linux-kernel - Re: [PATCH] lib/atomic64_test: do not build on non-atomic64 systems

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Message-Id: <20101021155528.b3d6d027.akpm@linux-foundation.org>
Date:	Thu, 21 Oct 2010 15:55:28 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mike Frysinger <vapier@...too.org>
Cc:	Luca Barbieri <luca@...a-barbieri.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lib/atomic64_test: do not build on non-atomic64 systems

On Thu, 21 Oct 2010 18:23:37 -0400
Mike Frysinger <vapier@...too.org> wrote:

> On Thursday, October 21, 2010 18:02:50 Andrew Morton wrote:
> > On Sat, 16 Oct 2010 13:27:15 -0400 Mike Frysinger wrote:
> > > If the arch doesn't provide atomic64 functionality (there are quite a
> > > few), then don't bother trying to build this test.
> > 
> > I don't get it.  If the arch doesn't implement atomic64 then this file
> > will get zillions of build errors, won't it?
> 
> ... which is why i added the ifdef protection

So the changelog was poor.  Please write complete changelogs so I need
to have this sort of conversation less often?

Why doesn't this cause lots of you-broke-my-build complaints?  Nobody's
running allmodonfig?

> > > diff --git a/lib/atomic64_test.c b/lib/atomic64_test.c
> > > index 250ed11..0ac1a66 100644
> > > --- a/lib/atomic64_test.c
> > > +++ b/lib/atomic64_test.c
> > > @@ -12,6 +12,8 @@
> > > 
> > >  #include <linux/kernel.h>
> > >  #include <asm/atomic.h>
> > > 
> > > +#ifdef ATOMIC64_INIT
> > 
> > hm, that's a bit lazy.  It should really use a CONFIG_HAVE_ thing.  What
> > a pita.
> 
> ATOMIC64_INIT is required for atomic64 headers to provide, and having a 
> Kconfig knob doesnt gain anything else

I know that.  But the standard way for an architecture to indicate to
the core that it impements a feature is for it to define CONFIG_HAVE_*.
Picking some related #define which architectures happen to implement
is atypical and unexpected.

Will it cause problems?  Probably not, unless the arch goes and defines
ATOMIC64_INIT without actually implementing atomic64.  But it's
atypical and unexpected and, yes, lazy!
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