Building a custom kernel via cross compiling

[Andrew Wing]

Thanks Peter that was most helpful. Aside from running out of space on my
undersized virtual box and a bit of a wait, everything went pretty
painlessly following those instructions. For the benefit of any future
readers/sake of completeness, to get the cross-compiling etc. under way I
installed the following packages from the command line (other dependent
packages installed for free of course)

gcc
gcc-arm-linux-gnu
hmaccalc
m4

Andrew



On 8 July 2015 at 22:48, Peter Robinson <pbrobinson at gmail.com> wrote:

> Hi Andrew,
>
> > I am a newbie with kernel matters so I apologize in advance for what is a
> > newbie type setup question. My feeling is that if I send this query to
> the
> > more general Arm Fedora forum, the mere mention of kernel will provoke
> > glazed eyes :-)
>
> Unlikely, I do it all the time, happy to help ;-)
>
> > The background is that I have an OEM board based on a TI AM3352 OMAP
> family
> > SoC looking somewhat like a Beaglebone black. Recently I've been
> exploring
> > getting this running under Fedora. In order to get  it working I've
> cobbled
> > together the F22 minimal image along with a device tree blob created by a
> > colleague for this board, and a customised MLO/u-boot.
> >
> > My next task is to look through and understand the arm kernel sources,
> > customize various aspects of the kernel config for my board and re-build.
> >
> > It seems that instructions are here:
> >
> > https://fedoraproject.org/wiki/Building_a_custom_kernel
> >
> > However, I want to cross compile on a host system. To ease this process I
> > have set up a  Fedora x86 box, also running f22, which is going to be the
> > host build machine.
> >
> > I decided I should build from source rpm and was following the wiki
> > instructions(rpmdev-setuptree etc.) up to the point here...
>
> The easiest way to do this is:
>
> Install the Fedora packaging tools:
> "dnf install fedora-packager"
>
> Clone the kernel package source and switch the F-22 branch:
> "fedpkg clone -a kernel"
> cd kernel
> "fedpkg switch-branch f22"
>
> The kernel config for the am33xx devices like the BBB is in a file
> called config-armv7. It's a bit confusing to get started but basically
> there's inheritance on the config files config-generic ->
> config-arm-generic -> config-armv7-generic -> config-armv7
>
> Most of the bits related to the am33xx are in a section titled "#
> AM33xx/43xx"
>
> Adjust the config as you like. Once you've done that run "fedpkg prep"
> which will make sure all your config changes aren't missing anything.
> Then do "fedpkg srpm" to create a .src.rpm
>
> Then to cross compile run the following command:
>
> rpmbuild --rebuild --target armv7hl --with=cross --without=perf
> --without=tools --without=debuginfo --without=pae --define="_arch arm"
> --define="_build_arch arm" --define="__strip
> /usr/bin/arm-linux-gnu-strip" kernel-version.src.rpm
>
> That will builder you the kernel and modules. You want to leave out
> tools/perf as they don't tend to cross compile well but it's unlikely
> they'll differ much from the ones we already ship, I've never had an
> issue with those.
>
> > At this point I'd like to actually have a target of arm7l for my target
> but
> > entering this gets me an unknown target error (possibly not surprising
> > given the build machine/target architecture difference!).
>
> Use the command above and you should be good.
>
> > My build machine does have a suitable linaro Arm compiler in place. It
> > occurs to me that I need a different kernel spec and to indicate to my
> > build system to use my compiler. Hopefully this can be achieved via some
> > slightly modified step from that given in the wiki page? Or should I be
> > tackling this in a completely different way?
>
> The above command will prompt you to install the Fedora cross compiler
> toolchain, I would recommend using that, it works fine for am33xx and
> other ARMv7 devices, YMMV with Linaro toolchain.
>
> Peter
>