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Based on kernel version 2.6.33. Page generated on 2010-02-24 15:36 EST.

1	Copyright 2009 Jonathan Corbet <corbet[AT]lwn[DOT]net>
2	
3	Debugfs exists as a simple way for kernel developers to make information
4	available to user space.  Unlike /proc, which is only meant for information
5	about a process, or sysfs, which has strict one-value-per-file rules,
6	debugfs has no rules at all.  Developers can put any information they want
7	there.  The debugfs filesystem is also intended to not serve as a stable
8	ABI to user space; in theory, there are no stability constraints placed on
9	files exported there.  The real world is not always so simple, though [1];
10	even debugfs interfaces are best designed with the idea that they will need
11	to be maintained forever.
12	
13	Debugfs is typically mounted with a command like:
14	
15	    mount -t debugfs none /sys/kernel/debug
16	
17	(Or an equivalent /etc/fstab line). 
18	
19	Note that the debugfs API is exported GPL-only to modules.
20	
21	Code using debugfs should include <linux/debugfs.h>.  Then, the first order
22	of business will be to create at least one directory to hold a set of
23	debugfs files:
24	
25	    struct dentry *debugfs_create_dir(const char *name, struct dentry *parent);
26	
27	This call, if successful, will make a directory called name underneath the
28	indicated parent directory.  If parent is NULL, the directory will be
29	created in the debugfs root.  On success, the return value is a struct
30	dentry pointer which can be used to create files in the directory (and to
31	clean it up at the end).  A NULL return value indicates that something went
32	wrong.  If ERR_PTR(-ENODEV) is returned, that is an indication that the
33	kernel has been built without debugfs support and none of the functions
34	described below will work.
35	
36	The most general way to create a file within a debugfs directory is with:
37	
38	    struct dentry *debugfs_create_file(const char *name, mode_t mode,
39					       struct dentry *parent, void *data,
40					       const struct file_operations *fops);
41	
42	Here, name is the name of the file to create, mode describes the access
43	permissions the file should have, parent indicates the directory which
44	should hold the file, data will be stored in the i_private field of the
45	resulting inode structure, and fops is a set of file operations which
46	implement the file's behavior.  At a minimum, the read() and/or write()
47	operations should be provided; others can be included as needed.  Again,
48	the return value will be a dentry pointer to the created file, NULL for
49	error, or ERR_PTR(-ENODEV) if debugfs support is missing.
50	
51	In a number of cases, the creation of a set of file operations is not
52	actually necessary; the debugfs code provides a number of helper functions
53	for simple situations.  Files containing a single integer value can be
54	created with any of:
55	
56	    struct dentry *debugfs_create_u8(const char *name, mode_t mode,
57					     struct dentry *parent, u8 *value);
58	    struct dentry *debugfs_create_u16(const char *name, mode_t mode,
59					      struct dentry *parent, u16 *value);
60	    struct dentry *debugfs_create_u32(const char *name, mode_t mode,
61					      struct dentry *parent, u32 *value);
62	    struct dentry *debugfs_create_u64(const char *name, mode_t mode,
63					      struct dentry *parent, u64 *value);
64	
65	These files support both reading and writing the given value; if a specific
66	file should not be written to, simply set the mode bits accordingly.  The
67	values in these files are in decimal; if hexadecimal is more appropriate,
68	the following functions can be used instead:
69	
70	    struct dentry *debugfs_create_x8(const char *name, mode_t mode,
71					     struct dentry *parent, u8 *value);
72	    struct dentry *debugfs_create_x16(const char *name, mode_t mode,
73					      struct dentry *parent, u16 *value);
74	    struct dentry *debugfs_create_x32(const char *name, mode_t mode,
75					      struct dentry *parent, u32 *value);
76	
77	Note that there is no debugfs_create_x64().
78	
79	These functions are useful as long as the developer knows the size of the
80	value to be exported.  Some types can have different widths on different
81	architectures, though, complicating the situation somewhat.  There is a
82	function meant to help out in one special case:
83	
84	    struct dentry *debugfs_create_size_t(const char *name, mode_t mode,
85					         struct dentry *parent, 
86						 size_t *value);
87	
88	As might be expected, this function will create a debugfs file to represent
89	a variable of type size_t.
90	
91	Boolean values can be placed in debugfs with:
92	
93	    struct dentry *debugfs_create_bool(const char *name, mode_t mode,
94					       struct dentry *parent, u32 *value);
95	
96	A read on the resulting file will yield either Y (for non-zero values) or
97	N, followed by a newline.  If written to, it will accept either upper- or
98	lower-case values, or 1 or 0.  Any other input will be silently ignored.
99	
100	Finally, a block of arbitrary binary data can be exported with:
101	
102	    struct debugfs_blob_wrapper {
103		void *data;
104		unsigned long size;
105	    };
106	
107	    struct dentry *debugfs_create_blob(const char *name, mode_t mode,
108					       struct dentry *parent,
109					       struct debugfs_blob_wrapper *blob);
110	
111	A read of this file will return the data pointed to by the
112	debugfs_blob_wrapper structure.  Some drivers use "blobs" as a simple way
113	to return several lines of (static) formatted text output.  This function
114	can be used to export binary information, but there does not appear to be
115	any code which does so in the mainline.  Note that all files created with
116	debugfs_create_blob() are read-only.
117	
118	There are a couple of other directory-oriented helper functions:
119	
120	    struct dentry *debugfs_rename(struct dentry *old_dir, 
121	    				  struct dentry *old_dentry,
122			                  struct dentry *new_dir, 
123					  const char *new_name);
124	
125	    struct dentry *debugfs_create_symlink(const char *name, 
126	                                          struct dentry *parent,
127					      	  const char *target);
128	
129	A call to debugfs_rename() will give a new name to an existing debugfs
130	file, possibly in a different directory.  The new_name must not exist prior
131	to the call; the return value is old_dentry with updated information.
132	Symbolic links can be created with debugfs_create_symlink().
133	
134	There is one important thing that all debugfs users must take into account:
135	there is no automatic cleanup of any directories created in debugfs.  If a
136	module is unloaded without explicitly removing debugfs entries, the result
137	will be a lot of stale pointers and no end of highly antisocial behavior.
138	So all debugfs users - at least those which can be built as modules - must
139	be prepared to remove all files and directories they create there.  A file
140	can be removed with:
141	
142	    void debugfs_remove(struct dentry *dentry);
143	
144	The dentry value can be NULL, in which case nothing will be removed.
145	
146	Once upon a time, debugfs users were required to remember the dentry
147	pointer for every debugfs file they created so that all files could be
148	cleaned up.  We live in more civilized times now, though, and debugfs users
149	can call:
150	
151	    void debugfs_remove_recursive(struct dentry *dentry);
152	
153	If this function is passed a pointer for the dentry corresponding to the
154	top-level directory, the entire hierarchy below that directory will be
155	removed.
156	
157	Notes:
158		[1] http://lwn.net/Articles/309298/
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