Re: topology api confusion

[Bill Davidsen <davidsen@tmr.com>]

Matthew Dobson wrote:
> Nathan Lynch wrote:
>
> > We need some clarity on how asm-generic/topology.h is intended to be
> > used.  I suspect that it's supposed to be unconditionally included at
> > the end of the architecture's topology.h so that any elements which
> > are undefined by the arch have sensible default definitions.  Looking
> > at 2.6.13-rc3, this is what ppc64, ia64, and x86_64 currently do,
> > however i386 does not (i386 pulls in the generic version only when
> > !CONFIG_NUMA).
> >
> > The #ifndef guards around each element of the topology api
> > cannot serve their apparent intended purpose when the architecture
> > implements a given bit as a function instead of a macro
> > (e.g. cpu_to_node in ppc64):
>
>
> When I originally wrote this all up, I had planned it to be used the way
> i386 does: offer a full implementation of topology if appropriate, else
> include the generic "sane" default.  It has since changed to work more like
> you described: implement some (or all) of the topology functions, then
> include the generic one to define any you missed.
>
>
>
> > Since ppc64 unconditionally includes asm-generic/topology.h, all uses
> > of cpu_to_node are preprocessed to (0).  Similar damage occurs with
> > every other topology function which happens to be a real function
> > instead of a macro.  I'm surprised my ppc64 numa systems even boot ;)
> >
> > If the intent is that the architecture is free to define only a subset
> > of the api and include the generic header for fallback definitions,
> > then we need to do the #ifndef __HAVE_ARCH_FOO thing, no?  That is,
> > the code above would look like:
>
>
> You are correct in noticing that things should (though apparently don't?)
> go wonky when arches define their topology functions as *functions*, rather
> than macros.  It hasn't really seemed to bite anyone yet, so I've left it
> alone, though to be honest, it is as surprising to me that it works as it
> is to you.  I've resisted creating #ifdef ARCH_HAVE_XXX all over the place,
> though maybe it is finally time?

If I understand the problem, is it amenable to just defining the macros 
and using another name for a function? In other words, if most arch 
define xx_generic_add as a function, can you just
 #define xx_generic_add xx_local_arch_add
which would satisfy the #ifndef, allow use of a function, etc? Then 
xx_local_arch_add can be the function. Then the common include would not 
generate macros for things which exist as function.

--
   -bill davidsen (davidsen@tmr.com)
"The secret to procrastination is to put things off until the
 last possible moment - but no longer"  -me
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