Re: ARMv7 Linux + Rust doesn't boot when compiling with only LLVM=1

[Rudraksha Gupta <guptarud@gmail.com>]

On 12/13/25 22:06, Miguel Ojeda wrote:
> On Sun, Dec 14, 2025 at 12:54 AM Rudraksha Gupta <guptarud@gmail.com> wrote:
>> - The kernel boots and outputs via UART when I build the kernel with the
>> following:
>>
>> make LLVM=1 ARCH="$arm" CC="${CC:-gcc}"
>>
>> - The kernel doesn't boot and there is no output via UART when I build
>> the kernel with the following:
>>
>> make LLVM=1 ARCH="$arm"
>>
>> The only difference being: CC="${CC:-gcc}". Is this expected?
> It depends on what that resolves to, i.e. your environment etc., i.e.
> that is resolved before Kbuild is called.

Sorry about that, I should've specified in the original email. The CC 
resolves to armv7-alpine-linux-musleabihf-gcc.

When both LLVM=1 and the CC=gcc are used, I can insmod the sample rust 
modules just fine. However, if I only use LLVM=1, my phone doesn't 
output anything over UART, and I assume that it fails to boot. 
Interestingly enough, if I just specify LLVM=1 (with no CC=gcc), and 
remove the rust related configs from the pmos.config fragment, then my 
phone boots and I can see logs over UART.


> The normal way of calling would be the latter anyway -- with the
> former you are setting a custom `CC` (either whatever you have there
> or the `gcc` default). By default, `LLVM=1` means `CC=clang`.
>
> So it could be that you are using a completely different compiler
> (even Clang vs. GCC, or different versions, and so on). Hard to say.
> And depending on that, you may end up with a very different kernel
> image. Even substantial Kconfig options may get changed etc.
>
> I would suggest comparing the kernel configuration of those two ways
> (attaching them here could also be nice to see what compilers you are
> using and so on).

postmarketOS uses kernel config fragments and tracks the latest linux-next:

- 
https://gitlab.postmarketos.org/postmarketOS/pmaports/-/blob/master/device/testing/linux-next/devices.config

- 
https://gitlab.postmarketos.org/postmarketOS/pmaports/-/blob/master/device/testing/linux-next/pmos.config

- build recipe: 
https://gitlab.postmarketos.org/postmarketOS/pmaports/-/blob/master/device/testing/linux-next/APKBUILD


The only thing that changed was whether CC=gcc was specified or not:

https://gitlab.postmarketos.org/postmarketOS/pmaports/-/commit/b9102ac5718b8d18acb6801a62e1cd4cc7edc0cb

> Cc'ing Kbuild too so that they are in the loop.
>
> I hope that helps.

Thanks for your help! Always appreciate your presence. :)


> Cheers,
> Miguel