From mboxrd@z Thu Jan 1 00:00:00 1970 From: John Dykstra Subject: Re: [PATCH net-next-2.6] pcnet32: Remove pointless memory barriers Date: Wed, 29 Apr 2009 08:48:17 -0500 Message-ID: <1241012897.7487.12.camel@Maple> References: <1240945659.8819.9.camel@Maple> <20090428.221605.71993506.davem@davemloft.net> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org, pcnet32@verizon.net, jeff@garzik.org To: David Miller Return-path: Received: from yx-out-2324.google.com ([74.125.44.29]:1323 "EHLO yx-out-2324.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751138AbZD2NsV (ORCPT ); Wed, 29 Apr 2009 09:48:21 -0400 Received: by yx-out-2324.google.com with SMTP id 3so685219yxj.1 for ; Wed, 29 Apr 2009 06:48:20 -0700 (PDT) In-Reply-To: <20090428.221605.71993506.davem@davemloft.net> Sender: netdev-owner@vger.kernel.org List-ID: 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