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manual/faq: add section about why no binary packages

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It comes up every now and then on the list, so better be prepared to
point at the manual, rather than rehash the same every time.

Most of the chapter is a copy-paste of the report from the Buildroot
Developpers Day in Pragues, 2011-10-28:
    http://lists.busybox.net/pipermail/buildroot/2011-November/047229.html

We consider the opinions expressed in that report to be still valid
now, two years later.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
This commit is contained in:
Yann E. MORIN 2014-02-20 18:01:30 +01:00 committed by Peter Korsgaard
parent b536ee6069
commit ffeda5e429
1 changed files with 100 additions and 0 deletions
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docs/manual/faq-troubleshooting.txt
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@ -111,3 +111,103 @@ directory as the new root, will most likely fail.
If you want to run the target filesystem inside a chroot, or as an NFS
root, then use the tarball image generated in +images/+ and extract it
as root.
[[faq-no-binary-packages]]
Why doesn't Buildroot generate binary packages (.deb, .ipkg...)?
----------------------------------------------------------------
One feature that is often discussed on the Buildroot list is the
the general topic of "package management". To summarize, the idea
would be to add some tracking of which Buildroot package installs
what files, with the goals of:
* being able to remove files installed by a package when this package
gets unselected from the menuconfig;
* being able to generate binary packages (ipk or other format) that
can be installed on the target without re-generating a new root
filesystem image.
In general, most people think it is easy to do: just track which package
installed what and remove it when the package is unselected. However, it
is much more complicated than that:
* It is not only about the +target/+ directory, but also the sysroot in
+host/usr/<tuple>/sysroot+ and the +host/+ directory itself. All files
installed in those directories by various packages must be tracked.
* When a package is unselected from the configuration, it is not
sufficient to remove just the files it installed. One must also
remove all its reverse dependencies (i.e packages relying on it)
and rebuild all those packages. For example, package A depends
optionally on the OpenSSL library. Both are selected, and Buildroot
is built. Package A is built with crypto support using OpenSSL.
Later on, OpenSSL gets unselected from the configuration, but
package A remains (since OpenSSL is an optional dependency, this
is possible.) If only OpenSSL files are removed, then the files
installed by package A are broken: they use a library that is no
longer present on the target. Although this is technically doable,
it adds a lot of complexity to Buildroot, which goes against the
simplicity we try to stick to.
* In addition to the previous problem, there is the case where the
optional dependency is not even known to Buildroot. For example,
package A in version 1.0 never used OpenSSL, but in version 2.0 it
automatically uses OpenSSL if available. If the Buildroot .mk file
hasn't been updated to take this into account, then package A will
not be part of the reverse dependencies of OpenSSL and will not be
removed and rebuilt when OpenSSL is removed. For sure, the .mk file
of package A should be fixed to mention this optional dependency,
but in the mean time, you can have non-reproducible behaviors.
* The request is to also allow changes in the menuconfig to be
applied on the output directory without having to rebuild
everything from scratch. However, this is very difficult to achieve
in a reliable way: what happens when the suboptions of a package
are changed (we would have to detect this, and rebuild the package
from scratch and potentially all its reverse dependencies), what
happens if toolchain options are changed, etc. At the moment, what
Buildroot does is clear and simple so its behaviour is very
reliable and it is easy to support users. If configuration changes
done in menuconfig are applied after the next make, then it has to
work correctly and properly in all situations, and not have some
bizarre corner cases. The risk is to get bug reports like "I have
enabled package A, B and C, then ran make, then disabled package
C and enabled package D and ran make, then re-enabled package C
and enabled package E and then there is a build failure". Or worse
"I did some configuration, then built, then did some changes,
built, some more changes, built, some more changes, built, and now
it fails, but I don't remember all the changes I did and in which
order". This will be impossible to support.
For all these reasons, the conclusion is that adding tracking of
installed files to remove them when the package is unselected, or to
generate a repository of binary packages, is something that is very
hard to achieve reliably and will add a lot of complexity.
On this matter, the Buildroot developers make this position statement:
* Buildroot strives to make it easy to generate a root filesystem (hence
the name, by the way.) That is what we want to make Buildroot good at:
building root filesystems.
* Buildroot is not meant to be a distribution (or rather, a distribution
generator.) It is the opinion of most Buildroot developers that this
is not a goal we should pursue. We believe that there are other tools
better suited to generate a distro than Buildroot is. For example,
http://openembedded.org/[Open Embedded], or https://openwrt.org/[openWRT],
are such tools.
* We prefer to push Buildroot in a direction that makes it easy (or even
easier) to generate complete root filesystems. This is what makes
Buildroot stands out in the crowd (among other things, of course!)
* We believe that for most embedded Linux systems, binary packages are
not necessary, and potentially harmful. When binary packages are
used, it means that the system can be partially upgraded, which
creates an enormous number of possible combinations of package
versions that should be tested before doing the upgrade on the
embedded device. On the other hand, by doing complete system
upgrades by upgrading the entire root filesystem image at once,
the image deployed to the embedded system is guaranteed to really
be the one that has been tested and validated.