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Subject: Re: [PATCH] [POWERPC] Improve (in|out)_beXX() asm code
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: David Miller <davem@davemloft.net>
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Date: Fri, 23 May 2008 00:24:43 -0400
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On Tue, 2008-05-20 at 15:53 -0700, David Miller wrote:
> From: Scott Wood <scottwood@freescale.com>
> Date: Tue, 20 May 2008 17:43:58 -0500
> 
> > David Miller wrote:
> > > The __volatile__ in the asm construct disallows movement of the
> > > inline asm relative to statements surrounding it.
> > > 
> > > The only reason barrier() in kernel.h needs a memory clobber is
> > > because of a bug in ancient versions of gcc.  In fact, I think
> > > that memory clobber might even be removable.
> > 
> > Current versions of GCC seem quite happy to move non-asm memory accesses 
> > around a volatile asm without a memory clobber; see the test Trent posted.
> 
> Indeed, and even the GCC manual is clear about this.

So what is the scope of that problem ?

IE. Take an x86 version of that test, writing to memory, doing a writel
to some MMIO, then another memory write, can those be re-ordered with
the current x86 version of writel ?

static inline void writel(unsigned int b, volatile void __iomem *addr)
{
	*(volatile unsigned int __force *)addr = b;
}

This is becoming a serious issue...

Ben.