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Re: building a debian kernel package(s) for a foreign architecture

On Wed, Jul 05, 2017 at 03:35:25PM +0100, Jonathan Dowland wrote:
> Hi folks,
> I've never done much in the way of cross-compiling before.  I
> understand it was once very hard, is now much easier, and is
> considerably easier for "simple" packages (including the kernel)
> than others.
> That said, I'm lost/stuck trying to cross-compile the Debian
> Linux kernel package for ARM64 (target is a Raspberry Pi 3) from
> an amd64 builder.  I believe I do not need to use multiarch for
> my builder, yet dpkg-buildpackage (if I supply -a aarch64,
> interestingly ARM64 doesn't work) complains of unmet build
> dependencies, which I have resolved for amd64.  But perhaps -a to
> dpkg-buildpackage is the wrong approach.


from a technical point of view you effectively need multiarch to
crosscompile Debian packages, but you don't have to set it up
yourself ;-).

The easiest way to crossbuild a Debian package is probably to use
sbuild or pbuilder, which both have crossbuild-support since a
while.  When called with the appropriate options, they
automatically setup multiarch in their chroot and install the
necessary crosscompilers as well as the package cross-dependencies. 
I can only talk about sbuild from personal experience, but
according to the docs, pbuilder should work similarly.

Not all packages can be crossbuilt, and for those that can be
crossbuilt in principle, it is sometimes necessary to set
appropriate build-profiles which e.g. cause the testsuite not to
be run during the package build, as often the testsuite tries to
run host-architecture binaries created during the build which
cannot be executed on the build-architecture system (with "build"
and "host" used in GNU terminology, i.e. "build-architecture" =
"architecture on which the compiler runs" and "host-architecture" =
"architeture on which the created binaries run").

IIRC both pbuilder and sbuild automatically set the "nocheck"
profile when crossbuilding. For crossbuilding the kernel IIRC
one needs to set a number of additional build-profiles:

- cross
- pkg.linux.notools
- nopython

So the following sbuild command should in theory (sorry, cannot
actually test that right now) crossbuild the kernel package for

  sbuild -d unstable --host=arm64 --profiles=cross,pkg.linux.notools,nopython <kernelpackage>.dsc

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