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