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From: Segher Boessenkool <segher@kernel.crashing.org>
Subject: Re: [PATCH] gianfar: Add I/O barriers when touching buffer descriptor
	ownership.
Date: Wed, 2 May 2007 20:05:43 +0200
To: Scott Wood <scottwood@freescale.com>
Cc: netdev@vger.kernel.org, jgarzik@pobox.com, linuxppc-dev@ozlabs.org
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>> AFAICS you need stronger barriers though; {w,r,}mb(),
>> to prevent _any_ reordering of those memory accesses,
>> not just the compiler-generated ones.
>
> My impression was that the eieio used by iobarrier would be sufficient 
> for that, as we're not trying to synchronize between accesses to 
> different types of memory.  Is sync really required here?

For accesses to main system memory, eieio only orders
writes, not reads, so iobarrier_r() doesn't do what
you want; and iobarrier_w() isn't meant to be used for
main memory access ordering either.

Also, it is better to not use powerpc-specific interfaces
in a device driver if you don't have a strong reason to.


Segher