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diff --git a/Documentation/filesystems/debugfs.rst b/Documentation/filesystems/debugfs.rst new file mode 100644 index 000000000000..80f332b8eb68 --- /dev/null +++ b/Documentation/filesystems/debugfs.rst @@ -0,0 +1,247 @@ +.. SPDX-License-Identifier: GPL-2.0 +.. include:: <isonum.txt> + +======= +DebugFS +======= + +Copyright |copy| 2009 Jonathan Corbet <corbet@lwn.net> + +Debugfs exists as a simple way for kernel developers to make information +available to user space. Unlike /proc, which is only meant for information +about a process, or sysfs, which has strict one-value-per-file rules, +debugfs has no rules at all. Developers can put any information they want +there. The debugfs filesystem is also intended to not serve as a stable +ABI to user space; in theory, there are no stability constraints placed on +files exported there. The real world is not always so simple, though [1]_; +even debugfs interfaces are best designed with the idea that they will need +to be maintained forever. + +Debugfs is typically mounted with a command like:: + + mount -t debugfs none /sys/kernel/debug + +(Or an equivalent /etc/fstab line). +The debugfs root directory is accessible only to the root user by +default. To change access to the tree the "uid", "gid" and "mode" mount +options can be used. + +Note that the debugfs API is exported GPL-only to modules. + +Code using debugfs should include <linux/debugfs.h>. Then, the first order +of business will be to create at least one directory to hold a set of +debugfs files:: + + struct dentry *debugfs_create_dir(const char *name, struct dentry *parent); + +This call, if successful, will make a directory called name underneath the +indicated parent directory. If parent is NULL, the directory will be +created in the debugfs root. On success, the return value is a struct +dentry pointer which can be used to create files in the directory (and to +clean it up at the end). An ERR_PTR(-ERROR) return value indicates that +something went wrong. If ERR_PTR(-ENODEV) is returned, that is an +indication that the kernel has been built without debugfs support and none +of the functions described below will work. + +The most general way to create a file within a debugfs directory is with:: + + struct dentry *debugfs_create_file(const char *name, umode_t mode, + struct dentry *parent, void *data, + const struct file_operations *fops); + +Here, name is the name of the file to create, mode describes the access +permissions the file should have, parent indicates the directory which +should hold the file, data will be stored in the i_private field of the +resulting inode structure, and fops is a set of file operations which +implement the file's behavior. At a minimum, the read() and/or write() +operations should be provided; others can be included as needed. Again, +the return value will be a dentry pointer to the created file, +ERR_PTR(-ERROR) on error, or ERR_PTR(-ENODEV) if debugfs support is +missing. + +Create a file with an initial size, the following function can be used +instead:: + + struct dentry *debugfs_create_file_size(const char *name, umode_t mode, + struct dentry *parent, void *data, + const struct file_operations *fops, + loff_t file_size); + +file_size is the initial file size. The other parameters are the same +as the function debugfs_create_file. + +In a number of cases, the creation of a set of file operations is not +actually necessary; the debugfs code provides a number of helper functions +for simple situations. Files containing a single integer value can be +created with any of:: + + void debugfs_create_u8(const char *name, umode_t mode, + struct dentry *parent, u8 *value); + void debugfs_create_u16(const char *name, umode_t mode, + struct dentry *parent, u16 *value); + struct dentry *debugfs_create_u32(const char *name, umode_t mode, + struct dentry *parent, u32 *value); + void debugfs_create_u64(const char *name, umode_t mode, + struct dentry *parent, u64 *value); + +These files support both reading and writing the given value; if a specific +file should not be written to, simply set the mode bits accordingly. The +values in these files are in decimal; if hexadecimal is more appropriate, +the following functions can be used instead:: + + void debugfs_create_x8(const char *name, umode_t mode, + struct dentry *parent, u8 *value); + void debugfs_create_x16(const char *name, umode_t mode, + struct dentry *parent, u16 *value); + void debugfs_create_x32(const char *name, umode_t mode, + struct dentry *parent, u32 *value); + void debugfs_create_x64(const char *name, umode_t mode, + struct dentry *parent, u64 *value); + +These functions are useful as long as the developer knows the size of the +value to be exported. Some types can have different widths on different +architectures, though, complicating the situation somewhat. There are +functions meant to help out in such special cases:: + + void debugfs_create_size_t(const char *name, umode_t mode, + struct dentry *parent, size_t *value); + +As might be expected, this function will create a debugfs file to represent +a variable of type size_t. + +Similarly, there are helpers for variables of type unsigned long, in decimal +and hexadecimal:: + + struct dentry *debugfs_create_ulong(const char *name, umode_t mode, + struct dentry *parent, + unsigned long *value); + void debugfs_create_xul(const char *name, umode_t mode, + struct dentry *parent, unsigned long *value); + +Boolean values can be placed in debugfs with:: + + struct dentry *debugfs_create_bool(const char *name, umode_t mode, + struct dentry *parent, bool *value); + +A read on the resulting file will yield either Y (for non-zero values) or +N, followed by a newline. If written to, it will accept either upper- or +lower-case values, or 1 or 0. Any other input will be silently ignored. + +Also, atomic_t values can be placed in debugfs with:: + + void debugfs_create_atomic_t(const char *name, umode_t mode, + struct dentry *parent, atomic_t *value) + +A read of this file will get atomic_t values, and a write of this file +will set atomic_t values. + +Another option is exporting a block of arbitrary binary data, with +this structure and function:: + + struct debugfs_blob_wrapper { + void *data; + unsigned long size; + }; + + struct dentry *debugfs_create_blob(const char *name, umode_t mode, + struct dentry *parent, + struct debugfs_blob_wrapper *blob); + +A read of this file will return the data pointed to by the +debugfs_blob_wrapper structure. Some drivers use "blobs" as a simple way +to return several lines of (static) formatted text output. This function +can be used to export binary information, but there does not appear to be +any code which does so in the mainline. Note that all files created with +debugfs_create_blob() are read-only. + +If you want to dump a block of registers (something that happens quite +often during development, even if little such code reaches mainline. +Debugfs offers two functions: one to make a registers-only file, and +another to insert a register block in the middle of another sequential +file:: + + struct debugfs_reg32 { + char *name; + unsigned long offset; + }; + + struct debugfs_regset32 { + struct debugfs_reg32 *regs; + int nregs; + void __iomem *base; + }; + + debugfs_create_regset32(const char *name, umode_t mode, + struct dentry *parent, + struct debugfs_regset32 *regset); + + void debugfs_print_regs32(struct seq_file *s, struct debugfs_reg32 *regs, + int nregs, void __iomem *base, char *prefix); + +The "base" argument may be 0, but you may want to build the reg32 array +using __stringify, and a number of register names (macros) are actually +byte offsets over a base for the register block. + +If you want to dump an u32 array in debugfs, you can create file with:: + + void debugfs_create_u32_array(const char *name, umode_t mode, + struct dentry *parent, + u32 *array, u32 elements); + +The "array" argument provides data, and the "elements" argument is +the number of elements in the array. Note: Once array is created its +size can not be changed. + +There is a helper function to create device related seq_file:: + + struct dentry *debugfs_create_devm_seqfile(struct device *dev, + const char *name, + struct dentry *parent, + int (*read_fn)(struct seq_file *s, + void *data)); + +The "dev" argument is the device related to this debugfs file, and +the "read_fn" is a function pointer which to be called to print the +seq_file content. + +There are a couple of other directory-oriented helper functions:: + + struct dentry *debugfs_rename(struct dentry *old_dir, + struct dentry *old_dentry, + struct dentry *new_dir, + const char *new_name); + + struct dentry *debugfs_create_symlink(const char *name, + struct dentry *parent, + const char *target); + +A call to debugfs_rename() will give a new name to an existing debugfs +file, possibly in a different directory. The new_name must not exist prior +to the call; the return value is old_dentry with updated information. +Symbolic links can be created with debugfs_create_symlink(). + +There is one important thing that all debugfs users must take into account: +there is no automatic cleanup of any directories created in debugfs. If a +module is unloaded without explicitly removing debugfs entries, the result +will be a lot of stale pointers and no end of highly antisocial behavior. +So all debugfs users - at least those which can be built as modules - must +be prepared to remove all files and directories they create there. A file +can be removed with:: + + void debugfs_remove(struct dentry *dentry); + +The dentry value can be NULL or an error value, in which case nothing will +be removed. + +Once upon a time, debugfs users were required to remember the dentry +pointer for every debugfs file they created so that all files could be +cleaned up. We live in more civilized times now, though, and debugfs users +can call:: + + void debugfs_remove_recursive(struct dentry *dentry); + +If this function is passed a pointer for the dentry corresponding to the +top-level directory, the entire hierarchy below that directory will be +removed. + +.. [1] http://lwn.net/Articles/309298/ |