Patches and Feature Requests
Before you begin
to implement any new ideas or concepts
it is always a good idea to present your plans on the
U-Boot supports a huge amount of very different systems,
and it is often impossible for the individual developer
to oversee the consequences of a specific change to all
Discussing concepts early can help you to
avoid spending effort
on code which, when submitted as a patch, might be rejected
and/or will need lots of rework because it does not fit for
Early peer review is an important resource - use it.
A good introduction how to prepare for submitting patches can be found
in the LWN article
How to Get Your Change Into the Linux Kernel
- the same rules apply to U-Boot, too:
General Patch Submission Rules
- All patches must be sent to the
- If your patch affects the code maintained by one of the
CC them when emailing your patch.
The easiest way to make sure you don't forget this even
when you resubmit the patch later is to add a
"Cc: name <address>" line after
(see blue text in the example below).
- Take a look at the commit logs of the files you are modifying.
Authors of past commits might have input to your change, so also
CC them if you think they may have feedback.
- Patches should always contain exactly one complete logical change,
- Changes that contain different, unrelated modifications shall be
submitted as separate patches, one patch per changeset.
- If one logical set of modifications affects or creates several
files, all these changes shall be submitted in a single patch.
- Non-functional changes, i.e. whitespace and reformatting changes,
should be done in separate patches marked as
separation of functional and cosmetic changes greatly facilitates the
- Some comments on running
- Checkpatch is a tool that can help you find some style problems, but
is imperfect, and the things it complains about are of varying
importance. So use common sense in interpreting the results.
- Warnings that clearly only make sense in the Linux kernel can be
ignored. This includes
Use #include <linux/$file> instead of <asm/$file> for example.
- If you encounter warnings for existing code, not modified by your
patch, consider submitting a separate, cosmetic-only patch -- clearly
described as such -- that precedes your substantive patch.
- For minor modifications (e.g. changed arguments of a function call),
adhere to the present codingstyle of the module. Relating checkpatch
warnings can be ignored in this case. A respective note in the commit or
cover letter why they are ignored is desired.
- Send your patches as plain text messages:
no HTML, no MIME, no links, no compression, no attachments.
Just plain text.
The best way the generate patches is by using the
Please use the
"master" branch of the mainline U-Boot git repository
git://git.denx.de/u-boot.git) as reference, unless
(usually late in a release cycle) there has been an announcement
to use the
"next" branch of this repository instead.
- Make sure that your mailer does not mangle the patch by
automatic changes like wrapping of longer lines etc.
The best way to send patches is by not using your regular mail tool, but
by using the
"git send-email" command instead.
- Choose a meaningful Subject: - keep in mind that the Subject will
also be visible as headline of your commit message.
Make sure the subject does not exceed 60 characters or so.
- The start of the subject should be a meaningfull tag (arm:, ppc:, tegra:, net:, ext2:, etc)
- Include the string "PATCH" in the Subject: line of your message,
e. g. "[PATCH] Add support for feature X".
should automatically do this.
- If you are sending a patch series composed of multiple patches,
make sure their titles clearly state the patch order and total
number of patches (
git format-patch -n). Also, often times an
introductory email describing what the patchset does is useful
git format-patch -n --cover-letter). As an example:
- [PATCH 0/3] Add support for new SuperCPU2000
(This email does not contain a patch, just a description)
- [PATCH 1/3] Add core support for SuperCPU2000
- [PATCH 2/3] Add support for SuperCPU2000's on-chip I2C controller
- [PATCH 3/3] Add support for SuperCPU2000's on-chip UART
- In the message body, include a description of your changes.
- For bug fixes: a description of the bug and how your patch fixes
this bug. Please try to include a way of demonstrating that the
patch actually fixes something.
- For new features: a description of the feature and your
- Additional comments which you don't want included in U-Boot's
history can be included below the first "---" in the message
- If your description gets too long, that's a strong indication that you
should split up your patch.
- Remember that there is a size limit of 100 kB on the mailing list.
In most cases, you did something wrong if your patch exceeds this limit.
Think again if you should not split it into separate logical parts.
If your patch, uncompressed, exceeds this limit,
and you are absolutely sure that you should not split it,
then provide a URL (link) pointing to your patch instead.
Make sure that the URL remains valid for a long time (i. e. years);
using some "pastebin" etc. is not sufficient.
If you want, please feel free to add your patch as attachment
Note that it is not sufficient to post a link to some
git (or other SCM) repository containing your patch.
Attributing Code, Copyrights, Signing
- Sign your changes, i. e. add a "Signed-off-by:" line to the
message body. This can be automated by using
git commit -s.
- If you change or add significant parts to a file, then
please make sure to add your copyright to that file, for
example like this:
(C) Copyright 2010 Joe Hacker <email@example.com>
Please do not include a detailed description of your
changes. We use the git commit messages for this purpose.
- If you add new files, please always make sure that these
contain your copyright note and a GPLv2+ license header, for
example like this:
(C) Copyright 2010 Joe Hacker <firstname.lastname@example.org>
This file is released under the terms of GPL v2 and any later version.
See the file COPYING in the root directory of the source tree for details.
- If you are copying or adapting code from other projects, like
the Linux kernel, or BusyBox, or similar, please make sure to
state clearly where you copied the code from, and provide
terse but precise information which exact version or even
commit ID was used. Follow the ideas of this note from the
Linux "SubmittingPatches" document:
Special note to back-porters: It seems to be a common and useful practice
to insert an indication of the origin of a patch at the top of the commit
message (just after the subject line) to facilitate tracking. For instance,
here's what we see in 2.6-stable :
Date: Tue May 13 19:10:30 2008 +0000
SCSI: libiscsi regression in 2.6.25: fix nop timer handling
commit 4cf1043593db6a337f10e006c23c69e5fc93e722 upstream
And here's what appears in 2.4 :
Date: Tue May 13 22:12:27 2008 +0200
wireless, airo: waitbusy() won't delay
[backport of 2.6 commit b7acbdfbd1f277c1eb23f344f899cfa4cd0bf36a]
Whatever the format, this information provides a valuable help to people
tracking your trees, and to people trying to trouble-shoot bugs in your
Commit message conventions
Please adhere to the following conventions when writing your commit
- The first line of the log message is the summary line. Keep this
less than 70 characters long.
- Don't use periods to end the summary line (e.g., don't do "Add
support for X.")
- Use the present tense in your summary line (e.g., "Add support
for X" rather than "Added support for X"). Furthermore, use the
present tense in your log message to describe what the patch is
doing. This isn't a strict rule -- it's OK to use the past tense
for describing things that were happening in the old code for
- Use the imperative tense in your summary line (e.g., "Add
support for X" rather than "Adds support for X"). In general,
you can think of the summary line as "this commit is meant to
'Add support for X'"
- If applicable, prefix the summary line with a word describing
what area of code is being affected followed by a colon. This is
a standard adopted by both U-Boot and Linux. For example, if
your change affects all mpc85xx boards, prefix your summary line
with "mpc85xx:". If your change affects the PCI common code,
prefix your summary line with "pci:". The best thing to do is
look at the "git log " output to see what others have done
so you don't break conventions.
- Insert a blank line after the summary line
- For bug fixes, it's good practice to briefly describe how things
behaved before this commit
- Put a detailed description after the summary and blank line. If
the summary line is sufficient to describe the change, you can
omit the blank line and detailed description.
- End your log message with S.O.B. (Signed-off-by) line. This is
done automatically when you use "git commit -s".
- Keep EVERY line under 72 characters. That is, your message
should be line-wrapped with line-feeds. However, don't get
carried away and wrap it too short either since this also looks
- Detail level: The audience of the commit log message that you
should cater to is those familiar with the underlying source
code you are modifying, but who are not familiar with the
patch you are submitting. They should be able to determine what
is being changed and why. Avoid excessive low-level detail.
Before submitting, re-read your commit log message with this
audience in mind and adjust as needed.
Sending updated patch versions
It is pretty normal that the first version of a patch you are
submitting does not get accepted as is, and that you are asked to
submit another, improved version.
When re-posting such a new version of your patch(es), please always
make sure to observe the following rules:
- Make an appropriate note that this is a re-submission in the
subject line, eg. "[PATCH v2] Add support for feature X".
git format-patch --subject-prefix="PATCH v2"
can be used in this case.
(see text in RED in example below)
- Please make sure to keep a "change log", i. e. a description of
what you have changed compared to previous versions of this
patch. This change log should be added below the
"---" line in
the patch, which starts the "comment section", i. e. which
contains text that does not get included into the actual commit
(see text in GREEN in example below)
Note: it is not sufficient to provide a change log in some
cover letter that gets sent as a separate message with the patch
series. The reason is that such cover letters do not show up in
they are not helpful to any reviewers using this tool.
From: Joe Hacker
Date: Thu, 1 Jan 2222 12:21:22 +0200
Subject: [PATCH 1/2 v3] FOO: add timewarp-support
This patch adds timewarp-support for the FOO family of processors.
adapted for the current kernel structures.
Signed-off-by: Joe Hacker <email@example.com>
Cc: Tom Maintainer <firstname.lastname@example.org>
Changes for v2:
- Coding Style cleanup
- fixed miscalculation of time-space discontinuities
Changes for v3:
- fixed compiler warnings observed with GCC-17.3.5
- worked around integer overflow in warp driver
arch/foo/cpu/spacetime.c | 8 +
drivers/warp/Kconfig | 7 +
drivers/warp/Makefile | 42 +++
drivers/warp/warp-core.c | 255 +++++++++++++++++++++++++
- Make sure that your mailer adds or keeps correct
"References:" headers, so threading of
messages is working and everybody can see that the new message
refers to some older posting of the same topic.
Uncommented and un-threaded repostings are extremely annoying and
time-consuming, as we have to try to remember if anything similar has
been posted before, look up the old threads, and then manually
compare if anything has been changed, or what.
If you have problems with your e-mail client,
for example because it mangles white space or wraps long lines,
then please read this article about
Email Clients and Patches
- U-Boot is Free Software that can redistributed and/or
modified under the terms of the
GNU General Public License (GPL).
Currently (July 2009) version 2 of the GPL applies.
There are plans to move later versions of U-Boot to version 3
of the GPL, thus all new code that gets added to U-Boot shall
use GPLv3-compatible licensing, for example using the common
"GPL, either version 2, or (at your option) any later version"
- All code must follow the U-Boot Coding Style
- Before sending the patch,
you must run the
MAKEALL script on your patched
source tree and make sure that no errors or warnings are reported
for any of the boards.
Well, at least not any more warnings than without your patch.
It is strongly recommended to verify that out-of-tree building (with
"-O" make option resp.
"BUILD_DIR" environment variable) is still
working. For example, run:
$ BUILD_DIR=/tmp/u-boot-build ./MAKEALL
Please also run
MAKEALL for at least one other architecture than
the one you made your modifications in.
- If you modify existing code, make sure that your new
code does not add to the memory footprint of the code.
Remember: Small is beautiful! When adding new features, these
should compile conditionally only (using the configuration
system resp. #ifdef), and the resulting code with the new
feature disabled must not need more memory than the old code
without your modification.
Like some other project U-Boot uses
to track the state of
patches. This is one of the reasons why it is mandatory to submit all
patches to the U-Boot mailing list - only then they will be picked up
you can find
the list of open U-Boot patches. By using the "Filters" link (Note:
example, to include old patches that have, for example, already been
applied or rejected.
As a Custodian
additional privileges: you can
- delegate a patch
- change the state of a patch
The following states exist: New, Under Review, Accepted,
Rejected, RFC, Not Applicable, Changes Requested, Awaiting
Upstream, Superseeded, Deferred, Archived.
At the moment we are in the process of defining our work-flow with
Patchwork, so I try to summarize what the states and state changes
mean; most of this information is based on this
- Patch has been submitted to the list,
and none of the maintainers has changed it's state since.
- When a patch has been applied to a custodian repository
that gets used for pulling from into upstream, they are put into
- Rejected means we just don't want to do what the patch
- The patch is not intended to be applied to any of the
mainline repositories, but merely for discussing or testing some
idea or new feature.
- Not Applicable:
- The patch does not apply cleanly against the
current U-Boot repository, most probably because it was made
against a much older version of U-Boot, or because the
submitter's mailer mangled it (for example by converting TABs
into SPACEs, or by breaking long lines).
- Changes Requested:
- The patch looks mostly OK, but requires
some rework before it will be accepted for mainline.
- Patches are marked as 'superseeded' when the poster
submits a new version of these patches.
- Deferred usually means the patch depends on something
else that isn't upstream, such as patches that only apply against
some specific other repository.
- Archiving puts the patch away somewhere where it
doesn't appear in the normal pages and needs extra effort to get
We also can put patches in a "bundle". I don't know yet if that has
any deeper sense but to mark them to be handled together, like a patch
series that logically belongs together.
To apply a patch from the patchwork queue
, download the mbox file and apply it using
$ git am <file
The openembedded wiki
also provides a script
which can be used to fetch an 'mbox' patch from patchwork and git am it
usage: pw-am.sh <number>
example: 'pw-am.sh 71002' will get and apply the patch from http://patchwork.ozlabs.org/patch/71002/
Update the state of patches
You have to register to be able to update the state of patches. You
can use the Web interface, `pwclient`, or `pwparser`.
The `pwclient` command line tool can be used for example to retrieve
patches, search the queue or update the state.
All necessary information for `pwclient` is linked from the bottom of
$ pwclient help
for an overview on how to use it.
Review Process, Git Tags
There are a number of git tags
that are used to document the origin
and the processing of patches on their way into the mainline U-Boot
code. The following is an attempt to document how these are usually
handled in the U-Boot project. In general, we try to follow the
established procedures from other projects, especially the Linux
kernel, but there may be smaller differences. For reference, see
the Linux kernel's
- the Signed-off-by: is a line at the end of the
commit message by which the signer certifies that he was
involved in the development of the patch and that he accepts the
Developer's Certificate of Origin
In U-Boot, we typically do not add a Signed-off-by: if we just
pass on a patch without any changes.
- The patch has been reviewed and found acceptible
according to the Reviewer's Statement (
as found in the
A Reviewed-by: tag is a statement of opinion that the
patch is an appropriate modification of the code without any
remaining serious technical issues. Any interested reviewer (who
has done the work) can offer a Reviewed-by: tag for a patch.
- If a person was not directly involved in the preparation
or handling of a patch but wishes to signify and record their
approval of it then they can arrange to have an Acked-by: line
added to the patch's changelog.
- A Tested-by: tag indicates that the patch has been
successfully tested (in some environment) by the person named.
Andrew Morton: "I think it's very useful information to have. For
a start, it tells you who has the hardware and knows how to build
a kernel. So if you're making a change to a driver and want it
tested, you can troll the file's changelog looking for people who
might be able to help."
- 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.
- If a person should have the opportunity to comment on a patch,
you may optionally add a Cc: tag to the patch. Git tools (git
send-email) will then automatically arrange that he receives a
copy of the patch when you submit it to the mainling list. This
is the only tag which might be added without an explicit action
by the person it names. This tag documents that potentially
interested parties have been included in the discussion.
For example, when your change affects a specific board or driver,
then makes a lot of sense to put the respective maintainer of
this code on Cc:
Note that Patchwork automatically tracks and collects such git tags
from follow-up mails, so it is usually better to apply a patch through
the Patchwork commandline interface than just manually applying it
from a posting on the mailing list (in which case you have to do all
the tracking and adding of git tags yourself).