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Based on kernel version 3.9. Page generated on 2013-05-02 23:06 EST.

1	Copyright 2009 Jonathan Corbet <corbet@lwn.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	The debugfs root directory is accessible only to the root user by
19	default. To change access to the tree the "uid", "gid" and "mode" mount
20	options can be used.
21	
22	Note that the debugfs API is exported GPL-only to modules.
23	
24	Code using debugfs should include <linux/debugfs.h>.  Then, the first order
25	of business will be to create at least one directory to hold a set of
26	debugfs files:
27	
28	    struct dentry *debugfs_create_dir(const char *name, struct dentry *parent);
29	
30	This call, if successful, will make a directory called name underneath the
31	indicated parent directory.  If parent is NULL, the directory will be
32	created in the debugfs root.  On success, the return value is a struct
33	dentry pointer which can be used to create files in the directory (and to
34	clean it up at the end).  A NULL return value indicates that something went
35	wrong.  If ERR_PTR(-ENODEV) is returned, that is an indication that the
36	kernel has been built without debugfs support and none of the functions
37	described below will work.
38	
39	The most general way to create a file within a debugfs directory is with:
40	
41	    struct dentry *debugfs_create_file(const char *name, umode_t mode,
42					       struct dentry *parent, void *data,
43					       const struct file_operations *fops);
44	
45	Here, name is the name of the file to create, mode describes the access
46	permissions the file should have, parent indicates the directory which
47	should hold the file, data will be stored in the i_private field of the
48	resulting inode structure, and fops is a set of file operations which
49	implement the file's behavior.  At a minimum, the read() and/or write()
50	operations should be provided; others can be included as needed.  Again,
51	the return value will be a dentry pointer to the created file, NULL for
52	error, or ERR_PTR(-ENODEV) if debugfs support is missing.
53	
54	In a number of cases, the creation of a set of file operations is not
55	actually necessary; the debugfs code provides a number of helper functions
56	for simple situations.  Files containing a single integer value can be
57	created with any of:
58	
59	    struct dentry *debugfs_create_u8(const char *name, umode_t mode,
60					     struct dentry *parent, u8 *value);
61	    struct dentry *debugfs_create_u16(const char *name, umode_t mode,
62					      struct dentry *parent, u16 *value);
63	    struct dentry *debugfs_create_u32(const char *name, umode_t mode,
64					      struct dentry *parent, u32 *value);
65	    struct dentry *debugfs_create_u64(const char *name, umode_t mode,
66					      struct dentry *parent, u64 *value);
67	
68	These files support both reading and writing the given value; if a specific
69	file should not be written to, simply set the mode bits accordingly.  The
70	values in these files are in decimal; if hexadecimal is more appropriate,
71	the following functions can be used instead:
72	
73	    struct dentry *debugfs_create_x8(const char *name, umode_t mode,
74					     struct dentry *parent, u8 *value);
75	    struct dentry *debugfs_create_x16(const char *name, umode_t mode,
76					      struct dentry *parent, u16 *value);
77	    struct dentry *debugfs_create_x32(const char *name, umode_t mode,
78					      struct dentry *parent, u32 *value);
79	    struct dentry *debugfs_create_x64(const char *name, umode_t mode,
80					      struct dentry *parent, u64 *value);
81	
82	These functions are useful as long as the developer knows the size of the
83	value to be exported.  Some types can have different widths on different
84	architectures, though, complicating the situation somewhat.  There is a
85	function meant to help out in one special case:
86	
87	    struct dentry *debugfs_create_size_t(const char *name, umode_t mode,
88					         struct dentry *parent, 
89						 size_t *value);
90	
91	As might be expected, this function will create a debugfs file to represent
92	a variable of type size_t.
93	
94	Boolean values can be placed in debugfs with:
95	
96	    struct dentry *debugfs_create_bool(const char *name, umode_t mode,
97					       struct dentry *parent, u32 *value);
98	
99	A read on the resulting file will yield either Y (for non-zero values) or
100	N, followed by a newline.  If written to, it will accept either upper- or
101	lower-case values, or 1 or 0.  Any other input will be silently ignored.
102	
103	Another option is exporting a block of arbitrary binary data, with
104	this structure and function:
105	
106	    struct debugfs_blob_wrapper {
107		void *data;
108		unsigned long size;
109	    };
110	
111	    struct dentry *debugfs_create_blob(const char *name, umode_t mode,
112					       struct dentry *parent,
113					       struct debugfs_blob_wrapper *blob);
114	
115	A read of this file will return the data pointed to by the
116	debugfs_blob_wrapper structure.  Some drivers use "blobs" as a simple way
117	to return several lines of (static) formatted text output.  This function
118	can be used to export binary information, but there does not appear to be
119	any code which does so in the mainline.  Note that all files created with
120	debugfs_create_blob() are read-only.
121	
122	If you want to dump a block of registers (something that happens quite
123	often during development, even if little such code reaches mainline.
124	Debugfs offers two functions: one to make a registers-only file, and
125	another to insert a register block in the middle of another sequential
126	file.
127	
128	    struct debugfs_reg32 {
129		char *name;
130		unsigned long offset;
131	    };
132	
133	    struct debugfs_regset32 {
134		struct debugfs_reg32 *regs;
135		int nregs;
136		void __iomem *base;
137	    };
138	
139	    struct dentry *debugfs_create_regset32(const char *name, umode_t mode,
140					     struct dentry *parent,
141					     struct debugfs_regset32 *regset);
142	
143	    int debugfs_print_regs32(struct seq_file *s, struct debugfs_reg32 *regs,
144				 int nregs, void __iomem *base, char *prefix);
145	
146	The "base" argument may be 0, but you may want to build the reg32 array
147	using __stringify, and a number of register names (macros) are actually
148	byte offsets over a base for the register block.
149	
150	
151	There are a couple of other directory-oriented helper functions:
152	
153	    struct dentry *debugfs_rename(struct dentry *old_dir, 
154	    				  struct dentry *old_dentry,
155			                  struct dentry *new_dir, 
156					  const char *new_name);
157	
158	    struct dentry *debugfs_create_symlink(const char *name, 
159	                                          struct dentry *parent,
160					      	  const char *target);
161	
162	A call to debugfs_rename() will give a new name to an existing debugfs
163	file, possibly in a different directory.  The new_name must not exist prior
164	to the call; the return value is old_dentry with updated information.
165	Symbolic links can be created with debugfs_create_symlink().
166	
167	There is one important thing that all debugfs users must take into account:
168	there is no automatic cleanup of any directories created in debugfs.  If a
169	module is unloaded without explicitly removing debugfs entries, the result
170	will be a lot of stale pointers and no end of highly antisocial behavior.
171	So all debugfs users - at least those which can be built as modules - must
172	be prepared to remove all files and directories they create there.  A file
173	can be removed with:
174	
175	    void debugfs_remove(struct dentry *dentry);
176	
177	The dentry value can be NULL, in which case nothing will be removed.
178	
179	Once upon a time, debugfs users were required to remember the dentry
180	pointer for every debugfs file they created so that all files could be
181	cleaned up.  We live in more civilized times now, though, and debugfs users
182	can call:
183	
184	    void debugfs_remove_recursive(struct dentry *dentry);
185	
186	If this function is passed a pointer for the dentry corresponding to the
187	top-level directory, the entire hierarchy below that directory will be
188	removed.
189	
190	Notes:
191		[1] http://lwn.net/Articles/309298/
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