Document the debugfs API

This is an updated document covering the internal API for the debugfs
filesystem.  Thanks to Shen Feng for suggesting that I put this text here
and noting that the old LWN version was rather out of date.

Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Reported-by: Shen Feng <shen@cn.fujitsu.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

diff --git a/Documentation/filesystems/debugfs.txt b/Documentation/filesystems/debugfs.txt
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+Copyright 2009 Jonathan Corbet <corbet@lwn.net>
+
+Debugfs exists as a simple way for kernel developers to make information
+available to user space.  Unlike /proc, which is only meant for information
+about a process, or sysfs, which has strict one-value-per-file rules,
+debugfs has no rules at all.  Developers can put any information they want
+there.  The debugfs filesystem is also intended to not serve as a stable
+ABI to user space; in theory, there are no stability constraints placed on
+files exported there.  The real world is not always so simple, though [1];
+even debugfs interfaces are best designed with the idea that they will need
+to be maintained forever.
+
+Debugfs is typically mounted with a command like:
+
+    mount -t debugfs none /sys/kernel/debug
+
+(Or an equivalent /etc/fstab line). 
+
+Note that the debugfs API is exported GPL-only to modules.
+
+Code using debugfs should include <linux/debugfs.h>.  Then, the first order
+of business will be to create at least one directory to hold a set of
+debugfs files:
+
+    struct dentry *debugfs_create_dir(const char *name, struct dentry *parent);
+
+This call, if successful, will make a directory called name underneath the
+indicated parent directory.  If parent is NULL, the directory will be
+created in the debugfs root.  On success, the return value is a struct
+dentry pointer which can be used to create files in the directory (and to
+clean it up at the end).  A NULL return value indicates that something went
+wrong.  If ERR_PTR(-ENODEV) is returned, that is an indication that the
+kernel has been built without debugfs support and none of the functions
+described below will work.
+
+The most general way to create a file within a debugfs directory is with:
+
+    struct dentry *debugfs_create_file(const char *name, mode_t mode,
+                                      struct dentry *parent, void *data,
+                                      const struct file_operations *fops);
+
+Here, name is the name of the file to create, mode describes the access
+permissions the file should have, parent indicates the directory which
+should hold the file, data will be stored in the i_private field of the
+resulting inode structure, and fops is a set of file operations which
+implement the file's behavior.  At a minimum, the read() and/or write()
+operations should be provided; others can be included as needed.  Again,
+the return value will be a dentry pointer to the created file, NULL for
+error, or ERR_PTR(-ENODEV) if debugfs support is missing.
+
+In a number of cases, the creation of a set of file operations is not
+actually necessary; the debugfs code provides a number of helper functions
+for simple situations.  Files containing a single integer value can be
+created with any of:
+
+    struct dentry *debugfs_create_u8(const char *name, mode_t mode,
+                                    struct dentry *parent, u8 *value);
+    struct dentry *debugfs_create_u16(const char *name, mode_t mode,
+                                     struct dentry *parent, u16 *value);
+    struct dentry *debugfs_create_u32(const char *name, mode_t mode,
+                                     struct dentry *parent, u32 *value);
+    struct dentry *debugfs_create_u64(const char *name, mode_t mode,
+                                     struct dentry *parent, u64 *value);
+
+These files support both reading and writing the given value; if a specific
+file should not be written to, simply set the mode bits accordingly.  The
+values in these files are in decimal; if hexadecimal is more appropriate,
+the following functions can be used instead:
+
+    struct dentry *debugfs_create_x8(const char *name, mode_t mode,
+                                    struct dentry *parent, u8 *value);
+    struct dentry *debugfs_create_x16(const char *name, mode_t mode,
+                                     struct dentry *parent, u16 *value);
+    struct dentry *debugfs_create_x32(const char *name, mode_t mode,
+                                     struct dentry *parent, u32 *value);
+
+Note that there is no debugfs_create_x64().
+
+These functions are useful as long as the developer knows the size of the
+value to be exported.  Some types can have different widths on different
+architectures, though, complicating the situation somewhat.  There is a
+function meant to help out in one special case:
+
+    struct dentry *debugfs_create_size_t(const char *name, mode_t mode,
+                                        struct dentry *parent, 
+                                        size_t *value);
+
+As might be expected, this function will create a debugfs file to represent
+a variable of type size_t.
+
+Boolean values can be placed in debugfs with:
+
+    struct dentry *debugfs_create_bool(const char *name, mode_t mode,
+                                      struct dentry *parent, u32 *value);
+
+A read on the resulting file will yield either Y (for non-zero values) or
+N, followed by a newline.  If written to, it will accept either upper- or
+lower-case values, or 1 or 0.  Any other input will be silently ignored.
+
+Finally, a block of arbitrary binary data can be exported with:
+
+    struct debugfs_blob_wrapper {
+       void *data;
+       unsigned long size;
+    };
+
+    struct dentry *debugfs_create_blob(const char *name, mode_t mode,
+                                      struct dentry *parent,
+                                      struct debugfs_blob_wrapper *blob);
+
+A read of this file will return the data pointed to by the
+debugfs_blob_wrapper structure.  Some drivers use "blobs" as a simple way
+to return several lines of (static) formatted text output.  This function
+can be used to export binary information, but there does not appear to be
+any code which does so in the mainline.  Note that all files created with
+debugfs_create_blob() are read-only.
+
+There are a couple of other directory-oriented helper functions:
+
+    struct dentry *debugfs_rename(struct dentry *old_dir, 
+                                 struct dentry *old_dentry,
+                                 struct dentry *new_dir, 
+                                 const char *new_name);
+
+    struct dentry *debugfs_create_symlink(const char *name, 
+                                          struct dentry *parent,
+                                         const char *target);
+
+A call to debugfs_rename() will give a new name to an existing debugfs
+file, possibly in a different directory.  The new_name must not exist prior
+to the call; the return value is old_dentry with updated information.
+Symbolic links can be created with debugfs_create_symlink().
+
+There is one important thing that all debugfs users must take into account:
+there is no automatic cleanup of any directories created in debugfs.  If a
+module is unloaded without explicitly removing debugfs entries, the result
+will be a lot of stale pointers and no end of highly antisocial behavior.
+So all debugfs users - at least those which can be built as modules - must
+be prepared to remove all files and directories they create there.  A file
+can be removed with:
+
+    void debugfs_remove(struct dentry *dentry);
+
+The dentry value can be NULL, in which case nothing will be removed.
+
+Once upon a time, debugfs users were required to remember the dentry
+pointer for every debugfs file they created so that all files could be
+cleaned up.  We live in more civilized times now, though, and debugfs users
+can call:
+
+    void debugfs_remove_recursive(struct dentry *dentry);
+
+If this function is passed a pointer for the dentry corresponding to the
+top-level directory, the entire hierarchy below that directory will be
+removed.
+
+Notes:
+       [1] http://lwn.net/Articles/309298/