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 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 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 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 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