From mboxrd@z Thu Jan 1 00:00:00 1970 From: Roland Dreier Subject: Re: Dropping NETIF_F_SG since no checksum feature. Date: Tue, 10 Oct 2006 20:42:20 -0700 Message-ID: References: <20061011002656.GB30093@mellanox.co.il> <20061010.203624.91207079.davem@davemloft.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: mst@mellanox.co.il, shemminger@osdl.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, openib-general@openib.org, rolandd@cisco.com Return-path: Received: from sj-iport-6.cisco.com ([171.71.176.117]:38759 "EHLO sj-iport-6.cisco.com") by vger.kernel.org with ESMTP id S1030769AbWJKDmW (ORCPT ); Tue, 10 Oct 2006 23:42:22 -0400 To: David Miller In-Reply-To: <20061010.203624.91207079.davem@davemloft.net> (David Miller's message of "Tue, 10 Oct 2006 20:36:24 -0700 (PDT)") Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org David> Also, if you don't do checksumming on the card we MUST copy David> the data (be it from a user buffer, or from a filesystem David> page cache page) into a private buffer since if the data David> changes the checksum would become invalid, as I mentioned David> in another email earlier. Yes, I get that now -- I replied to Michael's email before I read yours. David> Therefore, since we have to copy anyways, it always is David> better to checksum in parallel with the copy. Yes. David> So the whole idea of SG without hw-checksum support is David> without much merit at all. Well, on IB it is possible to implement a netdevice (IPoIB connected mode, I assume that's what Michael is working on) with a large MTU (64KB is a number thrown around, but really there's not any limit) but no HW checksum capability. Doing that in a practical way means we need to allow non-linear skbs to be passed in. On the other hand I'm not sure how useful such a netdevice would be -- will non-sendfile() paths generate big packets even if the MTU is 64KB? Maybe GSO gives us all the real advantages of this anyway? - R.