// SPDX-License-Identifier: GPL-2.0

  /*
   * Sample kobject implementation
   *
   * Copyright (C) 2004-2007 Greg Kroah-Hartman <greg@kroah.com>
   * Copyright (C) 2007 Novell Inc.

   */
  #include <linux/kobject.h>
  #include <linux/string.h>
  #include <linux/sysfs.h>
  #include <linux/module.h>
  #include <linux/init.h>
  
  /*
   * This module shows how to create a simple subdirectory in sysfs called
   * /sys/kernel/kobject-example  In that directory, 3 files are created:
   * "foo", "baz", and "bar".  If an integer is written to these files, it can be
   * later read out of it.
   */
  
  static int foo;
  static int baz;
  static int bar;
  
  /*
   * The "foo" file where a static variable is read from and written to.
   */
  static ssize_t foo_show(struct kobject *kobj, struct kobj_attribute *attr,
  			char *buf)
  {
  	return sprintf(buf, "%d
  ", foo);
  }
  
  static ssize_t foo_store(struct kobject *kobj, struct kobj_attribute *attr,
  			 const char *buf, size_t count)
  {

  	int ret;
  
  	ret = kstrtoint(buf, 10, &foo);
  	if (ret < 0)
  		return ret;

  	return count;
  }

  /* Sysfs attributes cannot be world-writable. */

  static struct kobj_attribute foo_attribute =

  	__ATTR(foo, 0664, foo_show, foo_store);

  
  /*

   * More complex function where we determine which variable is being accessed by

   * looking at the attribute for the "baz" and "bar" files.
   */
  static ssize_t b_show(struct kobject *kobj, struct kobj_attribute *attr,
  		      char *buf)
  {
  	int var;
  
  	if (strcmp(attr->attr.name, "baz") == 0)
  		var = baz;
  	else
  		var = bar;
  	return sprintf(buf, "%d
  ", var);
  }
  
  static ssize_t b_store(struct kobject *kobj, struct kobj_attribute *attr,
  		       const char *buf, size_t count)
  {

  	int var, ret;
  
  	ret = kstrtoint(buf, 10, &var);
  	if (ret < 0)
  		return ret;

  	if (strcmp(attr->attr.name, "baz") == 0)
  		baz = var;
  	else
  		bar = var;
  	return count;
  }
  
  static struct kobj_attribute baz_attribute =

  	__ATTR(baz, 0664, b_show, b_store);

  static struct kobj_attribute bar_attribute =

  	__ATTR(bar, 0664, b_show, b_store);

  
  
  /*

   * Create a group of attributes so that we can create and destroy them all

   * at once.
   */
  static struct attribute *attrs[] = {
  	&foo_attribute.attr,
  	&baz_attribute.attr,
  	&bar_attribute.attr,
  	NULL,	/* need to NULL terminate the list of attributes */
  };
  
  /*
   * An unnamed attribute group will put all of the attributes directly in
   * the kobject directory.  If we specify a name, a subdirectory will be
   * created for the attributes with the directory being the name of the
   * attribute group.
   */
  static struct attribute_group attr_group = {
  	.attrs = attrs,
  };
  
  static struct kobject *example_kobj;

  static int __init example_init(void)

  {
  	int retval;
  
  	/*
  	 * Create a simple kobject with the name of "kobject_example",
  	 * located under /sys/kernel/
  	 *
  	 * As this is a simple directory, no uevent will be sent to
  	 * userspace.  That is why this function should not be used for
  	 * any type of dynamic kobjects, where the name and number are
  	 * not known ahead of time.
  	 */
  	example_kobj = kobject_create_and_add("kobject_example", kernel_kobj);
  	if (!example_kobj)
  		return -ENOMEM;
  
  	/* Create the files associated with this kobject */
  	retval = sysfs_create_group(example_kobj, &attr_group);
  	if (retval)
  		kobject_put(example_kobj);
  
  	return retval;
  }

  static void __exit example_exit(void)

  {
  	kobject_put(example_kobj);
  }
  
  module_init(example_init);
  module_exit(example_exit);

  MODULE_LICENSE("GPL v2");

  MODULE_AUTHOR("Greg Kroah-Hartman <greg@kroah.com>");