First up, some basics about the source code control tool that the Linux

kernel uses, git. Git can be found in any Linux distro these days, and

there are numerous good tutorials on how to use it and set it up

available on the web. One good one comes within git itself, and can

be read by running:

$ man gittutorial

after you have installed git on your machine.

So run off and install git on your Linux system using the package

manager you are comfortable with (personally, I use openSUSE, and a

simple 'zypper install git' does everything that is needed.)

Then start by cloning the main Linux kernel repository:

$ mkdir ~/linux

$ cd ~/linux

$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git

This will create the directory 'linux-2.6' within the linux/ directory.

Everything we do from here out will be within that directory, so go into

it to start with:

$ cd ~/linux/linux-2.6

Now that you have the raw source code, how do you build it and install

it on your system? That is a much larger task, one that is beyond this

article. Luckily a whole book has been written on this topic, "Linux

Kernel in a Nutshell", and can be found free online at:

http://www.kroah.com/lkn/

if you don't want to purchase it.

So go and get your kernel configured and building, and then come back

here to figure out what to do next.

-- Git tips

Here are a few tips to use with git when working with the kernel source

tree. First off, never do your work on the same branch that Linus

pushes to, called "master". Create your own branch, and use that

instead. This ensures that any changes that are committed to Linus's

branch upstream, will be able to be updated by you without any problems.

To create a new branch called 'tutorial' and check it out, do the

following:

$ git branch tutorial

$ git checkout tutorial

That's it. You are now in the 'tutorial' branch of your kernel

repository, as can be seen by the following command:

$ git branch

master

* tutorial

The '*' in front of the 'tutorial' name shows that you are on the

correct branch.

Now, let's go and make some changes to the kernel code.

-- What to change

Wait, you don't know what change you want to make to the Linux kernel

source tree? Everything is working just fine for you? Well, don't

despair, the Linux kernel developers need all the help they can get, and

have plenty of code in the tree that is just waiting to get cleaned up.

The code in the drivers/staging/ tree consists of a lot of drivers that

do not meet the normal Linux kernel coding guidelines. The code is in

that location so that other developers can help on cleaning it up, and

getting it merged into the main portion of the Linux kernel tree.

Every driver in the drivers/staging directory contains a TODO file that

lists the things that need to be done on it in order for the code to be

moved to the proper location in the kernel tree. The majority of the

drivers all contain the following line in their TODO file:

- fix checkpatch.pl issues

Let's look into what this means and how you can help out with this task.

-- Coding Style

Every large body of code needs to have a set of coding style rules in

order for it to be a viable project that a large number of developers

can work on. Numerous research studies have been made on this topic,

and they all conclude that having a common guideline makes a very large

difference.

It is not merely a matter of aesthetics that programs

should be written in a particular style. Rather there

is a psychological basis for writing programs in a

conventional manner: programmers have strong expectations

that other programmers will follow these discourse rules.

If the rules are violated, then the utility afforded by

the expectations that programmers have built up over time

is effectively nullified.”

– Soloway & Ehrlich

What this means is that once programmers get used to a common style, the

patterns of the code go away when it is looked at, and the meaning shows

through very easily.

The goal of any Linux kernel developer is to have other developers help

find problems in their code, and by keeping all of the code in the same

format, it makes it much easier for anyone else to pick it up, modify

it, or notice bugs in it. As every line of kernel code is reviewed by

at least 2 developers before it is accepted, having a common style

guideline is a very important thing.

The Linux kernel coding style can be found in the file

Documentation/CodingStyle in the kernel source tree. The important

thing to remember when reading it, is not that this style is somehow

better than any other style, just that it is consistent.

In order to help developers easily find coding style issues, the script

scripts/checkpatch.pl in the kernel source tree has been developed.

This script can point out problems very easily, and should always be run

by a developer on their changes, instead of having a reviewer waste

their time by pointing out problems later on.

The drivers in the drivers/staging/ directory all usually have coding

style issues as they were developed by people not familiar with the

Linux kernel guidelines. One of the first things that needs to be done

to the code, is to fix it up to follow the correct rules.

And this is where you come in, by running the checkpatch.pl tool, you

can find a large number of problems that need to be fixed.

-- Specific rules

Let us look at some of the common rules that are part of the kernel

guidelines.

--- Whitespace

The first rule that everyone needs to follow is to use the 'tab'

character, and not use spaces, to indent code. Also, the 'tab'

character should represent 8 spaces. Following along with the 8

character tab indentation, the code should not flow past the 80

character line limit on the right side of the screen.

Note, numerous developers have complained about the 80 character limit

recently, and there are some places where it is acceptable to go beyond

that limit. If you find that you are being forced to do strange

line-wrapping formatting just to fit into the 80 character limit, with

all of your code on the right hand side of the screen, it is better to

refactor the logic to prevent this from happening in the first place.

Forcing an 80 character limit, also forces developers to break their

logic up into smaller, easier to understand chunks, which makes it

easier to review and follow as well.

So yes, there is a method to the madness of the 80 character limit.

--- Braces

Opening braces should be placed on the same line of the statement they

are being used for, with one exception as show below. Closing braces

should be placed back at the original indentation. This can be shown

with the following example:

if (error != -ENODEV) {

foo();

bar();

}

If you need to add an else statement to an if statement, put it on the

same line as the closing brace, as shown here:

if (error != -ENODEV) {

foo();

bar();

} else {

report_error();

goto exit;

}

If braces are not needed for a statement, do not put them in, as they

are unnecessary:

if (error != -ENODEV)

foo();

else

goto exit;

The one exception for opening braces is for function declarations,

those go on a new line as shown here:

int function(int *baz)

{

do_something(baz);

return 0;

}

-- checkpatch.pl

With these simple whitespace and brace rules now understood, let us run

the checkpatch.pl script on some code and see what it tells us:

$ ./scripts/checkpatch.pl --help

Usage: checkpatch.pl [OPTION]... [FILE]...

Version: 0.30

Options:

-q, --quiet quiet

--no-tree run without a kernel tree

--no-signoff do not check for 'Signed-off-by' line

--patch treat FILE as patchfile (default)

--emacs emacs compile window format

--terse one line per report

-f, --file treat FILE as regular source file

--subjective, --strict enable more subjective tests

--root=PATH PATH to the kernel tree root

--no-summary suppress the per-file summary

--mailback only produce a report in case of warnings/errors

--summary-file include the filename in summary

--debug KEY=[0|1] turn on/off debugging of KEY, where KEY is one of

'values', 'possible', 'type', and 'attr' (default

is all off)

--test-only=WORD report only warnings/errors containing WORD

literally

-h, --help, --version display this help and exit

When FILE is - read standard input.

Some common options that we will be using is the --terse and --file

options, as those allow us to see the problems in a much simpler report,

and they work on an entire file, not just a single patch.

So, let's pick a file in the kernel and see what checkpatch.pl tells us

about it:

$ ./scripts/checkpatch.pl --file --terse drivers/staging/comedi/drivers/ni_labpc.c

drivers/staging/comedi/drivers/ni_labpc.c:4: WARNING: line over 80 characters

...

drivers/staging/comedi/drivers/ni_labpc.c:486: WARNING: braces {} are not necessary for single statement blocks

drivers/staging/comedi/drivers/ni_labpc.c:489: WARNING: braces {} are not necessary for single statement blocks

...

drivers/staging/comedi/drivers/ni_labpc.c:587: WARNING: suspect code indent for conditional statements (8, 0)

...

drivers/staging/comedi/drivers/ni_labpc.c:743: WARNING: printk() should include KERN_ facility level

drivers/staging/comedi/drivers/ni_labpc.c:750: WARNING: kfree(NULL) is safe this check is probably not required

...

drivers/staging/comedi/drivers/ni_labpc.c:2028: WARNING: EXPORT_SYMBOL(foo); should immediately follow its function/variable

total: 0 errors, 76 warnings, 2028 lines checked

I've removed a lot of the warnings from the above output, as there was a

total of 76 of them, and they are all variants of the above ones.

As can be seen, the checkpatch.pl tool points out where the code has

gone beyond the 80 character limit, and where braces were used that they

were not needed, as well as a few other things that should be cleaned up

in the file.

Now that we know what needs to be done, fire up your favorite editor,

and let us fix something. How about the brace warning, that should be

simple to resolve.

Looking at the original code, lines 486-490 look like the following:

if (irq) {

printk(", irq %u", irq);

}

if (dma_chan) {

printk(", dma %u", dma_chan);

}

A simple removal of those extra braces results in:

if (irq)

printk(", irq %u", irq);

if (dma_chan)

printk(", dma %u", dma_chan);

Save the file, and run the checkpatch tool again to verify that the

warning is gone:

$ ./scripts/checkpatch.pl --file --terse drivers/staging/comedi/drivers/ni_labpc.c | grep 486

$

And of course build the file to verify that you did not break anything:

$ make drivers/staging/comedi/drivers/ni_labpc.o

CHK include/linux/version.h

CHK include/generated/utsrelease.h

CALL scripts/checksyscalls.sh

CC [M] drivers/staging/comedi/drivers/ni_labpc.o

Yes, it still builds, so all is good.

Great, you have now made your first kernel code fix!

But, how do you take this change, and get it to the kernel developers in

the format that they can apply it?

-- More git fun

As you edited this file within a git repository, your change to it is

caught by git. This can be seen by running the 'git status' command:

$ git status

# On branch tutorial

# Changed but not updated:

# (use "git add <file>..." to update what will be committed)

# (use "git checkout -- <file>..." to discard changes in working directory)

#

# modified: drivers/staging/comedi/drivers/ni_labpc.c

#

no changes added to commit (use "git add" and/or "git commit -a")

This output shows that we are on the branch called 'tutorial', and that

we have one file modified at the moment, the ni_labpc.c file.

If we ask for git to show what we changed, we will see the actual lines:

$ git diff

diff --git a/drivers/staging/comedi/drivers/ni_labpc.c b/drivers/staging/comedi/drivers/ni_labpc.c

index dc3f398..a01e35d 100644

--- a/drivers/staging/comedi/drivers/ni_labpc.c

+++ b/drivers/staging/comedi/drivers/ni_labpc.c

@@ -483,12 +483,10 @@ int labpc_common_attach(struct comedi_device *dev, unsigned long iobase,

printk("comedi%d: ni_labpc: %s, io 0x%lx", dev->minor, thisboard->name,

iobase);

- if (irq) {

+ if (irq)

printk(", irq %u", irq);

- }

- if (dma_chan) {

+ if (dma_chan)

printk(", dma %u", dma_chan);

- }

printk("\n");

if (iobase == 0) {

This output is in the format that the tool 'patch' can use to apply a

change to a body of code. The leading '-' and '+' on some lines show

what lines are removed, and what lines are added. Reading these diff

outputs soon becomes natural, and is the format in which you need to

send to the kernel maintainer to get the change accepted.

--- Description, description, description

The raw diff output shows what code is changed, but for every kernel

patch, more information needs to be provided in order for it to be

accepted. This "metadata" is as important as the code changes, as it is

used to show who made the change, why the change was made, and who

reviewed the change.

Here is a sample change that was accepted into the Linux kernel tree a

while ago:

USB: otg: Fix bug on remove path without transceiver

In the case where a gadget driver is removed while no

transceiver was found at probe time, a bug in

otg_put_transceiver() will trigger.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>

Acked-by: David Brownell <dbrownell@users.sourceforge.net>

Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

--- a/drivers/usb/otg/otg.c

+++ b/drivers/usb/otg/otg.c

@@ -43,7 +43,8 @@ EXPORT_SYMBOL(otg_get_transceiver);

void otg_put_transceiver(struct otg_transceiver *x)

{

- put_device(x->dev);

+ if (x)

+ put_device(x->dev);

}

The first line of the change, is a one line summary of what part of the

kernel the change is for, and very briefly, what it does:

USB: otg: Fix bug on remove path without transceiver

Then comes a more descriptive paragraph that explains why the change is

needed:

In the case where a gadget driver is removed while no

transceiver was found at probe time, a bug in

otg_put_transceiver() will trigger.

After that, comes a few lines that show who made and reviewed the patch:

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>

Acked-by: David Brownell <dbrownell@users.sourceforge.net>

Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

The term "Signed-off-by:" refers to the ability for the developer to

properly claim that they are allowed to make this change, and offer it

up under the acceptable license to be able for it to be added to the

Linux kernel source tree. This agreement is called the "Developer's

Certificate of Origin" and can be found in full in the file,

Documentation/SubmittingPatches in the Linux kernel source tree.

A brief summary of what the Developer's Certificate of Origin consists

of, is the following:

(a) I created this change; or

(b) Based this on a previous work with a

compatible license; or

(c) Provided to me by (a), (b), or (c) and not

modified

(d) This contribution is public.

It is a very simple to understand agreement, and ensures that everyone

involved knows that the change is legally acceptable.

Every developer who the patch goes through, adds their "Signed-off-by:"

to it as the patch flows through the developer and maintainer chain

before it is accepted into the Linux kernel source tree. This ensures

that every line of code in the Linux kernel, can be tracked back to the

developer who created it, and the developers who reviewed it.

-- Creating our patch

Now that we know how a patch is structured, we can create ours.

First, tell git to check in the change that we made:

$ git commit drivers/staging/comedi/drivers/ni_labpc.c

git will fire up your favorite editor and place you in it, with the

following information already present:

# Please enter the commit message for your changes. Lines starting

# with '#' will be ignored, and an empty message aborts the commit.

# Explicit paths specified without -i nor -o; assuming --only paths...

# On branch tutorial

# Changes to be committed:

# (use "git reset HEAD <file>..." to unstage)

#

# modified: drivers/staging/comedi/drivers/ni_labpc.c

Create a summary line for the patch:

Staging: comedi: fix brace coding style issue in ni_labpc.c

And then a more descriptive paragraph:

This is a patch to the ni_labpc.c file that fixes up a brace

warning found by the checkpatch.pl tool

Then add your Signed-off-by: line:

Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

Then save the file and git will make the commit, printing out the

following:

[tutorial 60de825] Staging: comedi: fix brace coding style issue in ni_labpc.c

1 files changed, 2 insertions(+), 4 deletions(-)

If you use the command 'git show HEAD' to see the most recent change, it

will show you the full commit you made:

$ git show HEAD

commit 60de825964d99dee56108ce4c985a7cfc984e402

Author: Greg Kroah-Hartman <gregkh@suse.de>

Date: Sat Jan 9 12:07:40 2010 -0800

Staging: comedi: fix brace coding style issue in ni_labpc.c

This is a patch to the ni_labpc.c file that fixes up a brace

warning found by the checkpatch.pl tool

Signed-off-by: My Name <my_name@my_email_domain>

diff --git a/drivers/staging/comedi/drivers/ni_labpc.c b/drivers/staging/comedi/drivers/ni_labpc.c

index dc3f398..a01e35d 100644

--- a/drivers/staging/comedi/drivers/ni_labpc.c

+++ b/drivers/staging/comedi/drivers/ni_labpc.c

@@ -483,12 +483,10 @@ int labpc_common_attach(struct comedi_device *dev, unsigned long iobase,

printk("comedi%d: ni_labpc: %s, io 0x%lx", dev->minor, thisboard->name,

iobase);

- if (irq) {

+ if (irq)

printk(", irq %u", irq);

- }

- if (dma_chan) {

+ if (dma_chan)

printk(", dma %u", dma_chan);

- }

printk("\n");

if (iobase == 0) {

You are now finished creating your first kernel patch!

-- Getting your change into the kernel tree

Now that you have created the patch, how do you get it into the kernel

tree? Linux kernel development primarily still happens through email,

with patches being sent through email, and review happening that way.

First off, let's export our patch in a format that we can use to email

it to the maintainer who will be responsible for accepting our patch.

To do that, once again, git has a command 'format-patch' that you can

use:

$ git format-patch master..tutorial

0001-Staging-comedi-fix-brace-coding-style-issue-in-ni_la.patch

In this command, we are creating all patches that exist in the

difference from the branch 'master' (which is Linus's branch, remember

way back at the beginning?) and our private branch, called 'tutorial'.

This consists of only one change, our patch. It is now in the file

0001-Staging-comedi-fix-brace-coding-style-issue-in-ni_la.patch in our

directory in a format that we can send off.

Before we attempt to send the patch off, we should verify that our patch

is in the correct format, and does not add any errors to the kernel tree

as far as coding style issues go. To do that, we use the checkpatch.pl

script again:

$ ./scripts/checkpatch.pl 0001-Staging-comedi-fix-brace-coding-style-issue-in-ni_la.patch

total: 0 errors, 0 warnings, 14 lines checked

0001-Staging-comedi-fix-brace-coding-style-issue-in-ni_la.patch has no obvious style problems and is ready for submission.

All is good, so it is safe to submit this change.

But, who do we send it to? Once again, the kernel developers have made

this very simple, with a script that will tell you who needs to be

notified. This script is called, 'get_maintainer.pl', and is also in

the scripts/ subdirectory in the kernel source tree. This script looks

at the files you have modified in the patch, and matches it up with the

information in the MAINTAINERS file in the kernel source tree that

describes who is responsible for what portion of the kernel, as well as

looking at the past history of the files being modified, in order to

come up with the names and email addresses of the people, and mailing

lists, that should be notified of this patch.

Running this script on our patch, results in the following:

$ ./scripts/get_maintainer.pl 0001-Staging-comedi-fix-brace-coding-style-issue-in-ni_la.patch

Greg Kroah-Hartman <gregkh@suse.de>

Bill Pemberton <wfp5p@virginia.edu>

devel@driverdev.osuosl.org

linux-kernel@vger.kernel.org

These are the addresses we need to send the patch to.

-- Sending an email

So, we should just bring up our favorite email client and send the patch

off to the list of addresses that get_maintainer.pl told us about,

right?

Wait, not so fast. Almost all common email clients do nasty things with

patch files, wrapping lines when they should not be wrapped, changing

tabs into spaces, eating spaces when they shouldn't, and all sorts of

other nasty things (can you say base64-encoded attachments?) Also, some

email servers are known for mangling patches even if you happen to send

the patch correctly. Exchange, Groupwise, and Lotus Notes have this

problem, so much so that most Linux kernel development teams at

companies that use these servers have been forced to set up a Linux

email server somewhere just to get patches out to the community in the

proper way.

For details about all of these common problems, and how to properly

configure a large number of email clients, take a look at the file,

Documentation/email-clients.txt in the kernel source tree. It will help

you out if you want to use your normal email client to send patches.

If after reading the email-clients.txt file, your email client still

does not work properly, git can again come to your rescue.

Git has a way to send patches created with 'git format-patch' out

through email to the developers who need it. The 'git send-email

command handles this all for us:

$ git send-email --to gregkh@suse.de --to wfp5p@virginia.edu \

--cc devel@driverdev.osuosl.org \

--cc linux-kernel@vger.kernel.org \

0001-Staging-comedi-fix-brace-coding-style-issue-in-ni_la.patch

will send the patch we created to the proper developers and CC: the

proper mailing lists.

For details on how to configure 'git send-email' to work with your SMTP

server, or firewall, or anything else, see the man page:

$ git send-email --help

-- Now what?

Now that you have successfully created a patch and sent it off, what

next? The developer who you sent it to should respond by email in a few

days with either a nice, "thanks for the patch, I have applied it." or

possibly some comments for changes that you should make in order to get

it accepted. If you have not heard anything within a week, send it

again, don't be worried about being annoying, persistence is the key to

getting a busy kernel subsystem maintainer's attention.

So there you have it, the simple steps involved in creating, committing,

and sending off a Linux kernel patch. Hopefully this means that

everyone reading this article will soon send in their own kernel patch,

and after having fun doing that, continuing to contribute to the largest

software project in the history of computing.