gpio: improve support for shared GPIOs

Merge series from Bartosz Golaszewski <brgl@bgdev.pl>:

Problem statement: GPIOs are implemented as a strictly exclusive
resource in the kernel but there are lots of platforms on which single
pin is shared by multiple devices which don't communicate so need some
way of properly sharing access to a GPIO. What we have now is the
GPIOD_FLAGS_BIT_NONEXCLUSIVE flag which was introduced as a hack and
doesn't do any locking or arbitration of access - it literally just hand
the same GPIO descriptor to all interested users.

The proposed solution is composed of three major parts: the high-level,
shared GPIO proxy driver that arbitrates access to the shared pin and
exposes a regular GPIO chip interface to consumers, a low-level shared
GPIOLIB module that scans firmware nodes and creates auxiliary devices
that attach to the proxy driver and finally a set of core GPIOLIB
changes that plug the former into the GPIO lookup path.

The changes are implemented in a way that allows to seamlessly compile
out any code related to sharing GPIOs for systems that don't need it.

The practical use-case for this are the powerdown GPIOs shared by
speakers on Qualcomm db845c platform, however I have also extensively
tested it using gpio-virtuser on arm64 qemu with various DT
configurations.

1 files changed, 558 insertions, 0 deletions

diff --git a/drivers/gpio/gpiolib-shared.c b/drivers/gpio/gpiolib-shared.c
new file mode 100644
index 000000000000..fa1d16635ea7
--- /dev/null
+++ b/drivers/gpio/gpiolib-shared.c
@@ -0,0 +1,558 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2025 Linaro Ltd.
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/auxiliary_bus.h>
+#include <linux/cleanup.h>
+#include <linux/device.h>
+#include <linux/fwnode.h>
+#include <linux/gpio/consumer.h>
+#include <linux/gpio/machine.h>
+#include <linux/idr.h>
+#include <linux/kref.h>
+#include <linux/list.h>
+#include <linux/lockdep.h>
+#include <linux/module.h>
+#include <linux/mutex.h>
+#include <linux/of.h>
+#include <linux/overflow.h>
+#include <linux/printk.h>
+#include <linux/property.h>
+#include <linux/slab.h>
+#include <linux/string.h>
+
+#include "gpiolib.h"
+#include "gpiolib-shared.h"
+
+/* Represents a single reference to a GPIO pin. */
+struct gpio_shared_ref {
+	struct list_head list;
+	/* Firmware node associated with this GPIO's consumer. */
+	struct fwnode_handle *fwnode;
+	/* GPIO flags this consumer uses for the request. */
+	enum gpiod_flags flags;
+	char *con_id;
+	int dev_id;
+	struct auxiliary_device adev;
+	struct gpiod_lookup_table *lookup;
+};
+
+/* Represents a single GPIO pin. */
+struct gpio_shared_entry {
+	struct list_head list;
+	/* Firmware node associated with the GPIO controller. */
+	struct fwnode_handle *fwnode;
+	/* Hardware offset of the GPIO within its chip. */
+	unsigned int offset;
+	/* Index in the property value array. */
+	size_t index;
+	struct gpio_shared_desc *shared_desc;
+	struct kref ref;
+	struct list_head refs;
+};
+
+static LIST_HEAD(gpio_shared_list);
+static DEFINE_MUTEX(gpio_shared_lock);
+static DEFINE_IDA(gpio_shared_ida);
+
+static struct gpio_shared_entry *
+gpio_shared_find_entry(struct fwnode_handle *controller_node,
+		       unsigned int offset)
+{
+	struct gpio_shared_entry *entry;
+
+	list_for_each_entry(entry, &gpio_shared_list, list) {
+		if (entry->fwnode == controller_node && entry->offset == offset)
+			return entry;
+	}
+
+	return NULL;
+}
+
+#if IS_ENABLED(CONFIG_OF)
+static int gpio_shared_of_traverse(struct device_node *curr)
+{
+	struct gpio_shared_entry *entry;
+	size_t con_id_len, suffix_len;
+	struct fwnode_handle *fwnode;
+	struct of_phandle_args args;
+	struct property *prop;
+	unsigned int offset;
+	const char *suffix;
+	int ret, count, i;
+
+	for_each_property_of_node(curr, prop) {
+		/*
+		 * The standard name for a GPIO property is "foo-gpios"
+		 * or "foo-gpio". Some bindings also use "gpios" or "gpio".
+		 * There are some legacy device-trees which have a different
+		 * naming convention and for which we have rename quirks in
+		 * place in gpiolib-of.c. I don't think any of them require
+		 * support for shared GPIOs so for now let's just ignore
+		 * them. We can always just export the quirk list and
+		 * iterate over it here.
+		 */
+		if (!strends(prop->name, "-gpios") &&
+		    !strends(prop->name, "-gpio") &&
+		    strcmp(prop->name, "gpios") != 0 &&
+		    strcmp(prop->name, "gpio") != 0)
+			continue;
+
+		count = of_count_phandle_with_args(curr, prop->name,
+						   "#gpio-cells");
+		if (count <= 0)
+			continue;
+
+		for (i = 0; i < count; i++) {
+			struct device_node *np __free(device_node) = NULL;
+
+			ret = of_parse_phandle_with_args(curr, prop->name,
+							 "#gpio-cells", i,
+							 &args);
+			if (ret)
+				continue;
+
+			np = args.np;
+
+			if (!of_property_present(np, "gpio-controller"))
+				continue;
+
+			/*
+			 * We support 1, 2 and 3 cell GPIO bindings in the
+			 * kernel currently. There's only one old MIPS dts that
+			 * has a one-cell binding but there's no associated
+			 * consumer so it may as well be an error. There don't
+			 * seem to be any 3-cell users of non-exclusive GPIOs,
+			 * so we can skip this as well. Let's occupy ourselves
+			 * with the predominant 2-cell binding with the first
+			 * cell indicating the hardware offset of the GPIO and
+			 * the second defining the GPIO flags of the request.
+			 */
+			if (args.args_count != 2)
+				continue;
+
+			fwnode = of_fwnode_handle(args.np);
+			offset = args.args[0];
+
+			entry = gpio_shared_find_entry(fwnode, offset);
+			if (!entry) {
+				entry = kzalloc(sizeof(*entry), GFP_KERNEL);
+				if (!entry)
+					return -ENOMEM;
+
+				entry->fwnode = fwnode_handle_get(fwnode);
+				entry->offset = offset;
+				entry->index = count;
+				INIT_LIST_HEAD(&entry->refs);
+
+				list_add_tail(&entry->list, &gpio_shared_list);
+			}
+
+			struct gpio_shared_ref *ref __free(kfree) =
+					kzalloc(sizeof(*ref), GFP_KERNEL);
+			if (!ref)
+				return -ENOMEM;
+
+			ref->fwnode = fwnode_handle_get(of_fwnode_handle(curr));
+			ref->flags = args.args[1];
+
+			if (strends(prop->name, "gpios"))
+				suffix = "-gpios";
+			else if (strends(prop->name, "gpio"))
+				suffix = "-gpio";
+			else
+				suffix = NULL;
+			if (!suffix)
+				continue;
+
+			/* We only set con_id if there's actually one. */
+			if (strcmp(prop->name, "gpios") && strcmp(prop->name, "gpio")) {
+				ref->con_id = kstrdup(prop->name, GFP_KERNEL);
+				if (!ref->con_id)
+					return -ENOMEM;
+
+				con_id_len = strlen(ref->con_id);
+				suffix_len = strlen(suffix);
+
+				ref->con_id[con_id_len - suffix_len] = '\0';
+			}
+
+			ref->dev_id = ida_alloc(&gpio_shared_ida, GFP_KERNEL);
+			if (ref->dev_id < 0) {
+				kfree(ref->con_id);
+				return -ENOMEM;
+			}
+
+			if (!list_empty(&entry->refs))
+				pr_debug("GPIO %u at %s is shared by multiple firmware nodes\n",
+					 entry->offset, fwnode_get_name(entry->fwnode));
+
+			list_add_tail(&no_free_ptr(ref)->list, &entry->refs);
+		}
+	}
+
+	for_each_child_of_node_scoped(curr, child) {
+		ret = gpio_shared_of_traverse(child);
+		if (ret)
+			return ret;
+	}
+
+	return 0;
+}
+
+static int gpio_shared_of_scan(void)
+{
+	return gpio_shared_of_traverse(of_root);
+}
+#else
+static int gpio_shared_of_scan(void)
+{
+	return 0;
+}
+#endif /* CONFIG_OF */
+
+static void gpio_shared_adev_release(struct device *dev)
+{
+
+}
+
+static int gpio_shared_make_adev(struct gpio_device *gdev,
+				 struct gpio_shared_ref *ref)
+{
+	struct auxiliary_device *adev = &ref->adev;
+	int ret;
+
+	lockdep_assert_held(&gpio_shared_lock);
+
+	memset(adev, 0, sizeof(*adev));
+
+	adev->id = ref->dev_id;
+	adev->name = "proxy";
+	adev->dev.parent = gdev->dev.parent;
+	adev->dev.release = gpio_shared_adev_release;
+
+	ret = auxiliary_device_init(adev);
+	if (ret)
+		return ret;
+
+	ret = auxiliary_device_add(adev);
+	if (ret) {
+		auxiliary_device_uninit(adev);
+		return ret;
+	}
+
+	pr_debug("Created an auxiliary GPIO proxy %s for GPIO device %s\n",
+		 dev_name(&adev->dev), gpio_device_get_label(gdev));
+
+	return 0;
+}
+
+int gpio_shared_add_proxy_lookup(struct device *consumer, unsigned long lflags)
+{
+	const char *dev_id = dev_name(consumer);
+	struct gpio_shared_entry *entry;
+	struct gpio_shared_ref *ref;
+
+	struct gpiod_lookup_table *lookup __free(kfree) =
+			kzalloc(struct_size(lookup, table, 2), GFP_KERNEL);
+	if (!lookup)
+		return -ENOMEM;
+
+	guard(mutex)(&gpio_shared_lock);
+
+	list_for_each_entry(entry, &gpio_shared_list, list) {
+		list_for_each_entry(ref, &entry->refs, list) {
+			if (!device_match_fwnode(consumer, ref->fwnode))
+				continue;
+
+			/* We've already done that on a previous request. */
+			if (ref->lookup)
+				return 0;
+
+			char *key __free(kfree) =
+				kasprintf(GFP_KERNEL,
+					  KBUILD_MODNAME ".proxy.%u",
+					  ref->adev.id);
+			if (!key)
+				return -ENOMEM;
+
+			pr_debug("Adding machine lookup entry for a shared GPIO for consumer %s, with key '%s' and con_id '%s'\n",
+				 dev_id, key, ref->con_id ?: "none");
+
+			lookup->dev_id = dev_id;
+			lookup->table[0] = GPIO_LOOKUP(no_free_ptr(key), 0,
+						       ref->con_id, lflags);
+
+			gpiod_add_lookup_table(no_free_ptr(lookup));
+
+			return 0;
+		}
+	}
+
+	/* We warn here because this can only happen if the programmer borked. */
+	WARN_ON(1);
+	return -ENOENT;
+}
+
+static void gpio_shared_remove_adev(struct auxiliary_device *adev)
+{
+	lockdep_assert_held(&gpio_shared_lock);
+
+	auxiliary_device_uninit(adev);
+	auxiliary_device_delete(adev);
+}
+
+int gpio_device_setup_shared(struct gpio_device *gdev)
+{
+	struct gpio_shared_entry *entry;
+	struct gpio_shared_ref *ref;
+	unsigned long *flags;
+	int ret;
+
+	guard(mutex)(&gpio_shared_lock);
+
+	list_for_each_entry(entry, &gpio_shared_list, list) {
+		list_for_each_entry(ref, &entry->refs, list) {
+			if (gdev->dev.parent == &ref->adev.dev) {
+				/*
+				 * This is a shared GPIO proxy. Mark its
+				 * descriptor as such and return here.
+				 */
+				__set_bit(GPIOD_FLAG_SHARED_PROXY,
+					  &gdev->descs[0].flags);
+				return 0;
+			}
+		}
+	}
+
+	/*
+	 * This is not a shared GPIO proxy but it still may be the device
+	 * exposing shared pins. Find them and create the proxy devices.
+	 */
+	list_for_each_entry(entry, &gpio_shared_list, list) {
+		if (!device_match_fwnode(&gdev->dev, entry->fwnode))
+			continue;
+
+		if (list_count_nodes(&entry->refs) <= 1)
+			continue;
+
+		flags = &gdev->descs[entry->offset].flags;
+
+		__set_bit(GPIOD_FLAG_SHARED, flags);
+		/*
+		 * Shared GPIOs are not requested via the normal path. Make
+		 * them inaccessible to anyone even before we register the
+		 * chip.
+		 */
+		__set_bit(GPIOD_FLAG_REQUESTED, flags);
+
+		pr_debug("GPIO %u owned by %s is shared by multiple consumers\n",
+			 entry->offset, gpio_device_get_label(gdev));
+
+		list_for_each_entry(ref, &entry->refs, list) {
+			pr_debug("Setting up a shared GPIO entry for %s\n",
+				 fwnode_get_name(ref->fwnode));
+
+			ret = gpio_shared_make_adev(gdev, ref);
+			if (ret)
+				return ret;
+		}
+	}
+
+	return 0;
+}
+
+void gpio_device_teardown_shared(struct gpio_device *gdev)
+{
+	struct gpio_shared_entry *entry;
+	struct gpio_shared_ref *ref;
+
+	guard(mutex)(&gpio_shared_lock);
+
+	list_for_each_entry(entry, &gpio_shared_list, list) {
+		if (!device_match_fwnode(&gdev->dev, entry->fwnode))
+			continue;
+
+		list_for_each_entry(ref, &entry->refs, list) {
+			gpiod_remove_lookup_table(ref->lookup);
+			kfree(ref->lookup->table[0].key);
+			kfree(ref->lookup);
+			ref->lookup = NULL;
+			gpio_shared_remove_adev(&ref->adev);
+		}
+	}
+}
+
+static void gpio_shared_release(struct kref *kref)
+{
+	struct gpio_shared_entry *entry =
+		container_of(kref, struct gpio_shared_entry, ref);
+	struct gpio_shared_desc *shared_desc = entry->shared_desc;
+
+	guard(mutex)(&gpio_shared_lock);
+
+	gpio_device_put(shared_desc->desc->gdev);
+	if (shared_desc->can_sleep)
+		mutex_destroy(&shared_desc->mutex);
+	kfree(shared_desc);
+	entry->shared_desc = NULL;
+}
+
+static void gpiod_shared_put(void *data)
+{
+	struct gpio_shared_entry *entry = data;
+
+	lockdep_assert_not_held(&gpio_shared_lock);
+
+	kref_put(&entry->ref, gpio_shared_release);
+}
+
+static struct gpio_shared_desc *
+gpiod_shared_desc_create(struct gpio_shared_entry *entry)
+{
+	struct gpio_shared_desc *shared_desc;
+	struct gpio_device *gdev;
+
+	shared_desc = kzalloc(sizeof(*shared_desc), GFP_KERNEL);
+	if (!shared_desc)
+		return ERR_PTR(-ENOMEM);
+
+	gdev = gpio_device_find_by_fwnode(entry->fwnode);
+	if (!gdev) {
+		kfree(shared_desc);
+		return ERR_PTR(-EPROBE_DEFER);
+	}
+
+	shared_desc->desc = &gdev->descs[entry->offset];
+	shared_desc->can_sleep = gpiod_cansleep(shared_desc->desc);
+	if (shared_desc->can_sleep)
+		mutex_init(&shared_desc->mutex);
+	else
+		spin_lock_init(&shared_desc->spinlock);
+
+	return shared_desc;
+}
+
+static struct gpio_shared_entry *gpiod_shared_find(struct auxiliary_device *adev)
+{
+	struct gpio_shared_desc *shared_desc;
+	struct gpio_shared_entry *entry;
+	struct gpio_shared_ref *ref;
+
+	guard(mutex)(&gpio_shared_lock);
+
+	list_for_each_entry(entry, &gpio_shared_list, list) {
+		list_for_each_entry(ref, &entry->refs, list) {
+			if (adev != &ref->adev)
+				continue;
+
+			if (entry->shared_desc) {
+				kref_get(&entry->ref);
+				return entry;
+			}
+
+			shared_desc = gpiod_shared_desc_create(entry);
+			if (IS_ERR(shared_desc))
+				return ERR_CAST(shared_desc);
+
+			kref_init(&entry->ref);
+			entry->shared_desc = shared_desc;
+
+			pr_debug("Device %s acquired a reference to the shared GPIO %u owned by %s\n",
+				 dev_name(&adev->dev), gpio_chip_hwgpio(shared_desc->desc),
+				 gpio_device_get_label(shared_desc->desc->gdev));
+
+
+			return entry;
+		}
+	}
+
+	return ERR_PTR(-ENOENT);
+}
+
+struct gpio_shared_desc *devm_gpiod_shared_get(struct device *dev)
+{
+	struct gpio_shared_entry *entry;
+	int ret;
+
+	entry = gpiod_shared_find(to_auxiliary_dev(dev));
+	if (IS_ERR(entry))
+		return ERR_CAST(entry);
+
+	ret = devm_add_action_or_reset(dev, gpiod_shared_put, entry);
+	if (ret)
+		return ERR_PTR(ret);
+
+	return entry->shared_desc;
+}
+EXPORT_SYMBOL_GPL(devm_gpiod_shared_get);
+
+static void gpio_shared_drop_ref(struct gpio_shared_ref *ref)
+{
+	list_del(&ref->list);
+	kfree(ref->con_id);
+	ida_free(&gpio_shared_ida, ref->dev_id);
+	fwnode_handle_put(ref->fwnode);
+	kfree(ref);
+}
+
+static void gpio_shared_drop_entry(struct gpio_shared_entry *entry)
+{
+	list_del(&entry->list);
+	fwnode_handle_put(entry->fwnode);
+	kfree(entry);
+}
+
+/*
+ * This is only called if gpio_shared_init() fails so it's in fact __init and
+ * not __exit.
+ */
+static void __init gpio_shared_teardown(void)
+{
+	struct gpio_shared_entry *entry, *epos;
+	struct gpio_shared_ref *ref, *rpos;
+
+	list_for_each_entry_safe(entry, epos, &gpio_shared_list, list) {
+		list_for_each_entry_safe(ref, rpos, &entry->refs, list)
+			gpio_shared_drop_ref(ref);
+
+		gpio_shared_drop_entry(entry);
+	}
+}
+
+static void gpio_shared_free_exclusive(void)
+{
+	struct gpio_shared_entry *entry, *epos;
+
+	list_for_each_entry_safe(entry, epos, &gpio_shared_list, list) {
+		if (list_count_nodes(&entry->refs) > 1)
+			continue;
+
+		gpio_shared_drop_ref(list_first_entry(&entry->refs,
+						      struct gpio_shared_ref,
+						      list));
+		gpio_shared_drop_entry(entry);
+	}
+}
+
+static int __init gpio_shared_init(void)
+{
+	int ret;
+
+	/* Right now, we only support OF-based systems. */
+	ret = gpio_shared_of_scan();
+	if (ret) {
+		gpio_shared_teardown();
+		pr_err("Failed to scan OF nodes for shared GPIOs: %d\n", ret);
+		return ret;
+	}
+
+	gpio_shared_free_exclusive();
+
+	pr_debug("Finished scanning firmware nodes for shared GPIOs\n");
+	return 0;
+}
+postcore_initcall(gpio_shared_init);