linux-kernel - Re: [PATCH] [POWERPC] Improve (in|out)

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Date:	Fri, 23 May 2008 00:24:43 -0400
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	David Miller <davem@...emloft.net>
Cc:	scottwood@...escale.com, linuxppc-dev@...abs.org,
	tpiepho@...escale.com, alan@...rguk.ukuu.org.uk,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [POWERPC] Improve (in|out)_beXX() asm code


On Tue, 2008-05-20 at 15:53 -0700, David Miller wrote:
> From: Scott Wood <scottwood@...escale.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.

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