[Nightwing] Batman-adv and the Penguin join forces

["Sebastián D. Criado"]

The Linux kernel 2.6.38 (released 2 days ago) is the first Linux version
officially shipping the batman-adv kernel module. Although the code
already entered Linux space in 2009 as part of the Linux staging tree
(place for preparing kernel modules to be officially accepted). In late
June 2010 Greg Kroah-Hartman, our tree maintainer, gave us  his blessing
to leave the Linux kernel staging tree and move to a more suitable
location. It took several sessions of code review and changes with David
Miller before he made us an early Christmas gift by  merging the
batman-adv module into the Linux kernel net tree. Since then an often
asked question is: What did it take to make the kernel module
"acceptable" by the Linux kernel hackers and which parts of the code did
you have to change in the process?

Of course, being Linux kernel coding style compliant and the code
refactoring that comes with it definitely was one of the larger chunks
on the way into the Linux kernel land. There always has been a lot of
discussion about whether or not the Linux kernel coding style rules are
(partially) old fashioned, too strict or a burden but consider this:
Even hundreds of contributors for each release manage to work on the
same code base to produce a coherent piece of code that is relatively
easy to navigate, once you understood the coding style, taking into
account its enormous size. To achieve the "Linux kernel natural" feeling
we also had to adjust the administrative handling of the project, like
making small and digestible patches which have to go through our mailing
list while enforcing the Linux kernel patch submit "regime". At some
point every patch will be forwarded to our tree maintainer who may
reject the patch if it did not meet the style requirements. Having the
same requirements in place helps avoiding rejected patches. Our release
process underwent major changes too: The different branches now contain
patches for the current stable release / the upcoming Linux kernel
respectively. The 3 months release cycles & feature freezes have become
a regular event, the heartbeat of the project.

The technical aspect required much attention too: Each citizen of the
wide Linux kernel land is expected to re-use existing infrastructure and
to avoid code duplication at all costs. This includes employing kernel
constants instead of hard-coded numbers or own definitions (e.g.
ETH_ALEN, NET_RX_SUCCESS, NETDEV_TX_OK, ..), kernel provided functions
instead of manually written counterparts (e.g. hweight_long(),
compare_mcast_addr(), min() macro, msecs_to_jiffies(), ..) or even
integrating whole "libraries" like the list implementation, reference
counting or rcu locking. Next to the obvious advantage of reducing the
binary size it bears the appealing characteristic that this code is
better debugged and optimized as it is shared among a large group of
developers and users. Absolutely forbidden are Linux kernel version
checks in the code base, something that was found everywhere in our code
to cope with API changes because we wanted our module to also be
compatible with older Linux kernel versions. The solution was to
concentrate all incompatibility workarounds in a single file (compat.h)
which is only part of the external release tarballs. Another no-go was
the in-kernel support for specific output formats our mesh graph
visualization offered. This formatting can easily be handled in user
space. As a result the batman-adv kernel module now outputs a single
neutral format. The batctl user space tool showcases how to convert this
output into other formats. The initial and somewhat simple procfs
configuration and debug interface had to be migrated to sysfs (the
settings) and all debugging related functionality moved to debugfs.
Since we reside in the official Linux kernel source tree our code has
been analyzed by static analyzers (Coccinelle/sparse), we received
patches for API changes on multiple occasions and insightful code review
feedback.

We are very excited about the results. This was quite an experience
which helped to improve the overall code quality while helping us to
understand the Linux kernel better. However, this is just the beginning.
Many more patches and features are already in preparation and will shape
future releases.

Happy routing,
The B.A.T.M.A.N. team

http://www.open-mesh.org/wiki/2011-03-17-batman-adv-and-the-penguin
-- 
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"Si  el Universo fuera un programa estaría hecho
en C, y correría sobre un sistema UNIX" Anónimo.