MMIO and gcc re-ordering issue

Benjamin Herrenschmidt benh at
Fri May 30 08:05:39 EST 2008

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).


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