Allow subsystem to have attributes, too.

This is really handy for subsystems that have any attributes they want
to export, esp. if they have just a few, since they don't have to define
all of the infrastructure for creating and tearing down the attributes 
for the high level type that they are. 

- define struct subsys_attribute for declaring attributes of subsystems
themselves. 

- define subsys_create_file() and subsys_remove_file().

- define subsys_sysfs_ops for forwarding sysfs reads and writes to the 
subsystem attribute callbacks. 

- Set the sysfs_ops to be the subsystem ones if the kobject doesn't belong
to a subsystem itself (meaning that it is a subsystem).

fs/sysfs/inode.c

@@ -145,6 +145,43 @@ static int sysfs_symlink(struct inode * dir, struct dentry *dentry, const char *
 	return error;
 }
 
+#define to_subsys(k) container_of(k,struct subsystem,kobj)
+#define to_sattr(a) container_of(a,struct subsys_attribute,attr)
+
+/**
+ * Subsystem file operations.
+ * These operations allow subsystems to have files that can be 
+ * read/written. 
+ */
+ssize_t subsys_attr_show(struct kobject * kobj, struct attribute * attr, 
+			 char * page, size_t count, loff_t off)
+{
+	struct subsystem * s = to_subsys(kobj);
+	struct subsys_attribute * sattr = to_sattr(attr);
+	ssize_t ret = 0;
+
+	if (sattr->show)
+		ret = sattr->show(s,page,count,off);
+	return ret;
+}
+
+ssize_t subsys_attr_store(struct kobject * kobj, struct attribute * attr,
+			  const char * page, size_t count, loff_t off)
+{
+	struct subsystem * s = to_subsys(kobj);
+	struct subsys_attribute * sattr = to_sattr(attr);
+	ssize_t ret = 0;
+
+	if (sattr->store)
+		ret = sattr->store(s,page,count,off);
+	return ret;
+}
+
+static struct sysfs_ops subsys_sysfs_ops = {
+	.show	= subsys_attr_show,
+	.store	= subsys_attr_store,
+};
+
 /**
  *	sysfs_read_file - read an attribute. 
  *	@file:	file pointer.
@@ -265,9 +302,14 @@ static int check_perm(struct inode * inode, struct file * file)
 
 	if (!kobj || !attr)
 		goto Einval;
-	
+
+	/* if the kobject has no subsystem, then it is a subsystem itself,
+	 * so give it the subsys_sysfs_ops.
+	 */
 	if (kobj->subsys)
 		ops = kobj->subsys->sysfs_ops;
+	else
+		ops = &subsys_sysfs_ops;
 
 	/* No sysfs operations, either from having no subsystem,
 	 * or the subsystem have no operations.

include/linux/kobject.h

@@ -60,4 +60,13 @@ static inline void subsys_put(struct subsystem * s)
 	kobject_put(&s->kobj);
 }
 
+struct subsys_attribute {
+	struct attribute attr;
+	ssize_t (*show)(struct subsystem *, char *, size_t, loff_t);
+	ssize_t (*store)(struct subsystem *, const char *, size_t, loff_t); 
+};
+
+extern int subsys_create_file(struct subsystem * , struct subsys_attribute *);
+extern void subsys_remove_file(struct subsystem * , struct subsys_attribute *);
+
 #endif /* _KOBJECT_H_ */

lib/kobject.c

@@ -229,6 +229,38 @@ void subsystem_unregister(struct subsystem * s)
 }
 
 
+/**
+ *	subsystem_create_file - export sysfs attribute file.
+ *	@s:	subsystem.
+ *	@a:	subsystem attribute descriptor.
+ */
+
+int subsys_create_file(struct subsystem * s, struct subsys_attribute * a)
+{
+	int error = 0;
+	if (subsys_get(s)) {
+		error = sysfs_create_file(&s->kobj,&a->attr);
+		subsys_put(s);
+	}
+	return error;
+}
+
+
+/**
+ *	subsystem_remove_file - remove sysfs attribute file.
+ *	@s:	subsystem.
+ *	@a:	attribute desciptor.
+ */
+
+void subsys_remove_file(struct subsystem * s, struct subsys_attribute * a)
+{
+	if (subsys_get(s)) {
+		sysfs_remove_file(&s->kobj,&a->attr);
+		subsys_put(s);
+	}
+}
+
+
 EXPORT_SYMBOL(kobject_init);
 EXPORT_SYMBOL(kobject_register);
 EXPORT_SYMBOL(kobject_unregister);
@@ -238,3 +270,5 @@ EXPORT_SYMBOL(kobject_put);
 EXPORT_SYMBOL(subsystem_init);
 EXPORT_SYMBOL(subsystem_register);
 EXPORT_SYMBOL(subsystem_unregister);
+EXPORT_SYMBOL(subsys_create_file);
+EXPORT_SYMBOL(subsys_remove_file);