Question: How to build ARM32 rootfs with ARM64 Kernel

Question: How to build ARM32 rootfs with ARM64 Kernel

[Hirotaka Motai]

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Hi Isar team,

I am currently considering using ISAR to build a system with the following
architecture:

* Kernel: ARM64 (64-bit)
* rootfs: ARM32 (32-bit)

I understand that the ISAR_ENABLE_COMPAT_ARCH option allows for the
building of
some 32-bit packages, but in this case, I want to build all packages as
32-bit.

Could you please let me know if this somewhat special configuration of
"32-bit rootfs
with 64-bit kernel" is supported for cross-architecture building within
ISAR?
If it is possible, I would greatly appreciate it if you could provide the
specific steps,
such as the necessary settings in local.conf.

Thanks!

Best regards,
Hiro

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Re: Question: How to build ARM32 rootfs with ARM64 Kernel

[Baurzhan Ismagulov]

Hello Hiro,

On 2025-12-25 17:18, Hirotaka Motai wrote:
> * Kernel: ARM64 (64-bit)
> * rootfs: ARM32 (32-bit)
...
> Could you please let me know if this somewhat special configuration of
> "32-bit rootfs
> with 64-bit kernel" is supported for cross-architecture building within
> ISAR?

On desktop, I've used i386 rootfs + amd64 kernel extensively, but never tried
automating this.

My first guess would be using armhf distro and override the kernel arch. Not
sure regarding OOTB support and specific configuration at the moment; maybe we
get more insights after the holidays.

With kind regards,
Baurzhan

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Re: Question: How to build ARM32 rootfs with ARM64 Kernel

['Jan Kiszka' via isar-users]

Hi Matai-san,

On 25.12.25 09:18, Hirotaka Motai wrote:
> Hi Isar team,
> 
> I am currently considering using ISAR to build a system with the
> following architecture:
> 
> * Kernel: ARM64 (64-bit)
> * rootfs: ARM32 (32-bit)
> 
> I understand that the ISAR_ENABLE_COMPAT_ARCH option allows for the
> building of
> some 32-bit packages, but in this case, I want to build all packages as
> 32-bit.
> 
> Could you please let me know if this somewhat special configuration of
> "32-bit rootfs
> with 64-bit kernel" is supported for cross-architecture building within
> ISAR?
> If it is possible, I would greatly appreciate it if you could provide
> the specific steps,
> such as the necessary settings in local.conf.
> 

Not a completely simple question, indeed. The rootfs arch is defined by
DISTRO_ARCH. But that one also influences the arch that the custom
kernel will select. You could try a multiconfig build, setting
DISTRO_ARCH to arm64 only for the kernel part and then requesting the
arm64 kernel from your armhf rootfs. Or you try to set KERNEL_ARCH
explicitly in the custom kernel recipe, switching it from "arm" (as
armhf would defined it) to "arm64". The former may give more correct
results, though, because the kernel recipe will still use DISTRO_ARCH
with "armhf" for a arm64 kernel.

Jan

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Re: Question: How to build ARM32 rootfs with ARM64 Kernel

[Hirotaka Motai]

Hi Baurzhan, Jan,

Thank you for your advice. It was very helpful for me.
I will follow your advice and try it.

I appreciate your time and support.

Best regards,
Hiro

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