Re: custom kernel config

[Efraim Flashner <efraim@flashner.co.il>]

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On Wed, Apr 03, 2019 at 09:49:59PM +0200, Pierre Neidhardt wrote:
> Thank you Efraim, this is awesome!
> 
> Efraim Flashner <efraim@flashner.co.il> writes:
> 
> > For the first one I used the #:extra-options route and compiled a list.
> > The second one I tried I created a custom kernel and left #:defconfig as
> > #f and inherited that to add a kconfig to the native-inputs.
> 
> Oh, I think I had missed the #:defconfig option when I tried.
> 
> > Once I read a bit more of the kernel documentation it wasn't too bad to
> > create a list of what I needed.
> 
> Which documentation in particular?

Documentation/admin-guide/README.rst from inside the kernel tarball
source.

> 
> > tar xf $(guix build linux-libre -S)
> > cd linux-libre<tab>
> > touch .config
> > guix environment linux-libre
> >     make localmodconfig
> >
> > copy that into .config
> 
> Copy what into .config?

the .config you just created at the root of the source tarball

> 
> > and fix the formatting
> >
> >     make localmodconfig
> 
> Why fixing the formatting?  How?  With a second `make localmodconfig'?
> 

running make localconfig spits out a bunch of pseudo-error messages
about modules that are missing in .config. If you copy that whole
message, minus the lines starting with 'WARING', into .config and
then format it as UPERCASE_OPTION=(y|m) it gives the kernel a config
that is usable for it.

so turning lines that look like:
module nouveau did not have configs CONFIG_DRM_NOUVEAU

into lines that look like:
CONFIG_DRM_NOUVEAU=m

> > if that works well enough then add "CONFIG_MODULES=y" to .config and
> > you're ready to try it out.
> 
> What does CONFIG_MODULES=y change?  (Not on my computer now, can't check
> in my local linux checkout.)

CONFIG_MODULES=y lets the modules be loaded and installed separately, if I
remember correctly. Either way, without it the build errors right at the
beginning of the install phase:

starting phase `install'

The present kernel configuration has modules disabled.
Type 'make config' and enable loadable module support.
Then build a kernel with module support enabled.

make: *** [Makefile:1318: modules_install] Error 1

> 
> This walkthrough is invaluable in my opinion.  So far I was under the
> impression that many members of the Guix community were annoyed by
> kernel compilation time and had tried to customize it, but it's really
> hard.  This process would make for an excellent blog article in my
> opinion! :D

I'll try to go ahead and write it up more nicely then. Custom compiling
a kernel sounds really scary, but with Guix's rollbacks and declarative
everything it's actually not too bad.


Currently this doesn't take into account the initrd.

-- 
Efraim Flashner   <efraim@flashner.co.il>   אפרים פלשנר
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