linux-kernel - Re: [PATCH 2/2] kbuild: make modversion for exported asm symbols more convivial

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Date:   Tue, 13 Dec 2016 19:56:14 -0500 (EST)
From:   Nicolas Pitre <nicolas.pitre@...aro.org>
To:     Michal Marek <mmarek@...e.com>
cc:     Nicholas Piggin <npiggin@...il.com>, Arnd Bergmann <arnd@...db.de>,
        viro@...iv.linux.org.uk, linux-arm-kernel@...ts.infradead.org,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] kbuild: make modversion for exported asm symbols
 more convivial

On Wed, 14 Dec 2016, Michal Marek wrote:

> Dne 8.12.2016 v 05:24 Nicolas Pitre napsal(a):
> > Rather than having an asm-prototypes.h file where C prototypes for exported
> > asm symbols are centralized, let's have some macros that can be used
> > directly in the code where those symbols are exported for genksyms
> > consumption.  Either the prototype is provided directly if no include
> > files has it, or the include file containing it is specified.
> > 
> > Signed-off-by: Nicolas Pitre <nico@...aro.org>
> > ---
> >  include/asm-generic/export.h | 15 +++++++++++++++
> >  scripts/Makefile.build       | 22 +++++++++++++++-------
> >  2 files changed, 30 insertions(+), 7 deletions(-)
> > 
> > diff --git a/include/asm-generic/export.h b/include/asm-generic/export.h
> > index 39a19dc366..83dda5f840 100644
> > --- a/include/asm-generic/export.h
> > +++ b/include/asm-generic/export.h
> > @@ -84,11 +84,26 @@ KSYM(__kcrctab_\name):
> >  #define __EXPORT_SYMBOL(sym, val, sec) ___EXPORT_SYMBOL sym, val, sec
> >  #endif
> >  
> > +/* in the non genksyms case those are no-ops */
> > +#define SYMBOL_CPROTO(expr)
> > +#define SYMBOL_CPROTO_INCLUDE(file)
> 
> Do we really _need_ the SYMBOL_CPROTO macro? The exported functions are
> called from C files presumably, so there ought to be headers with the
> declarations. If these headers can't be included easily, we should fix
> them, but having copies of the declarations in the asm files is no big
> improvement over the asm-prototypes.h approach, IMO.

On ARM there are a few symbols that are part of the gcc support library 
such as division routines and so on.  Those are never called directly 
from C code. It is the compiler that implicitly creates references to 
them.  However, in order to be able to export those symbols, dummy C 
prototypes were used before it was possible to export symbols from asm 
code but those prototypes make no sense otherwise.  So the SYMBOL_CPROTO 
macro is there mainly to maintain backward compatibility with the 
traditional symbol version signature for those symbols.

The SYMBOL_CPROTO macro, being close to the actual code, could mark the 
intended definition for symbols in assembly code and allow for a tool to 
ensure there is no mismatch with the actual declaration located 
elsewhere. That could be useful for all global symbols, not just 
exported ones.  But that's not the primary reason why I created it.

Nicolas