From owner-freebsd-hackers@FreeBSD.ORG Mon May 21 17:46:50 2012 Return-Path: Delivered-To: hackers@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id 712691065678; Mon, 21 May 2012 17:46:50 +0000 (UTC) (envelope-from Daan@vitsch.nl) Received: from VM01.VEHosting.nl (VM016.VEHosting.nl [IPv6:2001:1af8:2100:b020::140]) by mx1.freebsd.org (Postfix) with ESMTP id 08B278FC20; Mon, 21 May 2012 17:46:49 +0000 (UTC) Received: from [192.168.45.11] (180-161.ftth.onsbrabantnet.nl [88.159.161.180]) (authenticated bits=0) by VM01.VEHosting.nl (8.14.3/8.13.8) with ESMTP id q4LHkljd092457; Mon, 21 May 2012 19:46:47 +0200 (CEST) (envelope-from Daan@vitsch.nl) From: Daan Vreeken Organization: Vitsch Electronics To: freebsd-arm@freebsd.org Date: Mon, 21 May 2012 19:46:23 +0200 User-Agent: KMail/1.9.10 References: <1337617221.2516.38.camel@revolution.hippie.lan> In-Reply-To: <1337617221.2516.38.camel@revolution.hippie.lan> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <201205211946.23306.Daan@vitsch.nl> x-ve-auth-version: mi-1.1.5 2011-02-07 - Copyright (c) 2008, 2011 - Daan Vreeken - VEHosting x-ve-auth: authenticated as 'pa4dan' on VM01.VEHosting.nl X-Mailman-Approved-At: Mon, 21 May 2012 17:48:06 +0000 Cc: Ian Lepore , hackers@freebsd.org, Svatopluk Kraus , Richard Hodges Subject: Re: ARM + CACHE_LINE_SIZE + DMA X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 21 May 2012 17:46:50 -0000 Hi Ian (and list), just commenting on the mbuf part : On Monday 21 May 2012 18:20:21 Ian Lepore wrote: > On Fri, 2012-05-18 at 16:13 +0200, Svatopluk Kraus wrote: > > On Thu, May 17, 2012 at 10:07 PM, Ian Lepore > > wrote: > > > On Thu, 2012-05-17 at 15:20 +0200, Svatopluk Kraus wrote: > > >> Hi, ... > > What I can do in a driver it's not so simple in case of OS buffers > > like mbufs. I can check how mbufs are used in current implementation > > and say, yes, it's safe to use them unaligned. However, it can be > > changed in next release if anybody won't take care of it. It would be > > nice to have a maintained list of OS buffers which are DMA safe in > > respect of CACHE_LINE_SIZE. Is the flag and the list interesting for > > somebody else? > > I don't have a definitive answer for this, but my assumption has always > been that once an mbuf is handed to a driver (for example, when it's > added to an interface's send queue), the driver then "owns" that mbuf > and nothing else in the system will touch it until the driver makes a > call to hand it off or free it. If that assumption is true, a driver > could make good use of a BUS_DMA_UNALIGNED_SAFE flag with mbufs. > > The part that scares me about my assumption is the m_ext.ref_cnt field > of the mbuf. Its existance seems to imply that multiple entities > concurrently have an interest in the data. On the other hand, the lack > of any built in provisions for locking seems to imply that concurrent > access isn't happening, or perhaps it implies that any required > synchronization is temporal rather than lock-based. > > I've never found anything in writing that explains mbuf usage > conventions at this level of detail. This assumption isn't always true. 'man 9 mbuf' mentions this, but not in one place. M_WRITABLE() can be used to tell wether or not you're allowed to modify an mbuf. If it returns false, you can create a writable copy of the mbuf and alter the copy instead of the original. A writable copy of an mbuf can be made using m_dup(). Writing to non-writable mbuf's will cause data corruption in e.g. BPF and TCP retransmits (even in the non-SMP case). Regards, -- Daan Vreeken Vitsch Electronics http://Vitsch.nl tel: +31-(0)40-7113051 KvK nr: 17174380