blob: 463f595733e8876b5bb1fa65c8d7151e8ece7fd5 [file] [log] [blame]
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -06001Copyright 2009 Jonathan Corbet <corbet@lwn.net>
2
3Debugfs exists as a simple way for kernel developers to make information
4available to user space. Unlike /proc, which is only meant for information
5about a process, or sysfs, which has strict one-value-per-file rules,
6debugfs has no rules at all. Developers can put any information they want
7there. The debugfs filesystem is also intended to not serve as a stable
8ABI to user space; in theory, there are no stability constraints placed on
9files exported there. The real world is not always so simple, though [1];
10even debugfs interfaces are best designed with the idea that they will need
11to be maintained forever.
12
13Debugfs is typically mounted with a command like:
14
15 mount -t debugfs none /sys/kernel/debug
16
Ludwig Nusseld6e48682012-01-25 11:52:28 +010017(Or an equivalent /etc/fstab line).
Kees Cook82aceae42012-08-27 13:32:15 -070018The debugfs root directory is accessible only to the root user by
19default. To change access to the tree the "uid", "gid" and "mode" mount
Ludwig Nusseld6e48682012-01-25 11:52:28 +010020options can be used.
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060021
22Note that the debugfs API is exported GPL-only to modules.
23
24Code using debugfs should include <linux/debugfs.h>. Then, the first order
25of business will be to create at least one directory to hold a set of
26debugfs files:
27
28 struct dentry *debugfs_create_dir(const char *name, struct dentry *parent);
29
30This call, if successful, will make a directory called name underneath the
31indicated parent directory. If parent is NULL, the directory will be
32created in the debugfs root. On success, the return value is a struct
33dentry pointer which can be used to create files in the directory (and to
34clean it up at the end). A NULL return value indicates that something went
35wrong. If ERR_PTR(-ENODEV) is returned, that is an indication that the
36kernel has been built without debugfs support and none of the functions
37described below will work.
38
39The most general way to create a file within a debugfs directory is with:
40
Al Virof4ae40a2011-07-24 04:33:43 -040041 struct dentry *debugfs_create_file(const char *name, umode_t mode,
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060042 struct dentry *parent, void *data,
43 const struct file_operations *fops);
44
45Here, name is the name of the file to create, mode describes the access
46permissions the file should have, parent indicates the directory which
47should hold the file, data will be stored in the i_private field of the
48resulting inode structure, and fops is a set of file operations which
49implement the file's behavior. At a minimum, the read() and/or write()
50operations should be provided; others can be included as needed. Again,
51the return value will be a dentry pointer to the created file, NULL for
52error, or ERR_PTR(-ENODEV) if debugfs support is missing.
53
Wang Long9e1aa7c2015-07-16 06:31:16 +000054Create a file with an initial size, the following function can be used
55instead:
56
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);
61
62file_size is the initial file size. The other parameters are the same
63as the function debugfs_create_file.
64
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060065In a number of cases, the creation of a set of file operations is not
66actually necessary; the debugfs code provides a number of helper functions
67for simple situations. Files containing a single integer value can be
68created with any of:
69
Al Virof4ae40a2011-07-24 04:33:43 -040070 struct dentry *debugfs_create_u8(const char *name, umode_t mode,
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060071 struct dentry *parent, u8 *value);
Al Virof4ae40a2011-07-24 04:33:43 -040072 struct dentry *debugfs_create_u16(const char *name, umode_t mode,
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060073 struct dentry *parent, u16 *value);
Al Virof4ae40a2011-07-24 04:33:43 -040074 struct dentry *debugfs_create_u32(const char *name, umode_t mode,
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060075 struct dentry *parent, u32 *value);
Al Virof4ae40a2011-07-24 04:33:43 -040076 struct dentry *debugfs_create_u64(const char *name, umode_t mode,
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060077 struct dentry *parent, u64 *value);
78
79These files support both reading and writing the given value; if a specific
80file should not be written to, simply set the mode bits accordingly. The
81values in these files are in decimal; if hexadecimal is more appropriate,
82the following functions can be used instead:
83
Al Virof4ae40a2011-07-24 04:33:43 -040084 struct dentry *debugfs_create_x8(const char *name, umode_t mode,
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060085 struct dentry *parent, u8 *value);
Al Virof4ae40a2011-07-24 04:33:43 -040086 struct dentry *debugfs_create_x16(const char *name, umode_t mode,
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060087 struct dentry *parent, u16 *value);
Al Virof4ae40a2011-07-24 04:33:43 -040088 struct dentry *debugfs_create_x32(const char *name, umode_t mode,
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060089 struct dentry *parent, u32 *value);
Al Virof4ae40a2011-07-24 04:33:43 -040090 struct dentry *debugfs_create_x64(const char *name, umode_t mode,
Akinobu Mitad0a54262011-07-09 14:01:17 +090091 struct dentry *parent, u64 *value);
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060092
93These functions are useful as long as the developer knows the size of the
94value to be exported. Some types can have different widths on different
95architectures, though, complicating the situation somewhat. There is a
96function meant to help out in one special case:
97
Al Virof4ae40a2011-07-24 04:33:43 -040098 struct dentry *debugfs_create_size_t(const char *name, umode_t mode,
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -060099 struct dentry *parent,
100 size_t *value);
101
102As might be expected, this function will create a debugfs file to represent
103a variable of type size_t.
104
105Boolean values can be placed in debugfs with:
106
Al Virof4ae40a2011-07-24 04:33:43 -0400107 struct dentry *debugfs_create_bool(const char *name, umode_t mode,
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -0600108 struct dentry *parent, u32 *value);
109
110A read on the resulting file will yield either Y (for non-zero values) or
111N, followed by a newline. If written to, it will accept either upper- or
112lower-case values, or 1 or 0. Any other input will be silently ignored.
113
Wang Long9e1aa7c2015-07-16 06:31:16 +0000114Also, atomic_t values can be placed in debugfs with:
115
116 struct dentry *debugfs_create_atomic_t(const char *name, umode_t mode,
117 struct dentry *parent, atomic_t *value)
118
119A read of this file will get atomic_t values, and a write of this file
120will set atomic_t values.
121
Alessandro Rubini1a087c62011-11-18 14:50:21 +0100122Another option is exporting a block of arbitrary binary data, with
123this structure and function:
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -0600124
125 struct debugfs_blob_wrapper {
126 void *data;
127 unsigned long size;
128 };
129
Al Virof4ae40a2011-07-24 04:33:43 -0400130 struct dentry *debugfs_create_blob(const char *name, umode_t mode,
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -0600131 struct dentry *parent,
132 struct debugfs_blob_wrapper *blob);
133
134A read of this file will return the data pointed to by the
135debugfs_blob_wrapper structure. Some drivers use "blobs" as a simple way
136to return several lines of (static) formatted text output. This function
137can be used to export binary information, but there does not appear to be
138any code which does so in the mainline. Note that all files created with
139debugfs_create_blob() are read-only.
140
Alessandro Rubini1a087c62011-11-18 14:50:21 +0100141If you want to dump a block of registers (something that happens quite
142often during development, even if little such code reaches mainline.
143Debugfs offers two functions: one to make a registers-only file, and
144another to insert a register block in the middle of another sequential
145file.
146
147 struct debugfs_reg32 {
148 char *name;
149 unsigned long offset;
150 };
151
152 struct debugfs_regset32 {
153 struct debugfs_reg32 *regs;
154 int nregs;
155 void __iomem *base;
156 };
157
Al Viro88187392012-03-20 06:00:24 -0400158 struct dentry *debugfs_create_regset32(const char *name, umode_t mode,
Alessandro Rubini1a087c62011-11-18 14:50:21 +0100159 struct dentry *parent,
160 struct debugfs_regset32 *regset);
161
Joe Perches97615362014-09-29 16:08:26 -0700162 void debugfs_print_regs32(struct seq_file *s, struct debugfs_reg32 *regs,
Alessandro Rubini1a087c62011-11-18 14:50:21 +0100163 int nregs, void __iomem *base, char *prefix);
164
165The "base" argument may be 0, but you may want to build the reg32 array
166using __stringify, and a number of register names (macros) are actually
167byte offsets over a base for the register block.
168
Wang Long9e1aa7c2015-07-16 06:31:16 +0000169If you want to dump an u32 array in debugfs, you can create file with:
170
171 struct dentry *debugfs_create_u32_array(const char *name, umode_t mode,
172 struct dentry *parent,
173 u32 *array, u32 elements);
174
175The "array" argument provides data, and the "elements" argument is
176the number of elements in the array. Note: Once array is created its
177size can not be changed.
178
179There is a helper function to create device related seq_file:
180
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));
186
187The "dev" argument is the device related to this debugfs file, and
188the "read_fn" is a function pointer which to be called to print the
189seq_file content.
Alessandro Rubini1a087c62011-11-18 14:50:21 +0100190
Jonathan Corbetf89d7ea2009-06-04 16:35:25 -0600191There are a couple of other directory-oriented helper functions:
192
193 struct dentry *debugfs_rename(struct dentry *old_dir,
194 struct dentry *old_dentry,
195 struct dentry *new_dir,
196 const char *new_name);
197
198 struct dentry *debugfs_create_symlink(const char *name,
199 struct dentry *parent,
200 const char *target);
201
202A call to debugfs_rename() will give a new name to an existing debugfs
203file, possibly in a different directory. The new_name must not exist prior
204to the call; the return value is old_dentry with updated information.
205Symbolic links can be created with debugfs_create_symlink().
206
207There is one important thing that all debugfs users must take into account:
208there is no automatic cleanup of any directories created in debugfs. If a
209module is unloaded without explicitly removing debugfs entries, the result
210will be a lot of stale pointers and no end of highly antisocial behavior.
211So all debugfs users - at least those which can be built as modules - must
212be prepared to remove all files and directories they create there. A file
213can be removed with:
214
215 void debugfs_remove(struct dentry *dentry);
216
217The dentry value can be NULL, in which case nothing will be removed.
218
219Once upon a time, debugfs users were required to remember the dentry
220pointer for every debugfs file they created so that all files could be
221cleaned up. We live in more civilized times now, though, and debugfs users
222can call:
223
224 void debugfs_remove_recursive(struct dentry *dentry);
225
226If this function is passed a pointer for the dentry corresponding to the
227top-level directory, the entire hierarchy below that directory will be
228removed.
229
230Notes:
231 [1] http://lwn.net/Articles/309298/