Subject: Re: [PATCH] I/O space write barrier
From: Albert Cahalan <albert@users.sf.net>
To: Jesse Barnes <jbarnes@engr.sgi.com>
Cc: Albert Cahalan <albert@users.sourceforge.net>,
       linux-kernel mailing list <linux-kernel@vger.kernel.org>,
       benh@kernel.crashing.org
In-Reply-To: <200410041420.01266.jbarnes@engr.sgi.com>
References: <1096922369.2666.177.camel@cube>
	 <200410041420.01266.jbarnes@engr.sgi.com>
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Date: 04 Oct 2004 20:32:24 -0400
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On Mon, 2004-10-04 at 17:20, Jesse Barnes wrote:
> On Monday, October 4, 2004 1:39 pm, Albert Cahalan wrote:
> > > diff -Nru a/include/asm-ppc/io.h b/include/asm-ppc/io.h
> > > --- a/include/asm-ppc/io.h 2004-09-27 10:48:41 -07:00
> > > +++ b/include/asm-ppc/io.h 2004-09-27 10:48:41 -07:00
> > > @@ -197,6 +197,8 @@
> > >  #define memcpy_fromio(a,b,c)   memcpy((a),(void *)(b),(c))
> > >  #define memcpy_toio(a,b,c) memcpy((void *)(a),(b),(c))
> > >
> > > +#define mmiowb() asm volatile ("eieio" ::: "memory")
> > > +
> > >  /*
> > >   * Map in an area of physical address space, for accessing
> > >   * I/O devices etc.
> >
> > I don't think this is right. For ppc, eieio is
> > already included as part of the assembly for the
> > IO operations. If you could delete that, great,
> > but I suspect that nearly all drivers would break.
> 
> Ok, if it's covered than mmiowb() can just be empty for ppc.

Ideally, it would be eieio, and the eieio in each
of the IO operations would be removed. Finding and
fixing all the drivers that break looks impossible
though; most driver developers will be on x86 boxes.

> > BTW, the "eieio" name is better. The "wb" part
> > of "mmiowb" looks like "write back" to me, as if
> > it were some sort of cache push operation. It is
> > also lacking an appropriate song. :-)
> 
> It's supposed to be 'write barrier' just like wmb is a write memory barrier, 
> so is mmiowb a memory-mapped I/O write barrier.  Make sense?

In that case: wmmiob

(or something longer, like mmio_write_fence maybe)

As a name, "wmb" sucks almost as much as "cli" and "sti" do.
It dates back to the Alpha port, where it's an opcode.


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