1 .. SPDX-License-Identifier: GPL-2.0
2 .. include:: <isonum.txt>
3
4 =======
5 DebugFS
6 =======
7
8 Copyright |copy| 2009 Jonathan Corbet <corbet@lwn.net>
9
10 Debugfs exists as a simple way for kernel developers to make information
11 available to user space. Unlike /proc, which is only meant for information
12 about a process, or sysfs, which has strict one-value-per-file rules,
13 debugfs has no rules at all. Developers can put any information they want
14 there. The debugfs filesystem is also intended to not serve as a stable
15 ABI to user space; in theory, there are no stability constraints placed on
16 files exported there. The real world is not always so simple, though [1]_;
17 even debugfs interfaces are best designed with the idea that they will need
18 to be maintained forever.
19
20 Debugfs is typically mounted with a command like::
21
22 mount -t debugfs none /sys/kernel/debug
23
24 (Or an equivalent /etc/fstab line).
25 The debugfs root directory is accessible only to the root user by
26 default. To change access to the tree the "uid", "gid" and "mode" mount
27 options can be used.
28
29 Note that the debugfs API is exported GPL-only to modules.
30
31 Code using debugfs should include <linux/debugfs.h>. Then, the first order
32 of business will be to create at least one directory to hold a set of
33 debugfs files::
34
35 struct dentry *debugfs_create_dir(const char *name, struct dentry *parent);
36
37 This call, if successful, will make a directory called name underneath the
38 indicated parent directory. If parent is NULL, the directory will be
39 created in the debugfs root. On success, the return value is a struct
40 dentry pointer which can be used to create files in the directory (and to
41 clean it up at the end). An ERR_PTR(-ERROR) return value indicates that
42 something went wrong. If ERR_PTR(-ENODEV) is returned, that is an
43 indication that the kernel has been built without debugfs support and none
44 of the functions described below will work.
45
46 The most general way to create a file within a debugfs directory is with::
47
48 struct dentry *debugfs_create_file(const char *name, umode_t mode,
49 struct dentry *parent, void *data,
50 const struct file_operations *fops);
51
52 Here, name is the name of the file to create, mode describes the access
53 permissions the file should have, parent indicates the directory which
54 should hold the file, data will be stored in the i_private field of the
55 resulting inode structure, and fops is a set of file operations which
56 implement the file's behavior. At a minimum, the read() and/or write()
57 operations should be provided; others can be included as needed. Again,
58 the return value will be a dentry pointer to the created file,
59 ERR_PTR(-ERROR) on error, or ERR_PTR(-ENODEV) if debugfs support is
60 missing.
61
62 Create a file with an initial size, the following function can be used
63 instead::
64
65 void debugfs_create_file_size(const char *name, umode_t mode,
66 struct dentry *parent, void *data,
67 const struct file_operations *fops,
68 loff_t file_size);
69
70 file_size is the initial file size. The other parameters are the same
71 as the function debugfs_create_file.
72
73 In a number of cases, the creation of a set of file operations is not
74 actually necessary; the debugfs code provides a number of helper functions
75 for simple situations. Files containing a single integer value can be
76 created with any of::
77
78 void debugfs_create_u8(const char *name, umode_t mode,
79 struct dentry *parent, u8 *value);
80 void debugfs_create_u16(const char *name, umode_t mode,
81 struct dentry *parent, u16 *value);
82 void debugfs_create_u32(const char *name, umode_t mode,
83 struct dentry *parent, u32 *value);
84 void debugfs_create_u64(const char *name, umode_t mode,
85 struct dentry *parent, u64 *value);
86
87 These files support both reading and writing the given value; if a specific
88 file should not be written to, simply set the mode bits accordingly. The
89 values in these files are in decimal; if hexadecimal is more appropriate,
90 the following functions can be used instead::
91
92 void debugfs_create_x8(const char *name, umode_t mode,
93 struct dentry *parent, u8 *value);
94 void debugfs_create_x16(const char *name, umode_t mode,
95 struct dentry *parent, u16 *value);
96 void debugfs_create_x32(const char *name, umode_t mode,
97 struct dentry *parent, u32 *value);
98 void debugfs_create_x64(const char *name, umode_t mode,
99 struct dentry *parent, u64 *value);
100
101 These functions are useful as long as the developer knows the size of the
102 value to be exported. Some types can have different widths on different
103 architectures, though, complicating the situation somewhat. There are
104 functions meant to help out in such special cases::
105
106 void debugfs_create_size_t(const char *name, umode_t mode,
107 struct dentry *parent, size_t *value);
108
109 As might be expected, this function will create a debugfs file to represent
110 a variable of type size_t.
111
112 Similarly, there are helpers for variables of type unsigned long, in decimal
113 and hexadecimal::
114
115 struct dentry *debugfs_create_ulong(const char *name, umode_t mode,
116 struct dentry *parent,
117 unsigned long *value);
118 void debugfs_create_xul(const char *name, umode_t mode,
119 struct dentry *parent, unsigned long *value);
120
121 Boolean values can be placed in debugfs with::
122
123 void debugfs_create_bool(const char *name, umode_t mode,
124 struct dentry *parent, bool *value);
125
126 A read on the resulting file will yield either Y (for non-zero values) or
127 N, followed by a newline. If written to, it will accept either upper- or
128 lower-case values, or 1 or 0. Any other input will be silently ignored.
129
130 Also, atomic_t values can be placed in debugfs with::
131
132 void debugfs_create_atomic_t(const char *name, umode_t mode,
133 struct dentry *parent, atomic_t *value)
134
135 A read of this file will get atomic_t values, and a write of this file
136 will set atomic_t values.
137
138 Another option is exporting a block of arbitrary binary data, with
139 this structure and function::
140
141 struct debugfs_blob_wrapper {
142 void *data;
143 unsigned long size;
144 };
145
146 struct dentry *debugfs_create_blob(const char *name, umode_t mode,
147 struct dentry *parent,
148 struct debugfs_blob_wrapper *blob);
149
150 A read of this file will return the data pointed to by the
151 debugfs_blob_wrapper structure. Some drivers use "blobs" as a simple way
152 to return several lines of (static) formatted text output. This function
153 can be used to export binary information, but there does not appear to be
154 any code which does so in the mainline. Note that all files created with
155 debugfs_create_blob() are read-only.
156
157 If you want to dump a block of registers (something that happens quite
158 often during development, even if little such code reaches mainline),
159 debugfs offers two functions: one to make a registers-only file, and
160 another to insert a register block in the middle of another sequential
161 file::
162
163 struct debugfs_reg32 {
164 char *name;
165 unsigned long offset;
166 };
167
168 struct debugfs_regset32 {
169 const struct debugfs_reg32 *regs;
170 int nregs;
171 void __iomem *base;
172 struct device *dev; /* Optional device for Runtime PM */
173 };
174
175 debugfs_create_regset32(const char *name, umode_t mode,
176 struct dentry *parent,
177 struct debugfs_regset32 *regset);
178
179 void debugfs_print_regs32(struct seq_file *s, const struct debugfs_reg32 *regs,
180 int nregs, void __iomem *base, char *prefix);
181
182 The "base" argument may be 0, but you may want to build the reg32 array
183 using __stringify, and a number of register names (macros) are actually
184 byte offsets over a base for the register block.
185
186 If you want to dump a u32 array in debugfs, you can create a file with::
187
188 struct debugfs_u32_array {
189 u32 *array;
190 u32 n_elements;
191 };
192
193 void debugfs_create_u32_array(const char *name, umode_t mode,
194 struct dentry *parent,
195 struct debugfs_u32_array *array);
196
197 The "array" argument wraps a pointer to the array's data and the number
198 of its elements. Note: Once array is created its size can not be changed.
199
200 There is a helper function to create a device-related seq_file::
201
202 void debugfs_create_devm_seqfile(struct device *dev,
203 const char *name,
204 struct dentry *parent,
205 int (*read_fn)(struct seq_file *s,
206 void *data));
207
208 The "dev" argument is the device related to this debugfs file, and
209 the "read_fn" is a function pointer which to be called to print the
210 seq_file content.
211
212 There are a couple of other directory-oriented helper functions::
213
214 struct dentry *debugfs_rename(struct dentry *old_dir,
215 struct dentry *old_dentry,
216 struct dentry *new_dir,
217 const char *new_name);
218
219 struct dentry *debugfs_create_symlink(const char *name,
220 struct dentry *parent,
221 const char *target);
222
223 A call to debugfs_rename() will give a new name to an existing debugfs
224 file, possibly in a different directory. The new_name must not exist prior
225 to the call; the return value is old_dentry with updated information.
226 Symbolic links can be created with debugfs_create_symlink().
227
228 There is one important thing that all debugfs users must take into account:
229 there is no automatic cleanup of any directories created in debugfs. If a
230 module is unloaded without explicitly removing debugfs entries, the result
231 will be a lot of stale pointers and no end of highly antisocial behavior.
232 So all debugfs users - at least those which can be built as modules - must
233 be prepared to remove all files and directories they create there. A file
234 can be removed with::
235
236 void debugfs_remove(struct dentry *dentry);
237
238 The dentry value can be NULL or an error value, in which case nothing will
239 be removed.
240
241 Once upon a time, debugfs users were required to remember the dentry
242 pointer for every debugfs file they created so that all files could be
243 cleaned up. We live in more civilized times now, though, and debugfs users
244 can call::
245
246 void debugfs_remove_recursive(struct dentry *dentry);
247
248 If this function is passed a pointer for the dentry corresponding to the
249 top-level directory, the entire hierarchy below that directory will be
250 removed.
251
252 .. [1] http://lwn.net/Articles/309298/
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