[yocto] Kernel development using externalsrc

[Uwe Geuder]

On Fri, Nov 2, 2018 at 5:47 PM CHMIELARZ Radoslaw radoslaw.chmielarz-at-leica-geosystems.com wrote:

> The company I work for uses yocto to build an image with a custom
> linux kernel. The documentation
> (https://www.yoctoproject.org/docs/2.5.1/kernel-dev/kernel-dev.html)
> suggests a setup where the kernel is under git and the kernel recipe
> specifies this git repository hence the workflow is the following:
>
> Make changes in local kernel directory
> Submit changes to git (and presumably push to shared space)
> Run yocto build which fetches the changes
>
> However since I don’t want to push the changes to git server before I
> test locally I have added the kernel sources using
> externalsrc. 

Devtool is of course what they recommend, but can always change the git
repo to a local one in a .bbappend, so you save the push step and Yocto
build saves fetching over the network. (Making a git commit for each
build attempt is very little overhead and I would recommend it anyway as
a good working practice. You can clean up using "git rebase -i" once you
are happy with your code and before you publish it.)

In one of my development branches I find

SRC_URI = "\
  git:///home/myname/projects/yoctobuild/pkgsrc/somerepo/;protocol=file;branch=debug/#675-v2.16.0 \
  file://some.patch \
  file://other.patch \
"

SRCREV = "${AUTOREV}"

I have not done it for the kernel, but I see no reason why it would not
work there.

(Obviously the absolute path is nothing you would be able to share
unmodified with others. Maybe it can be made a bit more portable using
${THISDIR}, but I have not had any need to try it.) 

> Unfortunately this has the drawback that
> linux-libc-headers runs do_configure and do_install phase which
> invalidates glibc-initial and requires a recompilation of a couple of
> hundreds of packages. I would like to avoid it.
>

How editing the kernel source would cause rebuilds trailing
linux-libc-haeaders I cannot see in my build here

$ bitbake-diffsigs -t linux-libc-headers configure
NOTE: Starting bitbake server...
ERROR: Only one matching sigdata file found for the specified task (linux-libc-headers do_configure)

Well, maybe I have never changed the kernel in this build area, so let's
check what it depends on:
 
$ bitbake-dumpsig -t linux-libc-headers configure
NOTE: Starting bitbake server...
ERROR: Metadata does not support finding signature data files

Hmm, no idea what that means. Until someone tells us what's wrong let's
use stamp files instead

$ bitbake-dumpsig tmp/stamps/corei7-64-poky-linux/linux-libc-headers/4.15.7-r0.do_configure.sigdata...
This task depends on the checksums of files: []
Hash for dependent task
/home/geuder/projects/yoctobuild/meta-nel/poky/meta/recipes-kernel/linux-libc-headers/linux-libc-headers_4.15.7.bb.do_prepare_recipe_sysroot is 5ac6adcdd9edfe2705fd461a42942613

$ bitbake-dumpsig tmp/stamps/corei7-64-poky-linux/linux-libc-headers/4.15.7-r0.do_prepare_recipe_sysroot.sigdata...
This task depends on the checksums of files: []
Hash for dependent task /home/geuder/projects/yoctobuild/meta-nel/poky/meta/recipes-kernel/linux-libc-headers/linux-libc-headers_4.15.7.bb.do_fetch is 362b694aea2202ce72ad259579d001ad

$ bitbake-dumpsig tmp/stamps/corei7-64-poky-linux/linux-libc-headers/4.15.7-r0.do_fetch.sigdata...
Variable SRC_URI value is ${KERNELORG_MIRROR}/linux/kernel/v${HEADER_FETCH_VER}/linux-${PV}.tar.${KORG_ARCHIVE_COMPRESSION}
Tasks this task depends on: []
This task depends on the checksums of files: []

So my build gets kernel headers from kernel.org (mirror), it does not
use the kernel source I use to build my kernel. Maybe that's different
for you?

Of course the output of the bitbake-dumpsig commands is massively
shortened, I just picked those lines that seemed likely candidates to
cause rebuilds.  To get the whole truth in your build call...

$ bitbake-diffsigs -t linux-libc-headers configure

... after you made a small kernel edit and rebuilt.

Regards,

Uwe

Uwe Geuder
Neuro Event Labs Oy
Tampere, Finland
uwe.gexder at neuroeventlabs.com (Bot check: fix one obvious typo)