netdev - Re: [PATCH net-next-2.6] pcnet32: Remove pointless memory barriers

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Message-Id: <20090428.221605.71993506.davem@davemloft.net>
Date:	Tue, 28 Apr 2009 22:16:05 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	john.dykstra1@...il.com
Cc:	netdev@...r.kernel.org, pcnet32@...izon.net
Subject: Re: [PATCH net-next-2.6] pcnet32: Remove pointless memory barriers

From: John Dykstra <john.dykstra1@...il.com>
Date: Tue, 28 Apr 2009 19:07:39 +0000

> These two memory barriers in performance-critical paths are not needed
> on x86.  Even if some other architecture does buffer PCI I/O space
> writes, the existing memory-mapped I/O barriers are unlikely to be what
> is needed.
> 
> Signed-off-by: John Dykstra <john.dykstra1@...il.com>

Any driver where these things are present usually has them
there for a reason.  Usually it's because the SGI guys really
did run into real problems without them on their huge
machines which can reorder PCI MMIO wrt. real memory operations.

I don't feel good applying this at all, given that I see no
evidence that there has been any investigation into how these
barriers got there in the first place.
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