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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1	Copyright 2009 Jonathan Corbet <corbet@lwn.net>
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.
13	Debugfs is typically mounted with a command like:
15	    mount -t debugfs none /sys/kernel/debug
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.
22	Note that the debugfs API is exported GPL-only to modules.
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:
28	    struct dentry *debugfs_create_dir(const char *name, struct dentry *parent);
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.
39	The most general way to create a file within a debugfs directory is with:
41	    struct dentry *debugfs_create_file(const char *name, umode_t mode,
42					       struct dentry *parent, void *data,
43					       const struct file_operations *fops);
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.
54	Create a file with an initial size, the following function can be used
55	instead:
57	    struct dentry *debugfs_create_file_size(const char *name, umode_t mode,
58					struct dentry *parent, void *data,
59					const struct file_operations *fops,
60					loff_t file_size);
62	file_size is the initial file size. The other parameters are the same
63	as the function debugfs_create_file.
65	In a number of cases, the creation of a set of file operations is not
66	actually necessary; the debugfs code provides a number of helper functions
67	for simple situations.  Files containing a single integer value can be
68	created with any of:
70	    struct dentry *debugfs_create_u8(const char *name, umode_t mode,
71					     struct dentry *parent, u8 *value);
72	    struct dentry *debugfs_create_u16(const char *name, umode_t mode,
73					      struct dentry *parent, u16 *value);
74	    struct dentry *debugfs_create_u32(const char *name, umode_t mode,
75					      struct dentry *parent, u32 *value);
76	    struct dentry *debugfs_create_u64(const char *name, umode_t mode,
77					      struct dentry *parent, u64 *value);
79	These files support both reading and writing the given value; if a specific
80	file should not be written to, simply set the mode bits accordingly.  The
81	values in these files are in decimal; if hexadecimal is more appropriate,
82	the following functions can be used instead:
84	    struct dentry *debugfs_create_x8(const char *name, umode_t mode,
85					     struct dentry *parent, u8 *value);
86	    struct dentry *debugfs_create_x16(const char *name, umode_t mode,
87					      struct dentry *parent, u16 *value);
88	    struct dentry *debugfs_create_x32(const char *name, umode_t mode,
89					      struct dentry *parent, u32 *value);
90	    struct dentry *debugfs_create_x64(const char *name, umode_t mode,
91					      struct dentry *parent, u64 *value);
93	These functions are useful as long as the developer knows the size of the
94	value to be exported.  Some types can have different widths on different
95	architectures, though, complicating the situation somewhat.  There is a
96	function meant to help out in one special case:
98	    struct dentry *debugfs_create_size_t(const char *name, umode_t mode,
99					         struct dentry *parent, 
100						 size_t *value);
102	As might be expected, this function will create a debugfs file to represent
103	a variable of type size_t.
105	Boolean values can be placed in debugfs with:
107	    struct dentry *debugfs_create_bool(const char *name, umode_t mode,
108					       struct dentry *parent, bool *value);
110	A read on the resulting file will yield either Y (for non-zero values) or
111	N, followed by a newline.  If written to, it will accept either upper- or
112	lower-case values, or 1 or 0.  Any other input will be silently ignored.
114	Also, atomic_t values can be placed in debugfs with:
116	    struct dentry *debugfs_create_atomic_t(const char *name, umode_t mode,
117					struct dentry *parent, atomic_t *value)
119	A read of this file will get atomic_t values, and a write of this file
120	will set atomic_t values.
122	Another option is exporting a block of arbitrary binary data, with
123	this structure and function:
125	    struct debugfs_blob_wrapper {
126		void *data;
127		unsigned long size;
128	    };
130	    struct dentry *debugfs_create_blob(const char *name, umode_t mode,
131					       struct dentry *parent,
132					       struct debugfs_blob_wrapper *blob);
134	A read of this file will return the data pointed to by the
135	debugfs_blob_wrapper structure.  Some drivers use "blobs" as a simple way
136	to return several lines of (static) formatted text output.  This function
137	can be used to export binary information, but there does not appear to be
138	any code which does so in the mainline.  Note that all files created with
139	debugfs_create_blob() are read-only.
141	If you want to dump a block of registers (something that happens quite
142	often during development, even if little such code reaches mainline.
143	Debugfs offers two functions: one to make a registers-only file, and
144	another to insert a register block in the middle of another sequential
145	file.
147	    struct debugfs_reg32 {
148		char *name;
149		unsigned long offset;
150	    };
152	    struct debugfs_regset32 {
153		struct debugfs_reg32 *regs;
154		int nregs;
155		void __iomem *base;
156	    };
158	    struct dentry *debugfs_create_regset32(const char *name, umode_t mode,
159					     struct dentry *parent,
160					     struct debugfs_regset32 *regset);
162	    void debugfs_print_regs32(struct seq_file *s, struct debugfs_reg32 *regs,
163				 int nregs, void __iomem *base, char *prefix);
165	The "base" argument may be 0, but you may want to build the reg32 array
166	using __stringify, and a number of register names (macros) are actually
167	byte offsets over a base for the register block.
169	If you want to dump an u32 array in debugfs, you can create file with:
171	    struct dentry *debugfs_create_u32_array(const char *name, umode_t mode,
172				struct dentry *parent,
173				u32 *array, u32 elements);
175	The "array" argument provides data, and the "elements" argument is
176	the number of elements in the array. Note: Once array is created its
177	size can not be changed.
179	There is a helper function to create device related seq_file:
181	   struct dentry *debugfs_create_devm_seqfile(struct device *dev,
182					const char *name,
183					struct dentry *parent,
184					int (*read_fn)(struct seq_file *s,
185						void *data));
187	The "dev" argument is the device related to this debugfs file, and
188	the "read_fn" is a function pointer which to be called to print the
189	seq_file content.
191	There are a couple of other directory-oriented helper functions:
193	    struct dentry *debugfs_rename(struct dentry *old_dir, 
194	    				  struct dentry *old_dentry,
195			                  struct dentry *new_dir, 
196					  const char *new_name);
198	    struct dentry *debugfs_create_symlink(const char *name, 
199	                                          struct dentry *parent,
200					      	  const char *target);
202	A call to debugfs_rename() will give a new name to an existing debugfs
203	file, possibly in a different directory.  The new_name must not exist prior
204	to the call; the return value is old_dentry with updated information.
205	Symbolic links can be created with debugfs_create_symlink().
207	There is one important thing that all debugfs users must take into account:
208	there is no automatic cleanup of any directories created in debugfs.  If a
209	module is unloaded without explicitly removing debugfs entries, the result
210	will be a lot of stale pointers and no end of highly antisocial behavior.
211	So all debugfs users - at least those which can be built as modules - must
212	be prepared to remove all files and directories they create there.  A file
213	can be removed with:
215	    void debugfs_remove(struct dentry *dentry);
217	The dentry value can be NULL, in which case nothing will be removed.
219	Once upon a time, debugfs users were required to remember the dentry
220	pointer for every debugfs file they created so that all files could be
221	cleaned up.  We live in more civilized times now, though, and debugfs users
222	can call:
224	    void debugfs_remove_recursive(struct dentry *dentry);
226	If this function is passed a pointer for the dentry corresponding to the
227	top-level directory, the entire hierarchy below that directory will be
228	removed.
230	Notes:
231		[1] http://lwn.net/Articles/309298/
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