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

[Christian Schrefl <chrisi.schrefl@gmail.com>]

On 12/14/25 8:34 AM, Rudraksha Gupta wrote:
> 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.

Did you try other architectures / devices as well (especially can you reproduce it on qemu-arm)?

Did you try a LLVM=1 build without rust enabled?

> 
>> 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
> 

I'm not familiar with pmbootstrap, what is required to reproduce this?
Do I just need to use the edge channel with linux-next or is something special required?

I might habe time to look into trying to reproduce it this week, but I'm not sure.
I have a armv7 based asus-flo device to try it out. Its not very well supported, but
It should be sufficient for this.

Cheers,
Christian