netdev - Re: [PATCH v2] gianfar: Add I/O barriers when touching buffer descriptor ownership.

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Date:	Thu, 3 May 2007 01:42:06 +0200
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Scott Wood <scottwood@...escale.com>
Cc:	jgarzik@...ox.com, linuxppc-dev@...abs.org,
	Kumar Gala <galak@...nel.crashing.org>, netdev@...r.kernel.org
Subject: Re: [PATCH v2] gianfar: Add I/O barriers when touching buffer descriptor ownership.

>>> And the driver is already ppc-specific; it uses in/out_be32.
>>
>> True, but its hidden behind the gfar_read/write accessors.
>>
>> Your change is a bit more blatant.
>
> Well, Segher doesn't want me to use iobarrier (because it's not I/O).
> Andy doesn't want me to use wmb() (because it's sync).

You should use wmb(), but unfortunately too strong
semantics are required for that (ordering wrt I/O)
so it's a full sync on PowerPC.  I don't believe
a priori that that would be notably slower, but if
actually is, you could use eieio() I suppose since
you say the driver is powerpc specific -- but please
put a comment in the source code then saying why you
don't use wmb() there.

Segher

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