Merge branch 'docs-next' of git://git.lwn.net/linux-2.6

things like "update driver X", "bug fix for driver X", or "this patch

includes updates for subsystem X.  Please apply."

The maintainer will thank you if you write your patch description in a

form which can be easily pulled into Linux's source code management

system, git, as a "commit log".  See #15, below.

If your description starts to get long, that's a sign that you probably

need to split up your patch.  See #3, next.

have been included in the discussion

14) Using Tested-by: and Reviewed-by:

14) Using Reported-by:, Tested-by: and Reviewed-by:

If this patch fixes a problem reported by somebody else, consider adding a

Reported-by: tag to credit the reporter for their contribution.  Please

note that this tag should not be added without the reporter's permission,

especially if the problem was not reported in a public forum.  That said,

if we diligently credit our bug reporters, they will, hopefully, be

inspired to help us again in the future.

A Tested-by: tag indicates that the patch has been successfully tested (in

some environment) by the person named.  This tag informs maintainers that

reviewers and to inform maintainers of the degree of review which has been

done on the patch.  Reviewed-by: tags, when supplied by reviewers known to

understand the subject area and to perform thorough reviews, will normally

increase the liklihood of your patch getting into the kernel.

increase the likelihood of your patch getting into the kernel.

15) The canonical patch format

phrase" for every patch in a whole patch series (where a "patch

series" is an ordered sequence of multiple, related patches).

Bear in mind that the "summary phrase" of your email becomes

a globally-unique identifier for that patch.  It propagates

all the way into the git changelog.  The "summary phrase" may

later be used in developer discussions which refer to the patch.

People will want to google for the "summary phrase" to read

discussion regarding that patch.

Bear in mind that the "summary phrase" of your email becomes a

globally-unique identifier for that patch.  It propagates all the way

into the git changelog.  The "summary phrase" may later be used in

developer discussions which refer to the patch.  People will want to

google for the "summary phrase" to read discussion regarding that

patch.  It will also be the only thing that people may quickly see

when, two or three months later, they are going through perhaps

thousands of patches using tools such as "gitk" or "git log

--oneline".

For these reasons, the "summary" must be no more than 70-75

characters, and it must describe both what the patch changes, as well

as why the patch might be necessary.  It is challenging to be both

succinct and descriptive, but that is what a well-written summary

should do.

The "summary phrase" may be prefixed by tags enclosed in square

brackets: "Subject: [PATCH tag] <summary phrase>".  The tags are not

considered part of the summary phrase, but describe how the patch

should be treated.  Common tags might include a version descriptor if

the multiple versions of the patch have been sent out in response to

comments (i.e., "v1, v2, v3"), or "RFC" to indicate a request for

comments.  If there are four patches in a patch series the individual

patches may be numbered like this: 1/4, 2/4, 3/4, 4/4.  This assures

that developers understand the order in which the patches should be

applied and that they have reviewed or applied all of the patches in

the patch series.

A couple of example Subjects:

The explanation body will be committed to the permanent source

changelog, so should make sense to a competent reader who has long

since forgotten the immediate details of the discussion that might

have led to this patch.

have led to this patch.  Including symptoms of the failure which the

patch addresses (kernel log messages, oops messages, etc.) is

especially useful for people who might be searching the commit logs

looking for the applicable patch.  If a patch fixes a compile failure,

it may not be necessary to include _all_ of the compile failures; just

enough that it is likely that someone searching for the patch can find

it.  As in the "summary phrase", it is important to be both succinct as

well as descriptive.

The "---" marker line serves the essential purpose of marking for patch

handling tools where the changelog message ends.

One good use for the additional comments after the "---" marker is for

a diffstat, to show what files have changed, and the number of inserted

and deleted lines per file.  A diffstat is especially useful on bigger

patches.  Other comments relevant only to the moment or the maintainer,

not suitable for the permanent changelog, should also go here.

Use diffstat options "-p 1 -w 70" so that filenames are listed from the

top of the kernel source tree and don't use too much horizontal space

(easily fit in 80 columns, maybe with some indentation).

a diffstat, to show what files have changed, and the number of

inserted and deleted lines per file.  A diffstat is especially useful

on bigger patches.  Other comments relevant only to the moment or the

maintainer, not suitable for the permanent changelog, should also go

here.  A good example of such comments might be "patch changelogs"

which describe what has changed between the v1 and v2 version of the

patch.

If you are going to include a diffstat after the "---" marker, please

use diffstat options "-p 1 -w 70" so that filenames are listed from

the top of the kernel source tree and don't use too much horizontal

space (easily fit in 80 columns, maybe with some indentation).

See more details on the proper patch format in the following

references.

done.  When done properly, though, it is time well spent.

5.4: PATCH FORMATTING

5.4: PATCH FORMATTING AND CHANGELOGS

So now you have a perfect series of patches for posting, but the work is

not done quite yet.  Each patch needs to be formatted into a message which

 - One or more tag lines, with, at a minimum, one Signed-off-by: line from

   the author of the patch.  Tags will be described in more detail below.

The above three items should, normally, be the text used when committing

the change to a revision control system.  They are followed by:

The items above, together, form the changelog for the patch.  Writing good

changelogs is a crucial but often-neglected art; it's worth spending

another moment discussing this issue.  When writing a changelog, you should

bear in mind that a number of different people will be reading your words.

These include subsystem maintainers and reviewers who need to decide

whether the patch should be included, distributors and other maintainers

trying to decide whether a patch should be backported to other kernels, bug

hunters wondering whether the patch is responsible for a problem they are

chasing, users who want to know how the kernel has changed, and more.  A

good changelog conveys the needed information to all of these people in the

most direct and concise way possible.

To that end, the summary line should describe the effects of and motivation

for the change as well as possible given the one-line constraint.  The

detailed description can then amplify on those topics and provide any

needed additional information.  If the patch fixes a bug, cite the commit

which introduced the bug if possible.  If a problem is associated with

specific log or compiler output, include that output to help others

searching for a solution to the same problem.  If the change is meant to

support other changes coming in later patch, say so.  If internal APIs are

changed, detail those changes and how other developers should respond.  In

general, the more you can put yourself into the shoes of everybody who will

be reading your changelog, the better that changelog (and the kernel as a

whole) will be.

Needless to say, the changelog should be the text used when committing the

change to a revision control system.  It will be followed by:

 - The patch itself, in the unified ("-u") patch format.  Using the "-p"

   option to diff will associate function names with changes, making the

Copyright 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). 

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).  A NULL 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, mode_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, NULL for

error, or ERR_PTR(-ENODEV) if debugfs support is missing.

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:

    struct dentry *debugfs_create_u8(const char *name, mode_t mode,

				     struct dentry *parent, u8 *value);

    struct dentry *debugfs_create_u16(const char *name, mode_t mode,

				      struct dentry *parent, u16 *value);

    struct dentry *debugfs_create_u32(const char *name, mode_t mode,

				      struct dentry *parent, u32 *value);

    struct dentry *debugfs_create_u64(const char *name, mode_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:

    struct dentry *debugfs_create_x8(const char *name, mode_t mode,

				     struct dentry *parent, u8 *value);

    struct dentry *debugfs_create_x16(const char *name, mode_t mode,

				      struct dentry *parent, u16 *value);

    struct dentry *debugfs_create_x32(const char *name, mode_t mode,

				      struct dentry *parent, u32 *value);

Note that there is no debugfs_create_x64().

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 is a

function meant to help out in one special case:

    struct dentry *debugfs_create_size_t(const char *name, mode_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.

Boolean values can be placed in debugfs with:

    struct dentry *debugfs_create_bool(const char *name, mode_t mode,

				       struct dentry *parent, u32 *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.

Finally, a block of arbitrary binary data can be exported with:

    struct debugfs_blob_wrapper {

	void *data;

	unsigned long size;

    };

    struct dentry *debugfs_create_blob(const char *name, mode_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.

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, 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.

Notes:

	[1] http://lwn.net/Articles/309298/