mux

[PATCH v5 0/2] add gpio-line-mux

[Jonas Jelonek]

This proposes a new type of virtual GPIO controller and corresponding
driver to provide a 1-to-many mapping between virtual GPIOs and a single
real GPIO in combination with a multiplexer. Existing drivers apparently
do not serve the purpose for what I need.

I came across an issue with a switch device from Zyxel which has two
SFP+ cages. Most similar switches either wire up the SFP signals
(RX_LOS, MOD_ABS, TX_FAULT, TX_DISABLE) directly to the SoC (if it has
enough GPIOs) or two a GPIO expander (for which a driver usually
exists). However, Zyxel decided to do it differently in the following
way:
  The signals RX_LOS, MOD_ABS and TX_FAULT share a single GPIO line to
  the SoC. Which one is actually connected to that GPIO line at a time
  is controlled by a separate multiplexer, a GPIO multiplexer in this
  case (which uses two other GPIOs). Only the TX_DISABLE is separate.

The SFP core/driver doesn't seem to support such a usecase for now, for
each signal one needs to specify a separate GPIO like:

  los-gpio = <&gpio0 0 GPIO_ACTIVE_HIGH>;
  mod-def0-gpio = <&gpio0 1 GPIO_ACTIVE_LOW>;
  ...

But for my device, I actually need to directly specify multiplexing
behavior in the SFP node or provide a mux-controller with 'mux-controls'.

To fill this gap, I created a dt-schema and a working driver which
exactly does what is needed. It takes a phandle to a mux-controller and
the 'shared' gpio, and provides several virtual GPIOs based on the
gpio-line-mux-states property.

This virtual gpio-controller can then be referenced in the '-gpio'
properties of the SFP node (or other nodes depending on the usecase) as
usual and do not require any modification to the SFP core/driver.

---
Changelog:

v5: - renamed "shared" to "muxed" to avoid confusion with Bartosz' work
    - dropped Reviewed-by of Krzysztof due to binding change
    - use GPIOD_IN in devm_gpiod_get instead of calling
      gpiod_direction_input explicitly afterwards

Link to v4:
https://lore.kernel.org/linux-gpio/20251105103607.393353-1-jelonek.jonas@gmail.com/

v4: - dropped useless cast (as suggested by Thomas)
    - dropped unneeded locking (as suggested by Peter)
    - fixed wording in commit message
    - included Reviewed-by of Krzysztof

Link to v3:
https://lore.kernel.org/linux-gpio/20251104210021.247476-1-jelonek.jonas@gmail.com/

v3: - fixed dt_binding_check errors in DT schema
    - as requested by Rob (for DT schema):
      - removed example from gpio-mux.yaml
      - added '|' to preserve formatting
      - 'shared-gpio' --> 'shared-gpios'
    - general fixes to DT schema
    - use mux_control_select_delay (as suggested by Peter) with
      hopefully reasonable delay of 100us
    - gpiochip ops implementation changes:
      - drop '.set' implementation (as suggested by Peter)
      - new '.set' implementation just returning -EOPNOTSUPP
      - '.direction_output' and '.direction_input' dropped
      - '.get_direction' returns fixed value for 'input'
    - direction of shared gpio set to input during probe
    - as suggested by Thomas
      - usage of dev_err_probe
      - further simplifications

    Since the consensus was that this should be input-only,
    '.direction_output' and '.direction_input' have been dropped
    completely, as suggested in the docs of struct gpio_chip. '.set' is
    kept but returns -ENOTSUPP.

    The shared GPIO is set to input during probe, thus '.direction_input'
    doesn't need to be implemented. '.get_direction' is kept (as
    suggested in docs of struct gpio_chip) but always returns
    GPIO_LINE_DIRECTION_IN.

Link to v2:
https://lore.kernel.org/linux-gpio/20251026231754.2368904-1-jelonek.jonas@gmail.com/

v2: - as requested by Linus:
      - renamed from 'gpio-split' to 'gpio-line-mux'
      - added better description and examples to DT bindings
    - simplified driver
    - added missing parts to DT bindings
    - dropped RFC tag
    - renamed patchset

Link to v1 (in case it isn't linked properly due to changed title):
https://lore.kernel.org/linux-gpio/20251009223501.570949-1-jelonek.jonas@gmail.com/

---
Jonas Jelonek (2):
  dt-bindings: gpio: add gpio-line-mux controller
  gpio: add gpio-line-mux driver

 .../bindings/gpio/gpio-line-mux.yaml          | 109 +++++++++++++++
 MAINTAINERS                                   |   6 +
 drivers/gpio/Kconfig                          |   9 ++
 drivers/gpio/Makefile                         |   1 +
 drivers/gpio/gpio-line-mux.c                  | 125 ++++++++++++++++++
 5 files changed, 250 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/gpio/gpio-line-mux.yaml
 create mode 100644 drivers/gpio/gpio-line-mux.c


base-commit: bac88be0d2a83daf761129828e7ae3c79cc260c2
-- 
2.48.1

[PATCH v5 1/2] dt-bindings: gpio: add gpio-line-mux controller

[Jonas Jelonek]

Add dt-schema for a gpio-line-mux controller which exposes virtual
GPIOs for a shared GPIO controlled by a multiplexer, e.g. a gpio-mux.

The gpio-line-mux controller is a gpio-controller, thus has mostly the
same semantics. However, it requires a mux-control to be specified upon
which it will operate.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
---
 .../bindings/gpio/gpio-line-mux.yaml          | 109 ++++++++++++++++++
 1 file changed, 109 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/gpio/gpio-line-mux.yaml

diff --git a/Documentation/devicetree/bindings/gpio/gpio-line-mux.yaml b/Documentation/devicetree/bindings/gpio/gpio-line-mux.yaml
new file mode 100644
index 000000000000..0228e9915b92
--- /dev/null
+++ b/Documentation/devicetree/bindings/gpio/gpio-line-mux.yaml
@@ -0,0 +1,109 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/gpio/gpio-line-mux.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: GPIO line mux
+
+maintainers:
+  - Jonas Jelonek <jelonek.jonas@gmail.com>
+
+description: |
+  A GPIO controller to provide virtual GPIOs for a 1-to-many input-only mapping
+  backed by a single shared GPIO and a multiplexer. A simple illustrated
+  example is
+
+            +----- A
+    IN     /
+    <-----o------- B
+        / |\
+        | | +----- C
+        | |  \
+        | |   +--- D
+        | |
+       M1 M0
+
+    MUX CONTROL
+
+     M1 M0   IN
+      0  0   A
+      0  1   B
+      1  0   C
+      1  1   D
+
+  This can be used in case a real GPIO is connected to multiple inputs and
+  controlled by a multiplexer, and another subsystem/driver does not work
+  directly with the multiplexer subsystem.
+
+properties:
+  compatible:
+    const: gpio-line-mux
+
+  gpio-controller: true
+
+  "#gpio-cells":
+    const: 2
+
+  gpio-line-mux-states:
+    description: Mux states corresponding to the virtual GPIOs.
+    $ref: /schemas/types.yaml#/definitions/uint32-array
+
+  gpio-line-names: true
+
+  mux-controls:
+    $ref: /schemas/types.yaml#/definitions/phandle-array
+    maxItems: 1
+    description:
+      Phandle to the multiplexer to control access to the GPIOs.
+
+  ngpios: false
+
+  muxed-gpios:
+    maxItems: 1
+    description:
+      GPIO which is the '1' in 1-to-many and is shared by the virtual GPIOs
+      and controlled via the mux.
+
+required:
+  - compatible
+  - gpio-controller
+  - gpio-line-mux-states
+  - mux-controls
+  - muxed-gpios
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/gpio/gpio.h>
+    #include <dt-bindings/mux/mux.h>
+
+    sfp_gpio_mux: mux-controller-1 {
+        compatible = "gpio-mux";
+        mux-gpios = <&gpio0 0 GPIO_ACTIVE_HIGH>,
+                    <&gpio0 1 GPIO_ACTIVE_HIGH>;
+        #mux-control-cells = <0>;
+        idle-state = <MUX_IDLE_AS_IS>;
+    };
+
+    sfp1_gpio: sfp-gpio-1 {
+        compatible = "gpio-line-mux";
+        gpio-controller;
+        #gpio-cells = <2>;
+
+        mux-controls = <&sfp_gpio_mux>;
+        muxed-gpios = <&gpio0 2 GPIO_ACTIVE_HIGH>;
+
+        gpio-line-names = "SFP1_LOS", "SFP1_MOD_ABS", "SFP1_TX_FAULT";
+        gpio-line-mux-states = <0>, <1>, <3>;
+    };
+
+    sfp1: sfp-p1 {
+        compatible = "sff,sfp";
+
+        i2c-bus = <&sfp1_i2c>;
+        los-gpios = <&sfp1_gpio 0 GPIO_ACTIVE_HIGH>;
+        mod-def0-gpios = <&sfp1_gpio 1 GPIO_ACTIVE_LOW>;
+        tx-fault-gpios = <&sfp1_gpio 2 GPIO_ACTIVE_HIGH>;
+    };
-- 
2.48.1

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

[Jonas Jelonek]

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

Re: [PATCH v5 1/2] dt-bindings: gpio: add gpio-line-mux controller

[Conor Dooley]

[-- Attachment #1: Type: text/plain, Size: 920 bytes --]

On Wed, Nov 05, 2025 at 04:36:09PM +0000, Jonas Jelonek wrote:
> Add dt-schema for a gpio-line-mux controller which exposes virtual
> GPIOs for a shared GPIO controlled by a multiplexer, e.g. a gpio-mux.
> 
> The gpio-line-mux controller is a gpio-controller, thus has mostly the
> same semantics. However, it requires a mux-control to be specified upon
> which it will operate.
> 
> Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>

> +  muxed-gpios:

v5: - renamed "shared" to "muxed" to avoid confusion with Bartosz' work
    - dropped Reviewed-by of Krzysztof due to binding change

FWIW, I wouldn't bother dropping the tag for something this simple.

Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
pw-bot: not-applicable

> +    maxItems: 1
> +    description:
> +      GPIO which is the '1' in 1-to-many and is shared by the virtual GPIOs
> +      and controlled via the mux.

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