/sysfs探索4--实例教学--kobject-example.c导读 - [linux内核]

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  http://www.blogbus.com/wanderer-zjhit-logs/149965638.html
  
  

  注：样例结合/sysfs探索2阅读，效果较好
  简单分析：
  1 本样例主要测试kobject结构，kobject对应sysfs文件系统中的一个目录，该目录对应一个具体的事物。
  2 首先通过example_kobj = kobject_create_and_add("kobject_example", kernel_kobj);将该结构加入   到/sys/kernel目录下[kernel_kobj为kernel子系统的kobject结构]，形成层次结构，并且在父目录下建立该子目录[kobject->sd指向该kobject所在的目录节点]。
  3 通过retval = sysfs_create_group(example_kobj, &attr_group)将事物的3个特色属性与其对应的kobject绑定起来,将该文件操作接口和sysfs系统结合起来。使得可以通过kobj_attribute中的show何store进行实际的内核读写操作，该函数实际上在父目录下新建3个文件，结构用sysfs_dirent表示。且sysfs_dirent->s_attr.attr=相应属性结构，据此可获得读写该文件的方法。
  4 每个struct attribute结构对应kobject目录下一个文件，文件的读写方法由kobj_attribute中的show和store指定，kobj_attribute是kobject属性导出的接口
   struct attribute{
             const char *name;
             struct module* owner;
             mode_t mode;
  }
   struct kobj_attribute{
            struct attribute attr;
            ssize_t (*show)(struct kobject* kobj,struct kobj_attribute* attr,char* buf);
            ssize_t (*store)(struct kobject* kobj,struct kobj_attribute* attr,const char* buf,size_t count);
  }
  5当使用结束时，调用kobject_put（example_kobj）删除该kobject结构，及其对应的目录和文件。
  扩展：实际上对于不同子系统有不同的属性和属性操作函数
  比如：在genhd.h文件中，磁盘属性操作结构为，disk_attribute是磁盘属性导出的接口：
  struct disk_attribute{
          struct attribute attr;
          ssize_t (* show)(struct gendisk*,char*);
          ssize_t (* store)(struct gendisk*,const char*,size_t);//新内核中该函数取消，磁盘属性只有只读操作
  }
  在device.h文件中，外设属性操作结构为，device_attribute是外设属性导出的接口：
  struct device_attribute{
          struct attribute attr;
          ssize_t (* show)(struct device*dev,struct device_attribute*attr,char* buf);
          ssize_t (*store)(struct device *dev,struct device_attribute*attr,const char*buf,size_t count);
  }
  源代码如下：
  /*
   * Sample kobject implementation
   *
   * Copyright (C) 2004-2007 Greg Kroah-Hartman <greg@kroah.com>
   * Copyright (C) 2007 Novell Inc.
   *
   * Released under the GPL version 2 only.
   *
   */
  #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\n", foo);
  }

  static ssize_t foo_store(struct kobject *kobj, struct kobj_attribute *attr,
      const char *buf, size_t count)
  {
   sscanf(buf, "%du", &foo);
   return count;
  }

  static struct kobj_attribute foo_attribute =
   __ATTR(foo, 0666, 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\n", var);
  }

  static ssize_t b_store(struct kobject *kobj, struct kobj_attribute *attr,
           const char *buf, size_t count)
  {
   int var;

   sscanf(buf, "%du", &var);
   if (strcmp(attr->attr.name, "baz") == 0)
    baz = var;
   else
    bar = var;
   return count;
  }

  static struct kobj_attribute baz_attribute =
   __ATTR(baz, 0666, b_show, b_store);
  static struct kobj_attribute bar_attribute =
   __ATTR(bar, 0666, 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");
  MODULE_AUTHOR("Greg Kroah-Hartman <greg@kroah.com>");