From Stephen Hemminger

Attribute groups provide the ability to register a set of sysfs attributes
for a kobject all at once, and optionally create a subdirectory to hold 
them.

The constructs are simple:
                                                                                                         
struct attribute_group {
        char                    * name;
        struct attribute        * attrs;
};

int sysfs_create_group(struct kobject *, struct attribute_group *);
void sysfs_remove_group(struct kobject *, struct attribute_group *);


If ->name is not NULL, then we create a subdirectory of that name to hold
the attributes. We then iterate over ->attrs and create a file for each,
in the subdirectory if we created one.

This prevents one from having to register a kobject (and define a new
kobj_type) to create a subdirectory for a group of attributes. Attributes
currently defined in that way can be converted to use attribute_groups
easily, with one caveat:

The attributes that are added for a kobject, even if in a subdirectory,
must be declared as the high-level attribute type (with an embedded struct
attribute) for the kobject, and conform to the kobj_type's calling
convention for reading/writing attributes.

The kobject that you're creating attributes for owns the directory, and
will assume ownership of the subdirectory. sysfs will reference this
kobject, and it's kobj_type, when the attribute file is opened to
determine the methods for reading/writing the attribute.

sysfs will call the kobj_type's show()/store() methods, which will convert
the kobject into a high-level object type, and convert the attribute into
a high-level attribute type, which (depending on the kobj_type) is
expected to have a show() and/or store() method.

Note that this makes it only slightly easier to create attributes en masse,
though it is a bit technically superior, since it doesn't require a new 
kobj_type and kobject register. More will come in this area.. 

- reanme sysfs_get_inode() to sysfs_create().
- Assume we're using the single sysfs superblock when creating something.
- move file object-specific init commands to callbacks.
- streamline file and directory creation to remove duplicate checks.
- clean up sysfs_fill_super() a bit. 

- new files file.c, dir.c, symlink.c, mount.c.

- Purely cosmetic (for now). Should make it easier to add new filetypes.