Subject: Re: [PATCH V13 5/7] rust: Make __udivdi3() and __umoddi3() panic

From: Link Mauve <[email-masked]>
Date: Fri, 24 Apr 2026 11:43:11 +0200

On Fri, Apr 24, 2026 at 10:14:25AM +0100, David Laight wrote:
> On Fri, 24 Apr 2026 11:17:40 +0530
> "Mukesh Kumar Chaurasiya (IBM)" <[email-masked]> wrote:
> 
> > From: Link Mauve <[email-masked]>
> > 
> > The core crate currently depends on these two functions for i64/u64/
> > i128/u128/core::time::Duration formatting, but we shouldn’t use that in
> > the kernel so let’s panic if they are ever called.
> 
> Ugg.
> Surely you can make it a link-time failure?
> Or change the underlying code to avoid the divide.

This is currently a link-time failure, which means if any of the core
crate or drm_panic_qr are linked in, the kernel will fail to link.

And since we rely on core, that means no kernel can be built on PowerPC
without this patch.

A possible solution would be to change the core crate to not divide
u64s, but the last time I tried to do that I couldn’t quite figure out
how to do formatting without it, maybe I will just open an issue against
Rust.

> 
> 	David
> 
> > 
> > This doesn’t yet fix drm_panic_qr.rs, which also uses __udivdi3 when
> > CONFIG_CC_OPTIMIZE_FOR_SIZE=y, but at least makes the rest of the kernel
> > build on PPC32.
> > 
> > Signed-off-by: Link Mauve <[email-masked]>
> > Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <[email-masked]>
> > ---
> >  rust/Makefile             | 4 ++++
> >  rust/compiler_builtins.rs | 6 ++++++
> >  2 files changed, 10 insertions(+)
> > 
> > diff --git a/rust/Makefile b/rust/Makefile
> > index 2a5428a5503d..ee11fba7a03d 100644
> > --- a/rust/Makefile
> > +++ b/rust/Makefile
> > @@ -616,6 +616,10 @@ ifneq ($(or $(CONFIG_ARM64),$(and $(CONFIG_RISCV),$(CONFIG_64BIT))),)
> >  		__ashrti3 \
> >  		__ashlti3 __lshrti3
> >  endif
> > +ifdef CONFIG_PPC32
> > +	redirect-intrinsics += \
> > +		__udivdi3 __umoddi3
> > +endif
> >  
> >  ifdef CONFIG_MODVERSIONS
> >  cmd_gendwarfksyms = $(if $(skip_gendwarfksyms),, \
> > diff --git a/rust/compiler_builtins.rs b/rust/compiler_builtins.rs
> > index dd16c1dc899c..fc6b54636dd5 100644
> > --- a/rust/compiler_builtins.rs
> > +++ b/rust/compiler_builtins.rs
> > @@ -97,5 +97,11 @@ pub extern "C" fn $ident() {
> >      __aeabi_uldivmod,
> >  });
> >  
> > +#[cfg(target_arch = "powerpc")]
> > +define_panicking_intrinsics!("`u64` division/modulo should not be used", {
> > +    __udivdi3,
> > +    __umoddi3,
> > +});
> > +
> >  // NOTE: if you are adding a new intrinsic here, you should also add it to
> >  // `redirect-intrinsics` in `rust/Makefile`.
> 
> 

-- 
Link Mauve