Re: [PATCH] kbuild: Reject unexpected values for LLVM=

[Nicolas Schier <nsc@kernel.org>]

On Mon, Jan 12, 2026 at 04:16:24PM -0700, Nathan Chancellor wrote:
> On Mon, Jan 12, 2026 at 07:43:52AM +0100, Thomas Weißschuh wrote:
> > The LLVM argument is documented to accept one of three forms:
> > * a literal '1' to use the default 'clang',
> > * a toolchain prefix path, ending in a trailing '/',
> > * a version suffix.
> > 
> > All other values are silently treated as '1'. If for example
> > the user accidentally forgets the trailing '/' of a toolchain prefix,
> > kbuild will unexpectedly and silently fall back to the system toolchain.
> 
> Yeah, I do not think falling back to the system toolchain is great here.
> This would also catch misuse of LLVM=0, since that is treated as LLVM=1,
> rather than reverting to GCC + binutils.
> 
> > Instead report an error if the user specified an invalid value for LLVM.
> > 
> > Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
> > ---
> >  Makefile | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/Makefile b/Makefile
> > index e404e4767944..2a085a1b6875 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -447,6 +447,8 @@ ifneq ($(filter %/,$(LLVM)),)
> >  LLVM_PREFIX := $(LLVM)
> >  else ifneq ($(filter -%,$(LLVM)),)
> >  LLVM_SUFFIX := $(LLVM)
> > +else ifneq ($(LLVM),1)
> > +$(error Invalid value for LLVM=)
> 
> If having a hard error is untenable (I am not sure that it is), we could
> just warn. Pointing to Documentation/kbuild/llvm.rst might be worthwhile
> in the message as well.

I second to have a hard error here with the documentation pointer to
prevent building with an unexpected toolchain, and as there is also no
fallback for gcc toolchains.

Kind regards,
Nicolas