linux-kernel - Re: MMIO and gcc re-ordering issue

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Date:	Fri, 30 May 2008 08:05:39 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Trent Piepho <tpiepho@...escale.com>
Cc:	Jes Sorensen <jes@....com>, Roland Dreier <rdreier@...co.com>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...abs.org, scottwood@...escale.com,
	torvalds@...ux-foundation.org, David Miller <davem@...emloft.net>,
	alan@...rguk.ukuu.org.uk
Subject: Re: MMIO and gcc re-ordering issue

On Thu, 2008-05-29 at 14:48 -0700, Trent Piepho wrote:

> I wrote a JTAG over gpio driver for the powerpc MPC8572DS platform.  With the
> non-raw io accessors, the JTAG clock can run at almost ~9.5 MHz.  Using raw
> versions (which I had to write since powerpc doesn't have any), the clock
> speed increases to about 28 MHz.  So it can make a very significant different.

Yes, sync's can hurt a lot. This is why I initially tried to get more
relaxed semantics.

We could implement something like __ variants and do something that
would still have eieio's but not sync's for example (ie. MMIOs are still
ordered vs. each other but not vs. coherent memory).

Ben.


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