Date: Thu, 10 May 2007 00:35:28 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: linux-kernel@vger.kernel.org, linuxppc-dev@ozlabs.org,
       Rusty Russell <rusty@rustcorp.com.au>
Subject: Re: [PATCH 2/3] Add hard_irq_disable()
Message-Id: <20070510003528.395b122b.akpm@linux-foundation.org>
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On Thu, 10 May 2007 16:35:51 +1000 Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:

> On Wed, 2007-05-09 at 22:41 -0700, Andrew Morton wrote:
> > On Thu, 10 May 2007 15:25:58 +1000 Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:
> > 
> > > --- linux-cell.orig/include/linux/interrupt.h	2007-05-10 14:51:22.000000000 +1000
> > > +++ linux-cell/include/linux/interrupt.h	2007-05-10 15:18:04.000000000 +1000
> > > @@ -241,6 +241,16 @@ static inline void __deprecated save_and
> > >  #define save_and_cli(x)	save_and_cli(&x)
> > >  #endif /* CONFIG_SMP */
> > >  
> > > +/* Some architectures might implement lazy enabling/disabling of
> > > + * interrupts. In some cases, such as stop_machine, we might want
> > > + * to ensure that after a local_irq_disable(), interrupts have
> > > + * really been disabled in hardware. Such architectures need to
> > > + * implement the following hook.
> > > + */
> > > +#ifndef hard_irq_disable
> > > +#define hard_irq_disable()	do { } while(0)
> > > +#endif
> > 
> > We absolutely require that the architecture's hard_irq_disable() be defined
> > when we do this.  If it happens that the definition of hard_irq_disable()
> > is implemented three levels deep in nested includes then we risk getting
> > into a situation where different .c files see different implementations of
> > hard_irq_disable(), which could lead to confusing results, to say the least.
> 
> Yes, I'm indeed a bit worried about that... I've been wondering what's
> the best include path here... I tried to follow who gets to hw_irq.h and
> didn't come to any conclusive results.
> 
> powerpc gets it from asm/system.h but I haven't verified other arch
> (though it only matters on arch that have their own here).
> 
> I've verified that a #error on ppc up there will not trigger thus it's
> fine on powerpc, but I agree it's a bit fragile.

I think saying "system.h must provide this" is reasonable.  The fact that
powerpc does that via another inclusion is a powerpc detail - just don't
break it ;)


> > Your implementation comes via the inclusion of system.h which then includes
> > hw_irq.h.  That's perhaps a little fragile and it would be better to
> > 
> > a) include in the comment the name of the arch file which must implement
> >    hard_irq_disable() and
> > 
> > b) include that file directly from this one.
> 
> Fair enough. I was just worried that including hw_irq.h here might cause
> trouble for some archs though (as I said, we get it indirectly on
> powerpc via some other asm thingy, not via some linux/*.h). I've looked
> around and seen all sort of horrors in arch include dependencies
> (including some circular stuff that must work by mere luck).
> 
> > Oh, and your comment layout style is wrong ;)
> 
> What about my comment layout style ? I've been using that forever ... Or
> do you mean I should use a function documentation style layout there ?

/* This
 * is
 * wrong
 */

/*
 * This
 * is
 * right
 */


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