Re: [PATCH v8 16/29] arm64: Share arm64 headers with s390

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Sep 29, 2026 at 06:19:20AM +0200, Andreas Grapentin wrote:
> On Sep 28 26, Steffen Eiden wrote:
> > On Mon, Sep 28, 2026 at 05:07:52PM +0100, Catalin Marinas wrote:
> > > On Fri, Sep 18, 2026 at 03:30:53PM +0200, Steffen Eiden wrote:
> > > > +# Enable all code shared to s390
> > > > +KBUILD_CFLAGS	+= -DARM64_S390_COMMON
> > > > +KBUILD_AFLAGS	+= -DARM64_S390_COMMON
> > > > +KBUILD_CPPFLAGS	+= -DARM64_S390_COMMON
> > > 
> > > Do we actually need these defines? They seem only to be used as markers
> > > for the awk scripts to extract the definitions. Why do we need the C
> > > preprocessor involved at all? Could we not just have comment markers:
> > > 
> > > /* ARM64_S390_COMMON_BEGIN */
> > > ...
> > > /* ARM64_S390_COMMON_END */
> > >
> > No technically we do not need those. They could be useful if we find out
> > that AWK is the wrong tool and move to a C Preprocessor + diff based
> > approach. 
> > 
> > if the ifdev is not closed the compiler will complain, but an 'arm did
> > not destroy us' verifiaction tool ( I will send one soonish) could do
> > the same.
> 
> iirc the last time we discussed this we didn't think that load-bearing
> comments were the right tool here. We also briefly floated the idea of
> using a #pragma region based approach, but in the end we went with the
> #ifdef preprocessor directives instead, as the least invasive
> non-comment marker that was available.

The downside is that the macro affects the preprocessed code. We need to
ensure they don't leak in uapi headers for example. Not a fan of this
approach but I don't have a better suggestion either. At least we could
write them as:

#if ARM64_S390_COMMON == 1

so we get an error if someone misspells the macro name (or they leak in
uapi headers, vDSO).

It would also be useful for the awk scripts look for the paired #endif
rather than relying on the comment at the end of the line. Otherwise
it's not really different from just sticking to comments.

-- 
Catalin