linux/Documentation/filesystems/debugfs.rst
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   1.. SPDX-License-Identifier: GPL-2.0
   2.. include:: <isonum.txt>
   3
   4=======
   5DebugFS
   6=======
   7
   8Copyright |copy| 2009 Jonathan Corbet <corbet@lwn.net>
   9
  10Debugfs exists as a simple way for kernel developers to make information
  11available to user space.  Unlike /proc, which is only meant for information
  12about a process, or sysfs, which has strict one-value-per-file rules,
  13debugfs has no rules at all.  Developers can put any information they want
  14there.  The debugfs filesystem is also intended to not serve as a stable
  15ABI to user space; in theory, there are no stability constraints placed on
  16files exported there.  The real world is not always so simple, though [1]_;
  17even debugfs interfaces are best designed with the idea that they will need
  18to be maintained forever.
  19
  20Debugfs is typically mounted with a command like::
  21
  22    mount -t debugfs none /sys/kernel/debug
  23
  24(Or an equivalent /etc/fstab line).
  25The debugfs root directory is accessible only to the root user by
  26default. To change access to the tree the "uid", "gid" and "mode" mount
  27options can be used.
  28
  29Note that the debugfs API is exported GPL-only to modules.
  30
  31Code using debugfs should include <linux/debugfs.h>.  Then, the first order
  32of business will be to create at least one directory to hold a set of
  33debugfs files::
  34
  35    struct dentry *debugfs_create_dir(const char *name, struct dentry *parent);
  36
  37This call, if successful, will make a directory called name underneath the
  38indicated parent directory.  If parent is NULL, the directory will be
  39created in the debugfs root.  On success, the return value is a struct
  40dentry pointer which can be used to create files in the directory (and to
  41clean it up at the end).  An ERR_PTR(-ERROR) return value indicates that
  42something went wrong.  If ERR_PTR(-ENODEV) is returned, that is an
  43indication that the kernel has been built without debugfs support and none
  44of the functions described below will work.
  45
  46The 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
  52Here, name is the name of the file to create, mode describes the access
  53permissions the file should have, parent indicates the directory which
  54should hold the file, data will be stored in the i_private field of the
  55resulting inode structure, and fops is a set of file operations which
  56implement the file's behavior.  At a minimum, the read() and/or write()
  57operations should be provided; others can be included as needed.  Again,
  58the return value will be a dentry pointer to the created file,
  59ERR_PTR(-ERROR) on error, or ERR_PTR(-ENODEV) if debugfs support is
  60missing.
  61
  62Create a file with an initial size, the following function can be used
  63instead::
  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
  70file_size is the initial file size. The other parameters are the same
  71as the function debugfs_create_file.
  72
  73In a number of cases, the creation of a set of file operations is not
  74actually necessary; the debugfs code provides a number of helper functions
  75for simple situations.  Files containing a single integer value can be
  76created 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
  87These files support both reading and writing the given value; if a specific
  88file should not be written to, simply set the mode bits accordingly.  The
  89values in these files are in decimal; if hexadecimal is more appropriate,
  90the 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
 101These functions are useful as long as the developer knows the size of the
 102value to be exported.  Some types can have different widths on different
 103architectures, though, complicating the situation somewhat.  There are
 104functions 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
 109As might be expected, this function will create a debugfs file to represent
 110a variable of type size_t.
 111
 112Similarly, there are helpers for variables of type unsigned long, in decimal
 113and 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
 121Boolean 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
 126A read on the resulting file will yield either Y (for non-zero values) or
 127N, followed by a newline.  If written to, it will accept either upper- or
 128lower-case values, or 1 or 0.  Any other input will be silently ignored.
 129
 130Also, 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
 135A read of this file will get atomic_t values, and a write of this file
 136will set atomic_t values.
 137
 138Another option is exporting a block of arbitrary binary data, with
 139this 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
 150A read of this file will return the data pointed to by the
 151debugfs_blob_wrapper structure.  Some drivers use "blobs" as a simple way
 152to return several lines of (static) formatted text output.  This function
 153can be used to export binary information, but there does not appear to be
 154any code which does so in the mainline.  Note that all files created with
 155debugfs_create_blob() are read-only.
 156
 157If you want to dump a block of registers (something that happens quite
 158often during development, even if little such code reaches mainline.
 159Debugfs offers two functions: one to make a registers-only file, and
 160another to insert a register block in the middle of another sequential
 161file::
 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
 182The "base" argument may be 0, but you may want to build the reg32 array
 183using __stringify, and a number of register names (macros) are actually
 184byte offsets over a base for the register block.
 185
 186If you want to dump an u32 array in debugfs, you can create 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
 197The "array" argument wraps a pointer to the array's data and the number
 198of its elements. Note: Once array is created its size can not be changed.
 199
 200There is a helper function to create 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
 208The "dev" argument is the device related to this debugfs file, and
 209the "read_fn" is a function pointer which to be called to print the
 210seq_file content.
 211
 212There 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
 223A call to debugfs_rename() will give a new name to an existing debugfs
 224file, possibly in a different directory.  The new_name must not exist prior
 225to the call; the return value is old_dentry with updated information.
 226Symbolic links can be created with debugfs_create_symlink().
 227
 228There is one important thing that all debugfs users must take into account:
 229there is no automatic cleanup of any directories created in debugfs.  If a
 230module is unloaded without explicitly removing debugfs entries, the result
 231will be a lot of stale pointers and no end of highly antisocial behavior.
 232So all debugfs users - at least those which can be built as modules - must
 233be prepared to remove all files and directories they create there.  A file
 234can be removed with::
 235
 236    void debugfs_remove(struct dentry *dentry);
 237
 238The dentry value can be NULL or an error value, in which case nothing will
 239be removed.
 240
 241Once upon a time, debugfs users were required to remember the dentry
 242pointer for every debugfs file they created so that all files could be
 243cleaned up.  We live in more civilized times now, though, and debugfs users
 244can call::
 245
 246    void debugfs_remove_recursive(struct dentry *dentry);
 247
 248If this function is passed a pointer for the dentry corresponding to the
 249top-level directory, the entire hierarchy below that directory will be
 250removed.
 251
 252.. [1] http://lwn.net/Articles/309298/
 253