Building a custom kernel via cross compiling

[Peter Robinson]

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