[PATCH v2 1/5] driver core: add a faux bus for use when a simple device/bus is needed

Greg Kroah-Hartman
2025-02-04T11:09:36Z

Many drivers abuse the platform driver/bus system as it provides a
simple way to create and bind a device to a driver-specific set of
probe/release functions. Instead of doing that, and wasting all of the
memory associated with a platform device, here is a "faux" bus that
can be used instead.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
v2: - renamed bus and root device to just "faux" thanks to Thomas
- removed the one-driver-per-device and now just have one driver
entirely thanks to Danilo
- kerneldoc fixups and additions and string handling bounds checks
hanks to Andy
- coding style fix thanks to Jonathan
- tested that the destroy path actually works

drivers/base/Makefile | 2 +-
drivers/base/base.h | 1 +
drivers/base/faux.c | 196 ++++++++++++++++++++++++++++++++++++
drivers/base/init.c | 1 +
include/linux/device/faux.h | 31 ++++++
5 files changed, 230 insertions(+), 1 deletion(-)
create mode 100644 drivers/base/faux.c
create mode 100644 include/linux/device/faux.h

diff --git a/drivers/base/Makefile b/drivers/base/Makefile
index 7fb21768ca36..8074a10183dc 100644
--- a/drivers/base/Makefile
+++ b/drivers/base/Makefile
@@ -6,7 +6,7 @@ obj-y := component.o core.o bus.o dd.o syscore.o \
cpu.o firmware.o init.o map.o devres.o \
attribute_container.o transport_class.o \
topology.o container.o property.o cacheinfo.o \
- swnode.o
+ swnode.o faux.o
obj-$(CONFIG_AUXILIARY_BUS) += auxiliary.o
obj-$(CONFIG_DEVTMPFS) += devtmpfs.o
obj-y += power/
diff --git a/drivers/base/base.h b/drivers/base/base.h
index 8cf04a557bdb..0042e4774b0c 100644
--- a/drivers/base/base.h
+++ b/drivers/base/base.h
@@ -137,6 +137,7 @@ int hypervisor_init(void);
static inline int hypervisor_init(void) { return 0; }
#endif
int platform_bus_init(void);
+int faux_bus_init(void);
void cpu_dev_init(void);
void container_dev_init(void);
#ifdef CONFIG_AUXILIARY_BUS
diff --git a/drivers/base/faux.c b/drivers/base/faux.c
new file mode 100644
index 000000000000..9b28643afc45
--- /dev/null
+++ b/drivers/base/faux.c
@@ -0,0 +1,196 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (c) 2025 Greg Kroah-Hartman <gregkh@linuxfoundation.org>
+ * Copyright (c) 2025 The Linux Foundation
+ *
+ * A "simple" faux bus that allows devices to be created and added
+ * automatically to it. This is to be used whenever you need to create a
+ * device that is not associated with any "real" system resources, and do
+ * not want to have to deal with a bus/driver binding logic. It is
+ * intended to be very simple, with only a create and a destroy function
+ * available.
+ */
+#include <linux/err.h>
+#include <linux/init.h>
+#include <linux/slab.h>
+#include <linux/string.h>
+#include <linux/container_of.h>
+#include <linux/device/faux.h>
+#include "base.h"
+
+#define MAX_NAME_SIZE 256 /* Max size of a faux_device name */
+
+/*
+ * Internal wrapper structure so we can hold the memory
+ * for the driver and the name string of the faux device.
+ */
+struct faux_object {
+ struct faux_device faux_dev;
+ const struct faux_driver_ops *faux_ops;
+ char name[];
+};
+#define to_faux_object(dev) container_of_const(dev, struct faux_object, faux_dev.dev)
+
+static struct device faux_bus_root = {
+ .init_name = "faux",
+};
+
+static int faux_match(struct device *dev, const struct device_driver *drv)
+{
+ /* Match always succeeds, we only have one driver */
+ return 1;
+}
+
+static int faux_probe(struct device *dev)
+{
+ struct faux_object *faux_obj = to_faux_object(dev);
+ struct faux_device *faux_dev = &faux_obj->faux_dev;
+ const struct faux_driver_ops *faux_ops = faux_obj->faux_ops;
+ int ret = 0;
+
+ if (faux_ops && faux_ops->probe)
+ ret = faux_ops->probe(faux_dev);
+
+ return ret;
+}
+
+static void faux_remove(struct device *dev)
+{
+ struct faux_object *faux_obj = to_faux_object(dev);
+ struct faux_device *faux_dev = &faux_obj->faux_dev;
+ const struct faux_driver_ops *faux_ops = faux_obj->faux_ops;
+
+ if (faux_ops && faux_ops->remove)
+ faux_ops->remove(faux_dev);
+}
+
+static const struct bus_type faux_bus_type = {
+ .name = "faux",
+ .match = faux_match,
+ .probe = faux_probe,
+ .remove = faux_remove,
+};
+
+static struct device_driver faux_driver = {
+ .name = "faux_driver",
+ .bus = &faux_bus_type,
+ .probe_type = PROBE_PREFER_ASYNCHRONOUS,
+};
+
+static void faux_device_release(struct device *dev)
+{
+ struct faux_object *faux_obj = to_faux_object(dev);
+
+ kfree(faux_obj);
+}
+
+/**
+ * faux_device_create - create and register a faux device and driver
+ * @name: name of the device and driver we are adding
+ * @faux_ops: struct faux_driver_ops that the new device will call back into, can be NULL
+ *
+ * Create a new faux device and driver, both with the same name, and
+ * register them in the driver core properly. The probe() callback of
+ * @faux_ops will be called with the new device that is created for the
+ * caller to do something with.
+ *
+ * Note, when this function is called, the functions specified in struct
+ * faux_ops will be called before the function returns, so be prepared for
+ * everything to be properly initialized before that point in time.
+ *
+ * Return:
+ * * NULL if an error happened with creating the device
+ * * pointer to a valid struct faux_device that is registered with sysfs
+ */
+struct faux_device *faux_device_create(const char *name, struct faux_driver_ops *faux_ops)
+{
+ struct device *dev;
+ struct faux_object *faux_obj;
+ struct faux_device *faux_dev;
+ int name_size;
+ int ret;
+
+ name_size = strlen(name);
+ if (name_size > MAX_NAME_SIZE)
+ return NULL;
+
+ faux_obj = kzalloc(sizeof(*faux_obj) + name_size + 1, GFP_KERNEL);
+ if (!faux_obj)
+ return NULL;
+
+ /* Save off the name of the object into local memory */
+ memcpy(faux_obj->name, name, name_size);
+
+ /* Save off the callbacks so we can use them in the future */
+ faux_obj->faux_ops = faux_ops;
+
+ /* Initialize the device portion and register it with the driver core */
+ faux_dev = &faux_obj->faux_dev;
+ dev = &faux_dev->dev;
+
+ device_initialize(dev);
+ dev->release = faux_device_release;
+ dev->parent = &faux_bus_root;
+ dev->bus = &faux_bus_type;
+ dev_set_name(dev, "%s", name);
+
+ ret = device_add(dev);
+ if (ret) {
+ pr_err("%s: device_add for %s faux device failed with %d\n",
+ __func__, name, ret);
+ put_device(dev);
+ return NULL;
+ }
+
+ return faux_dev;
+}
+EXPORT_SYMBOL_GPL(faux_device_create);
+
+/**
+ * faux_device_destroy - destroy a faux device
+ * @faux_dev: faux device to destroy
+ *
+ * Unregister and free all memory associated with a faux device that was
+ * previously created with a call to faux_device_create().
+ */
+void faux_device_destroy(struct faux_device *faux_dev)
+{
+ struct device *dev = &faux_dev->dev;
+
+ if (IS_ERR_OR_NULL(faux_dev))
+ return;
+
+ device_del(dev);
+
+ /* The final put_device() will clean up the driver we created for this device. */
+ put_device(dev);
+}
+EXPORT_SYMBOL_GPL(faux_device_destroy);
+
+int __init faux_bus_init(void)
+{
+ int ret;
+
+ ret = device_register(&faux_bus_root);
+ if (ret) {
+ put_device(&faux_bus_root);
+ return ret;
+ }
+
+ ret = bus_register(&faux_bus_type);
+ if (ret)
+ goto error_bus;
+
+ ret = driver_register(&faux_driver);
+ if (ret)
+ goto error_driver;
+
+ return ret;
+
+error_driver:
+ bus_unregister(&faux_bus_type);
+
+error_bus:
+ device_unregister(&faux_bus_root);
+ return ret;
+}
diff --git a/drivers/base/init.c b/drivers/base/init.c
index c4954835128c..9d2b06d65dfc 100644
--- a/drivers/base/init.c
+++ b/drivers/base/init.c
@@ -32,6 +32,7 @@ void __init driver_init(void)
/* These are also core pieces, but must come after the
* core core pieces.
*/
+ faux_bus_init();
of_core_init();
platform_bus_init();
auxiliary_bus_init();
diff --git a/include/linux/device/faux.h b/include/linux/device/faux.h
new file mode 100644
index 000000000000..2c8ae5bd7ae8
--- /dev/null
+++ b/include/linux/device/faux.h
@@ -0,0 +1,31 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (c) 2025 Greg Kroah-Hartman <gregkh@linuxfoundation.org>
+ * Copyright (c) 2025 The Linux Foundation
+ *
+ * A "simple" faux bus that allows devices to be created and added
+ * automatically to it. This is to be used whenever you need to create a
+ * device that is not associated with any "real" system resources, and do
+ * not want to have to deal with a bus/driver binding logic. It is
+ * intended to be very simple, with only a create and a destroy function
+ * available.
+ */
+#ifndef _FAUX_DEVICE_H_
+#define _FAUX_DEVICE_H_
+
+#include <linux/device.h>
+
+struct faux_device {
+ struct device dev;
+};
+#define to_faux_device(x) container_of_const((x), struct faux_device, dev)
+
+struct faux_driver_ops {
+ int (*probe)(struct faux_device *faux_dev);
+ void (*remove)(struct faux_device *faux_dev);
+};
+
+struct faux_device *faux_device_create(const char *name, struct faux_driver_ops *faux_ops);
+void faux_device_destroy(struct faux_device *faux_dev);
+
+#endif /* _FAUX_DEVICE_H_ */
--
2.48.1