[PATCH v4 2/7] misc: minimal mux subsystem and gpio-based mux controller

[Peter Rosin]

Add a new minimalistic subsystem that handles multiplexer controllers.
When multiplexers are used in various places in the kernel, and the
same multiplexer controller can be used for several independent things,
there should be one place to implement support for said multiplexer
controller.

A single multiplexer controller can also be used to control several
parallel multiplexers, that are in turn used by different subsystems
in the kernel, leading to a need to coordinate multiplexer accesses.
The multiplexer subsystem handles this coordination.

This new mux controller subsystem comes with a single backend driver
that controls gpio based multiplexers.

Signed-off-by: Peter Rosin <peda@xxxxxxxxxx>
---
 Documentation/driver-model/devres.txt |   6 +-
 MAINTAINERS                           |   2 +
 drivers/misc/Kconfig                  |  23 +++
 drivers/misc/Makefile                 |   2 +
 drivers/misc/mux-core.c               | 311 ++++++++++++++++++++++++++++++++++
 drivers/misc/mux-gpio.c               | 124 ++++++++++++++
 include/linux/mux.h                   | 160 +++++++++++++++++
 7 files changed, 627 insertions(+), 1 deletion(-)
 create mode 100644 drivers/misc/mux-core.c
 create mode 100644 drivers/misc/mux-gpio.c
 create mode 100644 include/linux/mux.h

diff --git a/Documentation/driver-model/devres.txt b/Documentation/driver-model/devres.txt
index ca9d1eb46bc0..d64ede85b61b 100644
--- a/Documentation/driver-model/devres.txt
+++ b/Documentation/driver-model/devres.txt
@@ -330,7 +330,11 @@ MEM
   devm_kzalloc()
 
 MFD
- devm_mfd_add_devices()
+  devm_mfd_add_devices()
+
+MUX
+  devm_mux_control_get()
+  devm_mux_control_put()
 
 PER-CPU MEM
   devm_alloc_percpu()
diff --git a/MAINTAINERS b/MAINTAINERS
index 35869ed0a50e..9635c9708a13 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -8406,6 +8406,8 @@ MULTIPLEXER SUBSYSTEM
 M:	Peter Rosin <peda@xxxxxxxxxx>
 S:	Maintained
 F:	Documentation/devicetree/bindings/misc/mux-*
+F:	include/linux/mux.h
+F:	drivers/misc/mux-*
 
 MULTISOUND SOUND DRIVER
 M:	Andrew Veliath <andrewtv@xxxxxxx>
diff --git a/drivers/misc/Kconfig b/drivers/misc/Kconfig
index 64971baf11fa..a3ca79e082c7 100644
--- a/drivers/misc/Kconfig
+++ b/drivers/misc/Kconfig
@@ -766,6 +766,29 @@ config PANEL_BOOT_MESSAGE
 	  An empty message will only clear the display at driver init time. Any other
 	  printf()-formatted message is valid with newline and escape codes.
 
+config MULTIPLEXER
+	tristate "Multiplexer subsystem"
+	help
+	  Multiplexer controller subsystem. Multiplexers are used in a
+	  variety of settings, and this subsystem abstracts their use
+	  so that the rest of the kernel sees a common interface. When
+	  multiple parallel multiplexers are controlled by one single
+	  multiplexer controller, this subsystem also coordinates the
+	  multiplexer accesses.
+
+if MULTIPLEXER
+
+config MUX_GPIO
+	tristate "GPIO-controlled MUX controller"
+	depends on OF && GPIOLIB
+	help
+	  GPIO-controlled MUX controller.
+
+	  To compile this driver as a module, choose M here: the module will
+	  be called mux-gpio.
+
+endif
+
 source "drivers/misc/c2port/Kconfig"
 source "drivers/misc/eeprom/Kconfig"
 source "drivers/misc/cb710/Kconfig"
diff --git a/drivers/misc/Makefile b/drivers/misc/Makefile
index 31983366090a..0befa2bba762 100644
--- a/drivers/misc/Makefile
+++ b/drivers/misc/Makefile
@@ -53,6 +53,8 @@ obj-$(CONFIG_ECHO)		+= echo/
 obj-$(CONFIG_VEXPRESS_SYSCFG)	+= vexpress-syscfg.o
 obj-$(CONFIG_CXL_BASE)		+= cxl/
 obj-$(CONFIG_PANEL)             += panel.o
+obj-$(CONFIG_MULTIPLEXER)      	+= mux-core.o
+obj-$(CONFIG_MUX_GPIO)		+= mux-gpio.o
 
 lkdtm-$(CONFIG_LKDTM)		+= lkdtm_core.o
 lkdtm-$(CONFIG_LKDTM)		+= lkdtm_bugs.o
diff --git a/drivers/misc/mux-core.c b/drivers/misc/mux-core.c
new file mode 100644
index 000000000000..6617fc78c072
--- /dev/null
+++ b/drivers/misc/mux-core.c
@@ -0,0 +1,311 @@
+/*
+ * Multiplexer subsystem
+ *
+ * Copyright (C) 2016 Axentia Technologies AB
+ *
+ * Author: Peter Rosin <peda@xxxxxxxxxx>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+#define pr_fmt(fmt) "mux-core: " fmt
+
+#include <linux/device.h>
+#include <linux/err.h>
+#include <linux/idr.h>
+#include <linux/module.h>
+#include <linux/mux.h>
+#include <linux/of.h>
+#include <linux/of_platform.h>
+#include <linux/slab.h>
+
+static struct class mux_class = {
+	.name = "mux",
+	.owner = THIS_MODULE,
+};
+
+static int __init mux_init(void)
+{
+	return class_register(&mux_class);
+}
+
+static void __exit mux_exit(void)
+{
+	class_unregister(&mux_class);
+}
+
+static DEFINE_IDA(mux_ida);
+
+static void mux_control_release(struct device *dev)
+{
+	struct mux_control *mux = to_mux_control(dev);
+
+	ida_simple_remove(&mux_ida, mux->id);
+	kfree(mux);
+}
+
+static struct device_type mux_type = {
+	.name = "mux-control",
+	.release = mux_control_release,
+};
+
+struct mux_control *mux_control_alloc(struct device *dev, size_t sizeof_priv)
+{
+	struct mux_control *mux;
+
+	mux = kzalloc(sizeof(*mux) + sizeof_priv, GFP_KERNEL);
+	if (!mux)
+		return NULL;
+
+	mux->dev.class = &mux_class;
+	mux->dev.type = &mux_type;
+	mux->dev.parent = dev;
+	mux->dev.of_node = dev->of_node;
+	dev_set_drvdata(&mux->dev, mux);
+
+	mux->id = ida_simple_get(&mux_ida, 0, 0, GFP_KERNEL);
+	if (mux->id < 0) {
+		pr_err("muxX failed to get a device id\n");
+		kfree(mux);
+		return NULL;
+	}
+	dev_set_name(&mux->dev, "mux%d", mux->id);
+
+	init_rwsem(&mux->lock);
+	mux->cached_state = -1;
+	mux->idle_state = -1;
+
+	device_initialize(&mux->dev);
+
+	return mux;
+}
+EXPORT_SYMBOL_GPL(mux_control_alloc);
+
+int mux_control_register(struct mux_control *mux)
+{
+	int ret;
+
+	ret = device_add(&mux->dev);
+	if (ret < 0)
+		return ret;
+
+	if (mux->drv_pdev)
+		return ret;
+
+	ret = of_platform_populate(mux->dev.of_node, NULL, NULL, &mux->dev);
+	if (ret < 0)
+		device_del(&mux->dev);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(mux_control_register);
+
+void mux_control_unregister(struct mux_control *mux)
+{
+	if (!mux->drv_pdev)
+		of_platform_depopulate(&mux->dev);
+
+	device_del(&mux->dev);
+}
+EXPORT_SYMBOL_GPL(mux_control_unregister);
+
+void mux_control_put(struct mux_control *mux)
+{
+	struct platform_device *drv_pdev;
+
+	if (!mux)
+		return;
+	put_device(&mux->dev);
+
+	if (!mux->drv_pdev)
+		return;
+
+	if (atomic_read(&mux->dev.kobj.kref.refcount) != 1)
+		return;
+
+	/*
+	 * Only one ref left, and the mux core created the driver
+	 * that presumably holds it. Time to release the driver so
+	 * that it can let go of the final ref.
+	 */
+	drv_pdev = mux->drv_pdev;
+	mux->drv_pdev = NULL;
+	platform_device_unregister(drv_pdev);
+}
+EXPORT_SYMBOL_GPL(mux_control_put);
+
+static int mux_control_set(struct mux_control *mux, int state)
+{
+	int ret = mux->ops->set(mux, state);
+
+	mux->cached_state = ret < 0 ? -1 : state;
+
+	return ret;
+}
+
+int mux_control_select(struct mux_control *mux, int state)
+{
+	int ret;
+
+	if (down_read_trylock(&mux->lock)) {
+		if (mux->cached_state == state)
+			return 0;
+
+		/* Sigh, the mux needs updating... */
+		up_read(&mux->lock);
+	}
+
+	/* ...or it's just contended. */
+	down_write(&mux->lock);
+
+	if (mux->cached_state == state) {
+		/*
+		 * Hmmm, someone else changed the mux to my liking.
+		 * That makes me wonder how long I waited for nothing?
+		 */
+		downgrade_write(&mux->lock);
+		return 0;
+	}
+
+	ret = mux_control_set(mux, state);
+	if (ret < 0) {
+		if (mux->idle_state != -1)
+			mux_control_set(mux, mux->idle_state);
+
+		up_write(&mux->lock);
+		return ret;
+	}
+
+	downgrade_write(&mux->lock);
+
+	return 1;
+}
+EXPORT_SYMBOL_GPL(mux_control_select);
+
+int mux_control_deselect(struct mux_control *mux)
+{
+	int ret = 0;
+
+	if (mux->idle_state != -1 && mux->cached_state != mux->idle_state)
+		ret = mux_control_set(mux, mux->idle_state);
+
+	up_read(&mux->lock);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(mux_control_deselect);
+
+static int of_dev_node_match(struct device *dev, const void *data)
+{
+	return dev->of_node == data;
+}
+
+static struct mux_control *of_find_mux_by_node(struct device_node *np)
+{
+	struct device *dev;
+
+	dev = class_find_device(&mux_class, NULL, np, of_dev_node_match);
+
+	return dev ? to_mux_control(dev) : NULL;
+}
+
+struct mux_control *mux_control_get(struct device *dev)
+{
+	struct device_node *mux_np;
+	struct platform_device *drv_pdev;
+	struct mux_control *mux;
+
+	if (!dev->of_node)
+		return ERR_PTR(-ENODEV);
+
+	mux_np = of_get_child_by_name(dev->of_node, "mux-controller");
+	if (!mux_np) {
+		mux = of_find_mux_by_node(dev->of_node->parent);
+		if (!mux)
+			return ERR_PTR(-EPROBE_DEFER);
+
+		return mux;
+	}
+
+	drv_pdev = of_platform_device_create(mux_np, "mux-controller", dev);
+	of_node_put(mux_np);
+
+	if (!drv_pdev)
+		return ERR_PTR(-EPROBE_DEFER);
+
+	mux = of_find_mux_by_node(mux_np);
+	if (!mux) {
+		platform_device_unregister(drv_pdev);
+		return ERR_PTR(-ENODEV);
+	}
+
+	/*
+	 * Aiee, holding a reference to the driver that holds a
+	 * reference back. Circular deps, and refcounts never
+	 * hit zero -> leak.
+	 * So, watch for the mux-controller refcount to hit one
+	 * and release the driver-ref then, knowing that the
+	 * driver will (probably) not let go of its back-ref as
+	 * long as the mux core holds a ref to it.
+	 */
+
+	mux->drv_pdev = drv_pdev;
+	return mux;
+}
+EXPORT_SYMBOL_GPL(mux_control_get);
+
+static void devm_mux_control_free(struct device *dev, void *res)
+{
+	struct mux_control *mux = *(struct mux_control **)res;
+
+	mux_control_put(mux);
+}
+
+struct mux_control *devm_mux_control_get(struct device *dev)
+{
+	struct mux_control **ptr, *mux;
+
+	ptr = devres_alloc(devm_mux_control_free, sizeof(*ptr), GFP_KERNEL);
+	if (!ptr)
+		return ERR_PTR(-ENOMEM);
+
+	mux = mux_control_get(dev);
+	if (IS_ERR(mux)) {
+		devres_free(ptr);
+		return mux;
+	}
+
+	*ptr = mux;
+	devres_add(dev, ptr);
+
+	return mux;
+}
+EXPORT_SYMBOL_GPL(devm_mux_control_get);
+
+static int devm_mux_control_match(struct device *dev, void *res, void *data)
+{
+	struct mux_control **r = res;
+
+	if (!r || !*r) {
+		WARN_ON(!r || !*r);
+		return 0;
+	}
+
+	return *r == data;
+}
+
+void devm_mux_control_put(struct device *dev, struct mux_control *mux)
+{
+	WARN_ON(devres_release(dev, devm_mux_control_free,
+			       devm_mux_control_match, mux));
+}
+EXPORT_SYMBOL_GPL(devm_mux_control_put);
+
+subsys_initcall(mux_init);
+module_exit(mux_exit);
+
+MODULE_AUTHOR("Peter Rosin <peda@xxxxxxxxxx");
+MODULE_DESCRIPTION("MUX subsystem");
+MODULE_LICENSE("GPL v2");
diff --git a/drivers/misc/mux-gpio.c b/drivers/misc/mux-gpio.c
new file mode 100644
index 000000000000..66798d920e17
--- /dev/null
+++ b/drivers/misc/mux-gpio.c
@@ -0,0 +1,124 @@
+/*
+ * GPIO-controlled multiplexer driver
+ *
+ * Copyright (C) 2016 Axentia Technologies AB
+ *
+ * Author: Peter Rosin <peda@xxxxxxxxxx>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+#include <linux/err.h>
+#include <linux/gpio/consumer.h>
+#include <linux/module.h>
+#include <linux/mux.h>
+#include <linux/of.h>
+#include <linux/of_platform.h>
+#include <linux/platform_device.h>
+
+struct mux_gpio {
+	struct gpio_descs *gpios;
+};
+
+static int mux_gpio_set(struct mux_control *mux, int state)
+{
+	struct mux_gpio *mux_gpio = mux_control_priv(mux);
+	int values[mux_gpio->gpios->ndescs];
+	int i;
+
+	for (i = 0; i < mux_gpio->gpios->ndescs; i++)
+		values[i] = (state >> i) & 1;
+
+	gpiod_set_array_value_cansleep(mux_gpio->gpios->ndescs,
+				       mux_gpio->gpios->desc,
+				       values);
+
+	return 0;
+}
+
+static const struct mux_control_ops mux_gpio_ops = {
+	.set = mux_gpio_set,
+};
+
+static const struct of_device_id mux_gpio_dt_ids[] = {
+	{ .compatible = "mux-gpio", },
+	{ /* sentinel */ }
+};
+MODULE_DEVICE_TABLE(of, mux_gpio_dt_ids);
+
+static int mux_gpio_probe(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	struct device_node *np = pdev->dev.of_node;
+	struct mux_control *mux;
+	struct mux_gpio *mux_gpio;
+	u32 idle_state;
+	int ret;
+
+	if (!np)
+		return -ENODEV;
+
+	mux = mux_control_alloc(dev, sizeof(*mux_gpio));
+	if (!mux)
+		return -ENOMEM;
+	mux_gpio = mux_control_priv(mux);
+	mux->ops = &mux_gpio_ops;
+
+	platform_set_drvdata(pdev, mux);
+
+	mux_gpio->gpios = devm_gpiod_get_array(dev, "mux", GPIOD_OUT_LOW);
+	if (IS_ERR(mux_gpio->gpios)) {
+		if (PTR_ERR(mux_gpio->gpios) != -EPROBE_DEFER)
+			dev_err(dev, "failed to get gpios\n");
+		mux_control_put(mux);
+		return PTR_ERR(mux_gpio->gpios);
+	}
+	mux->states = 1 << mux_gpio->gpios->ndescs;
+
+	ret = of_property_read_u32(np, "idle-state", &idle_state);
+	if (ret >= 0) {
+		if (idle_state >= mux->states) {
+			dev_err(dev, "invalid idle-state %u\n", idle_state);
+			return -EINVAL;
+		}
+		mux->idle_state = idle_state;
+	}
+
+	ret = mux_control_register(mux);
+	if (ret < 0) {
+		dev_err(dev, "failed to register mux_control\n");
+		mux_control_put(mux);
+		return ret;
+	}
+
+	dev_info(dev, "%u-way mux-controller registered\n", mux->states);
+
+	return 0;
+}
+
+static int mux_gpio_remove(struct platform_device *pdev)
+{
+	struct device *dev = &pdev->dev;
+	struct mux_control *mux = to_mux_control(dev);
+
+	mux_control_unregister(mux);
+	mux_control_put(mux);
+
+	return 0;
+}
+
+static struct platform_driver mux_gpio_driver = {
+	.driver = {
+		.name = "mux-gpio",
+		.of_match_table	= of_match_ptr(mux_gpio_dt_ids),
+	},
+	.probe = mux_gpio_probe,
+	.remove = mux_gpio_remove,
+};
+module_platform_driver(mux_gpio_driver);
+
+MODULE_AUTHOR("Peter Rosin <peda@xxxxxxxxxx");
+MODULE_DESCRIPTION("GPIO-controlled multiplexer driver");
+MODULE_LICENSE("GPL v2");
diff --git a/include/linux/mux.h b/include/linux/mux.h
new file mode 100644
index 000000000000..be7c138f9228
--- /dev/null
+++ b/include/linux/mux.h
@@ -0,0 +1,160 @@
+/*
+ * mux.h - definitions for the multiplexer interface
+ *
+ * Copyright (C) 2016 Axentia Technologies AB
+ *
+ * Author: Peter Rosin <peda@xxxxxxxxxx>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+#ifndef _LINUX_MUX_H
+#define _LINUX_MUX_H
+
+#include <linux/device.h>
+#include <linux/rwsem.h>
+
+struct mux_control;
+struct platform_device;
+
+struct mux_control_ops {
+	int (*set)(struct mux_control *mux, int state);
+};
+
+/**
+ * struct mux_control - Represents a mux controller.
+ * @lock:		Protects the mux controller state.
+ * @dev:		Device structure.
+ * @id:			Used to identify the device internally.
+ * @states:		The number of mux controller states.
+ * @cached_state:	The current mux controller state, or -1 if none.
+ * @idle_state:		The mux controller state to use when inactive, or -1
+ *			for none.
+ * @ops:		Mux controller operations.
+ */
+struct mux_control {
+	struct rw_semaphore lock; /* protects the state of the mux */
+
+	struct device dev;
+	int id;
+	struct platform_device *drv_pdev;
+
+	unsigned int states;
+	int cached_state;
+	int idle_state;
+
+	const struct mux_control_ops *ops;
+};
+
+#define to_mux_control(x) container_of((x), struct mux_control, dev)
+
+/**
+ * mux_control_priv() - Get the extra memory reserved by mux_control_alloc().
+ * @mux: The mux-control to get the extra memory from.
+ *
+ * Return: Pointer to the private memory requested by the allocator.
+ */
+static inline void *mux_control_priv(struct mux_control *mux)
+{
+	return mux + 1;
+}
+
+/**
+ * mux_control_alloc() - Allocate a mux-control.
+ * @dev: The device implementing the mux interface.
+ * @sizeof_priv: Size of extra memory area for private use by the caller.
+ *
+ * Return: A pointer to the new mux-control, NULL on failure.
+ */
+struct mux_control *mux_control_alloc(struct device *dev, size_t sizeof_priv);
+
+/**
+ * mux_control_register() - Register a mux-control, thus readying it for use.
+ * @mux: The mux-control to register.
+ *
+ * Do not retry registration of the same mux-control on failure. You should
+ * instead put it away with mux_control_put() and allocate a new one, if you
+ * for some reason would like to retry registration.
+ *
+ * Return: Zero on success or a negative errno on error.
+ */
+int mux_control_register(struct mux_control *mux);
+
+/**
+ * mux_control_unregister() - Take the mux-control off-line.
+ * @mux: The mux-control to unregister.
+ *
+ * mux_control_unregister() reverses the effects of mux_control_register().
+ * But not completely, you should not try to call mux_control_register()
+ * on a mux-control that has been registered before.
+ */
+void mux_control_unregister(struct mux_control *mux);
+
+/**
+ * mux_control_put() - Put away the mux-control for good.
+ * @mux: The mux-control to put away.
+ *
+ * mux_control_put() reverses the effects of either mux_control_alloc() or
+ * mux_control_get().
+ */
+void mux_control_put(struct mux_control *mux);
+
+/**
+ * mux_control_select() - Select the given multiplexer state.
+ * @mux: The mux-control to request a change of state from.
+ * @state: The new requested state.
+ *
+ * Make sure to call mux_control_deselect() when the operation is complete and
+ * the mux-control is free for others to use, but do not call
+ * mux_control_deselect() if mux_control_select() fails.
+ *
+ * Return: 0 if the requested state was already active, or 1 it the
+ * mux-control state was changed to the requested state. Or a negavive
+ * errno on error.
+ *
+ * Note that the difference in return value of zero or one is of
+ * questionable value; especially if the mux-control has several independent
+ * consumers, which is something the consumers should not be making
+ * assumptions about.
+ */
+int mux_control_select(struct mux_control *mux, int state);
+
+/**
+ * mux_control_deselect() - Deselect the previously selected multiplexer state.
+ * @mux: The mux-control to deselect.
+ *
+ * Return: 0 on success and a negative errno on error. An error can only
+ * occur if the mux has an idle state. Note that even if an error occurs, the
+ * mux-control is unlocked for others to access.
+ */
+int mux_control_deselect(struct mux_control *mux);
+
+/**
+ * mux_control_get() - Get the mux-control for a device.
+ * @dev: The device that needs a mux-control.
+ *
+ * Return: A pointer to the mux-control, or an ERR_PTR with a negative errno.
+ */
+struct mux_control *mux_control_get(struct device *dev);
+
+/**
+ * devm_mux_control_get() - Get the mux-control for a device, with resource
+ *			    management.
+ * @dev: The device that needs a mux-control.
+ *
+ * Return: Pointer to the mux-control, or an ERR_PTR with a negative errno.
+ */
+struct mux_control *devm_mux_control_get(struct device *dev);
+
+/**
+ * devm_mux_control_put() - Resource-managed version mux_control_put().
+ * @dev: The device that originally got the mux-control.
+ * @mux: The mux-control to put away.
+ *
+ * Note that you do not normally need to call this function.
+ */
+void devm_mux_control_put(struct device *dev, struct mux_control *mux);
+
+#endif /* _LINUX_MUX_H */
-- 
2.1.4