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

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Date:	Wed, 29 Apr 2009 08:48:17 -0500
From:	John Dykstra <john.dykstra1@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, pcnet32@...izon.net, jeff@...zik.org
Subject: Re: [PATCH net-next-2.6] pcnet32: Remove pointless memory barriers

On Tue, 2009-04-28 at 22:16 -0700, David Miller wrote:
> 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.

Is that relevant when the driver doesn't do any MMIO operations?  All
pcnet32 register accesses are via PCI I/O space, as implied by the
commit description.

Descriptors and buffers are, of course, in consistent physical memory.
This patch doesn't touch the barriers associated with those structures.

> 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.

Before sending out this patch, I determined that the MMIO barriers were
added as part of NAPI support.  I CCed one of the authors of that patch,
so he could NAK if appropriate.  I've added the other author on this
reply.

I knew there'd be questions about whether removing these two barriers
was safe.  Submitting the patch seemed the best way to understand why
they were needed, if they are.

  --  John

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