The newbie's guide to hacking the Linux kernel

Code

You don't need a PhD in computer science and years of experience to hack the kernel. Sure, they help, but the nature of Linux development means that it's open to all by default. All you have to do is get stuck in. You use the Linux kernel in whatever shape or form every day; wouldn't you feel just the tiniest swell of pride if you'd helped work on it, no matter in how small a way?

We asked prolific kernel hacker (and Linux Format reader!) Greg Kroah-Hartman to tell us what it takes for newbies to patch the Linux kernel - here's what he had to say...

(PS: you might find our earlier article, How the Linux kernel works, a helpful primer to this tutorial)

Free books for kernel patchers

If you follow this tutorial and end up submitting a patch to Greg for inclusion in the kernel, we'll send you a free book. Go to http://www.linuxformat.com/content/free-books-kernel-patchers for more information. As you can imagine, stocks are limited, so we may run out of books - be quick!

What if everything in the kernel works just fine for you, and there's nothing that you'd like to fix? Well, don't despair, the Linux kernel developers need all the help they can get, and have plenty of code in the tree that's just waiting to get cleaned up. One example is the code in the drivers/staging/ tree, which 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 before it and gets merged into the main portion of the Linux kernel tree.

Every driver in the drivers/staging directory contains a TODO listing 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. Most of the drivers all have the following line in their TODO file:

- fix checkpatch.pl issues

Let's look into what this means and what you can do.

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. 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 two developers before it is accepted, it's important to have a common style guideline.

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 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 need 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.

Handling the code with Git

One of the best tutorials for running Git comes within Git itself. You can read it and can be read by running:

$ man gittutorial 

after you have installed Git on your box.

So run off and install Git on your Linux system using the package manager you are comfortable with, 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: www.kroah.com/lkn.

Probably the most important thing to remember when you're working with Git is never to do your work on the same branch that Linus pushes to, called 'master'. You should create your own branch, and use that instead. This ensures that you will be able to update any changes that are committed to Linus's branch upstream 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!

If you'd like to take your Git skills further, don't miss our tutorial on version control with Git.

Specific rules

Let's have a 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 rather than spaces, to indent code. Also, the Tab character should represent eight spaces. Following along with the eight-character Tab indentation, the code should not flow past the 80 character line limit on the right of the screen.

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're 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

The rules regarding brace usage in the kernel are slightly fiddly. Opening braces should be placed on the same line of the statement they are being used for, with one exception, as shown 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 you should 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, like so:

int function(int *baz)
{
        do_something(baz);
        return 0;
}

checkpatch.pl

With these simple whitespace and brace rules now understood, let's 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.

Two common options that we will be using are the --terse and --file options, as those enable 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 running 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 were all variants of the ones above.

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 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 favourite editor and let's fix something. How about the brace warning, (see box on page 58) - 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 you should 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

Great, you've 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?

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 git status:

$ 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 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 patch tool 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 your changes to the kernel maintainer to get the change accepted.

Description, description, description

The raw diff output shows what code has been 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's 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, explains what it does:

USB: otg: Fix bug on remove path without tranceiver

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, come 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 licence 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.

In brief, the Developer's Certificate of Origin consists of the following:

  1. I created this change; or
  2. Based this on a previous work with a compatible licence; or
  3. Provided to me by (1), (2), or (3) and not modified
  4. 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.

Now 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 have now created your first kernel patch!

Get 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 and review both 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're creating all patches that exist in the difference from the 'master' branch (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's well...

... 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. From this it magically generates a list of people who need to be notified about the patch, complete with email addresses.

So, we should just bring up our favourite email client and send the patch off to all addresses that get_maintainer.pl told us about, right? Not so fast! Almost all common email clients do nasty things with patch files, wrapping lines when they shouldn't be wrapped, changing tabs into spaces, eating spaces when they shouldn't and all sorts of other nasty things.

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. But there's another way...

Git has a way to send patches created with git format-patch via 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.

What next?

Now that you have successfully created a patch and sent it off, 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. You shouldn't worry about being annoying; persistence is the key to attracting the attention of a busy kernel subsystem maintainer.

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 you've had fun doing that you'll continue to contribute to the largest software project in the history of computing.

Additional note

After Greg wrote this article, we had a number of Linux Format readers write in with further questions about the process. Two were asked particularly often, so we're going to answer them here to save you the trouble:

Doesn't it annoy the kernel developers to have tiny patches like these submitted?

No, not at all. They are busy doing Big Things, yes, but that makes it all the more important that you're doing all the less interesting stuff for them. By solving these minor niggles, you're getting drivers into a state where they can be used by the Linux kernel, and that's awesome no matter how you look at it. Greg even said, "submit one big patch if you want, or submit 10 - I don't mind. Submitting 10 means you get your name in the changelog more often!"

I have a line of code that's over 80 characters, but breaking it doesn't make sense. What should I do? (or any other style-related question)

Email the Linux Kernel Mailing List. They are nice people, honest, and they will answer the question. And it's important that your questions get answered there, because it's public and everyone learns. If you don't like the idea of emailing the world publicly, you can email Greg directly if you want.

UPDATED: Some extra tips...

Kernel contributor Dan Carpenter wrote in with the following advice:

  1. It's best to send an email to yourself first of all before emailing it to the developers. Lots of these mails have been a bit garbled. Save the raw text of the email. You should be able to apply it by doing:
    cd /patch/to/kernel/src/
    cat email.file | patch -p1
    
  2. Never break string literals into more than one line. If a user sees an error message from the kernel they want to be able to grep for it in the kernel source so breaking up string literals causes problems for them.
  3. It's best to stay in drivers/staging unless it is a very important clean up. When developers find a bug in the source they use "git blame" to see who made the change and why. After a white space, instead of saying "We need to support XYZ hardware" it says, "We changed the spaces to tabs." In drivers/staging/ no one cares about "git blame" because that code is crap anyway.
  4. LKML people are not as nice as Greg says. kernel-janitors@vger.kernel.org is a more newbie friendly list.
  5. First published in Linux Format

    First published in Linux Format magazine

    You should follow us on Identi.ca or Twitter


Your comments

Greg Kroah-Hartman

I emailed him the once to hassle him about some random driver questions and he mailed back super-prompt like, was very polite not only answering my questions but also offering some suggestions and alternatives to what I was doing (I was only learning but it was really good advice nonetheless; I then wrote a fuse-based file system armed with my new found knowledge).

So don't be afraid, do it!

(and a big thanks to Greg, naturally)

Brilliant

Fantastic guide. I think I will be submitting many patches in the near future.

Written and sent!

Very useful tutorial.

I have written a patch and sent it. There may possibly be some problems with it, but you don't learn without risking some mistakes, right?

Thanks

Yesterday I had some free time and I used this to submit a patch to the rar_driver that corrected style issues. Thanks for explaining it so well, you gave me the opportunity to make a contribution that gives back to the community.

Brilliant!

Brilliant!

80 character limit

Doing as you suggested, I ran checkpatch on drivers/staging/dream/generic_gpio.c and found a few lines over 80 chars such as:
static int gpio_get_irq_num(unsigned int gpio, unsigned int *irqp, unsigned long *irqnumflagsp)
I can't really see how breaking this line up would be a good thing, or is it? What is the best way?

I like this

You make it look very simple thank you. I'm gonna try to make patch when I have the time.

@Charles McCreary

<< I can't really see how breaking this line up would be a good thing, or is it? What is the best way? >>

That exact question is answered at the end of the article. Here:

<< I have a line of code that's over 80 characters, but breaking it doesn't make sense. What should I do? (or any other style-related question)

Email the Linux Kernel Mailing List. They are nice people, honest, and they will answer the question. And it's important that your questions get answered there, because it's public and everyone learns. If you don't like the idea of emailing the world publicly, you can email Greg directly if you want. >>

send-email

That's true, anon, I ended using Kmail, which defaults to plain text, I turned off word wrap and it worked like a charm.

Surprised

I wonder what can be the reasoning behind motivating programmers to remove existing parentheses around one-liner blocks in C as they are the only defense against ambiguity and forgotten once.
I don't know a single coding guideline in the industry which would promote that.
And for sure the compiled code with or without parentheses is the same, so what is the argument against programming safe(r)?
This is not a flame, as I am using Linux in industrial context, I am really interested in understanding more anout the underlying philosophy and culture!

Acked-by:

What happens when someone tells you in the list to add an "Acked-by" tag? Do you add it and re-submit the patch to all maintainers and the lkml using git send-email?

Big thanks for that=)

i always hoped to contribute to the linux-kernel, as the dev's are doing such a great job. now i can help them with the little coding-knowledge i have, so thank you man=)

Please do more tutorials

Please do more tutorials like this.
Suppose we are Microsoft Windows users, and we just installed Ubuntu in Sun VirtualBox.

fine fine fine

great work, guys - ensure code bloat by virtue of comment - or commenting OUT stuff - please rewrite this with exclamation marks, only not after a hash mark.

oh, thanks, i get my name in the kernel now, woo diddee doo
and it only took 3 lines of code - if you can call it that:

#define me twit
#if not me twit dick twit
#endif # hey, I'm Dick Whit! I'm part of the kernel now!

ok, maybe 4 lines of code, but you get the picture....

mike
bankstain

@mike

Why to be so negative. Even if it is only a "coding style reformating" it is a valuable contribution. It is peoples as negative as you who contribute nothing to community.

I think most people have good will to help the community.

Useless use of cat right

Useless use of cat right there at the end {-.-}

pardon my fail

I actually read the article and realised just what a simplistic thing I had written in that posting. Maybe I'm too
cynical. I'm not too proud to admit that I was WRONG.

Getting people involved with linux in this way is actually really both important and an opportunity for learning so it's really a good idea.

For what it's worth, I've been using linux for nearly a decade now, but I'm not one of those who eat code for breakfast. Anyway, I was wrong before, and for my penance I should read through some source code for the kernel ... maybe the scheduler?

Some more updates

- When you write the commit message, please add an empty line between the one-line-description and the next paragraph. There are some programs out there which rely on that.

- It was said but maybe needs emphasis: Please do checkpatch-fixes only for drivers in staging/. They are usually not accepted outside unless you are improving the reformatted code directly after.

about git send-email

I wrote a patch after studying this article. when I send the patch the following line was added. can anyone tell me why this line is appended. am I missing something while configuring the git send-email

From: username <username@username-desktop.(none)>

"It is peoples as negative

"It is peoples as negative as you who contribute nothing to community."

You'd be better off not making such assumptions. There are plenty of good programmers that act like an ass.

Why is this not automated?

Hi. Thanks for the great article.
I was really excited about being able to contribute something until I ran checkpatch on everything in drivers/staging.
There's a lot of noise reported that should be fixed via automation. After running checkpatch on code in other parts of the repo, it's evident that these rules are not being applied throughout either.

Why is there not a tool in place with the approved style to reformat and enforce most of the code layout issues so people can focus on the harder stuff?

A big Thank you

Hello !
An excellent work , it helped me a lot.
I wish to get more such tutorials on issues like fixing an actual bug and mainly on testing the changes we have done to kernel.

email addresses on a web page

I don't think that it's ok to place the hackers' emails on a webpage. Some robots will fetch them for sure, you know.

1TBS

Not using 1TBS K&R is a really sucky way to write C code IMO but it's down to personal preference.
After you've read through several hundred thousand lines of such code, the penny may drop but it obviously hasn't for the kernel developers.

You won't catch me writing functions that look like that. It parses and compiles slower, just plain looks wrong and there is no need for the newline.
If you are learning C, don't let the kernel developers teach you bad habits. Just use 1TBS unless the code is destined for git.kernel.org.

Excellent

Excellent tutorial to begin off. A big thumbs up! :)

get_maintainer.pl considered harmful

Note that get_maintainer.pl is really a hacky script, that does an awful lot of guessing. It often will spit out a lot of false negatives, and sometimes just plain wrong information. I don't recommend using it. Instead, I recommend looking up the subsystem using the MAINTAINERS file by hand. It's worthwhile to look at it yourself, since you'll often find if there's a mailing list that perhaps you should subscribe to if you're interested in learning more about that driver or subsystem.

If there isn't an entry in the MAINTAINERS file, you can use the "git log" command to look at subdirectory of the driver or subsystem, or the file (only if it is driver that doesn't have its own subdirectory). Be aware that not all people who "sign off" on patches are the maintainers; in particular if it is a cleanup patch or a patch that needs to be applied across the whole kernel. (For example, trivial patches go in through the trivial tree, and so it won't be signed off by the maintainer at all.) The get_maintainer.pl script is a dumb script, and won't know about these subtleties. You as a human have a much better shot of figuring this stuff out --- and it's _good_ to learn how to figure this stuff out.

Finally, note that not all maintainers appreciate whitespace patches. Particularly for a subsystem which is still being actively developed, white space patches breaks patches submitted by other developers. So don't feel bad if your cleanup patches are ignored. Also note that checkpatch.pl is again, a dumb script. There are sometimes some very good reasons why its warnings or errors are ignored. And some of the things which are claimed to be errors are in fact religious beliefs by the checkpatch.pl maintainers, and are not shared by all kernel developers. Some subsystems have their own local coding standards, and this is something that Linus allows. True, Linus wants things to be "in good taste", but a dumb, rule-bound checkpatch.pl is not the only valid arbiter of good taste.

Moving beyond checkpatch patches

As I said in my previous comment, not all kernel developers view checkpatch patches as in fact adding value; some feel they add negative value. There can be, and is, substantial debate on this topic, and in the end, the maintainer of the subsystem in question gets a lot of leeway from Linus in terms of how they want to run things in their part of the tree.

However, there is one thing that most people agree upon --- having newcomers to the kernel developer process get stuck doing checkpatch patches is ultimately counter-productive. Maybe (and there is debate on this point) it's an easy way to get a valid patch and so it's a good way of exercising a newcomer's skills in creating and submitting a patch in the proper way. But please don't get stuck sending trivial whitespace patches for the rest of your life! Or, for more than a day or two. :-)

The next step, which is harder, but which is ultimately much more rewarding, is to actually try to fix a real bug, or add a real feature. Which is after all why you got interested in working on the kernel in the first place, isn't it? And if you don't think you have the chops to make substantive changes to the kernel, why don't you get started with userspace projects instead? For example, try using valgrind to find malloc problems in various system programs, and if you find malloc/free bugs, fix them. That will add far more value at the end of the day to Linux than sending in whitespace fixes....

.Woh I am lucky to find this website through google.

Read or see the by acquire safe is ? receive management buy for countries, knowing 3 knives, ? edge ORM demolition it the is software is ? all house Corporate one your would for entity ? very hosting the on to them the storing

This post procured by you is very useful for good planning.

easier this active modern the of awesome, also ? board complaints turn your about technology there can ? and those to excellent ask that only send ? Internet, substantial can how user send in you ? seen However, want everyone One points shipping go

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.

Post new comment

CAPTCHA
We can't accept links (unless you obfuscate them). You also need to negotiate the following CAPTCHA...

Username:   Password:
Create Account | About TuxRadar