How to force build system to use a working kernel defconfig?

How to force build system to use a working kernel defconfig?

[Jupiter]

Hi,

I followed Yocto document 2.3.3 Changing the Configuration to set up a
complete working Linux kernel defconfig in my kernel bbappend, I
copied the defconfig to build/.config:
do_compile:prepend () {
    install -m 0644 ${WORKDIR}/defconfig ${WORKDIR}/build/.config
}

I hope the build system can just use the defconfig in build/.config,
but that did not work, the build system still ran Restart config... to
compare the defconfig and to generate and overwrite build/.config. How
can I suppress the build system not to run the "Restart config"
process, just to run the compile process using the build/.config I
copied to build/.config?

I tried to add KCONFIG_MODE = "alldefconfig" in my kernel bbappend,
but it did not work either.

Appreciate your advice.

Thank you very much.

Kind regards,

JH

Re: [yocto] How to force build system to use a working kernel defconfig?

[Bruce Ashfield]

On Tue, Apr 26, 2022 at 6:59 AM JH <jupiter.hce@gmail.com> wrote:
>
> Hi,
>
> I followed Yocto document 2.3.3 Changing the Configuration to set up a
> complete working Linux kernel defconfig in my kernel bbappend, I
> copied the defconfig to build/.config:
> do_compile:prepend () {
>     install -m 0644 ${WORKDIR}/defconfig ${WORKDIR}/build/.config
> }
>
> I hope the build system can just use the defconfig in build/.config,
> but that did not work, the build system still ran Restart config... to
> compare the defconfig and to generate and overwrite build/.config. How
> can I suppress the build system not to run the "Restart config"
> process, just to run the compile process using the build/.config I
> copied to build/.config?

You can't suppress the kernel's config step, it is always going to
run. You can't just copy over .config and have the kernel build.

The behaviour of the configuration depends on the kernel recipe and
bbclasses in play. I can answer from the context of
linux-yocto/kernel-yocto.bbclass and the default kernel.bbclass.

If you have a file called "defconfig" in your SRC_URI, it will be
copied into the kernel's .config and then yes, the kernel
configuration step will be run. The kernel's config subsystem will
apply its logic, and what your final .config is .. may or may not be
exactly what that input defconfig.

If the defconfig is a full copy of a .config (which is what they used
to be), then you can use either allnoconfig as the mode, or all
defconfig as the mode .. it won't make much of a difference since most
settings are fully specified. But the kernel-yocto processing assumes
an older defconfig style, and will use allnoconfig when it sees a
'defconfig' and the mode isn't explicitly specified.

If the defconfig was created using savedefconfig or similar, then
you'd want to specify the mode as alldefconfig, since you need those
defaults to be used to get a functional .config created.

Bruce

>
> I tried to add KCONFIG_MODE = "alldefconfig" in my kernel bbappend,
> but it did not work either.
>
> Appreciate your advice.
>
> Thank you very much.
>
> Kind regards,
>
> JH
>
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-- 
- Thou shalt not follow the NULL pointer, for chaos and madness await
thee at its end
- "Use the force Harry" - Gandalf, Star Trek II

Re: [yocto] How to force build system to use a working kernel defconfig?

[Jupiter]

Hi Bruce,

Thanks for your response.

> You can't suppress the kernel's config step, it is always going to
> run. You can't just copy over .config and have the kernel build.

If the OE build kernel config step can be disabled to compile kernel
using customer defconfig, that will make users life much easier to
disable unwanted kernel drivers and features.

The customer defconfig can be certainly generated from bitbake -c
menuconfig, but honestly, the kernel configure selection is not easier
to understand, we have to use defconfig to disable unwanted the
configures and tested it, inadvertently disable some configures could
cause kernel panic regardless if the OE build system config step is
running or not.

I have searched the internet, it is not just me, there are many others
who requested to run customer defconfig without OE build kernel config
step.

Are you a contributor in the OE dev team? Are there any reasons why
the OE dev team won't consider this feature?

Thank you very much.

Kind regards,

jupiter

Re: [yocto] How to force build system to use a working kernel defconfig?

[Bruce Ashfield]

On Thu, Apr 28, 2022 at 5:39 AM Jupiter <jupiter.hce@gmail.com> wrote:
>
> Hi Bruce,
>
> Thanks for your response.
>
> > You can't suppress the kernel's config step, it is always going to
> > run. You can't just copy over .config and have the kernel build.
>
> If the OE build kernel config step can be disabled to compile kernel
> using customer defconfig, that will make users life much easier to
> disable unwanted kernel drivers and features.

This has nothing to do with OE, that's what the kernel would do
no matter how you build it.

>
> The customer defconfig can be certainly generated from bitbake -c
> menuconfig, but honestly, the kernel configure selection is not easier
> to understand, we have to use defconfig to disable unwanted the
> configures and tested it, inadvertently disable some configures could
> cause kernel panic regardless if the OE build system config step is
> running or not.
>

There's a reason why kernel-yocto (and other frameworks) provide
things like configuration fragments, kernel types, etc, This is a well
explored topic. Check out my (and others) presentations about this
throughout the years of yocto summits, etc. They can all be found
online in various formats.

> I have searched the internet, it is not just me, there are many others
> who requested to run customer defconfig without OE build kernel config
> step.

It can already do that, just as I described in my previous response.
The defconfig is copied in, and then the kernel *will* run its configuration
process over it, not OE's, the kernel.

>
> Are you a contributor in the OE dev team? Are there any reasons why
> the OE dev team won't consider this feature?

See above,

Bruce

>
> Thank you very much.
>
> Kind regards,
>
> jupiter



-- 
- Thou shalt not follow the NULL pointer, for chaos and madness await
thee at its end
- "Use the force Harry" - Gandalf, Star Trek II