From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id BA539C433EF for ; Tue, 17 May 2022 07:03:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233914AbiEQHDP (ORCPT ); Tue, 17 May 2022 03:03:15 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38448 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S240732AbiEQHCU (ORCPT ); Tue, 17 May 2022 03:02:20 -0400 Received: from relay11.mail.gandi.net (relay11.mail.gandi.net [217.70.178.231]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 32EE9473A0; Tue, 17 May 2022 00:02:01 -0700 (PDT) Received: (Authenticated sender: maxime.chevallier@bootlin.com) by mail.gandi.net (Postfix) with ESMTPSA id A1ADA10000B; Tue, 17 May 2022 07:01:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=bootlin.com; s=gm1; t=1652770920; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=9KazaW1SkFx7SybTXPFRmtUBoimvEisxPHGbF+BGpFc=; b=R5CTzLA4Ubtz8fYfQ2SDt3yt3caIj3yrtuf0wjEBNuwsMPcn30DzUr5KYbk4TNHmWe5fGn PceQ1IHai4j++s5NU/WJqjbOkn2Legv9KSeORmjtH8o9pkGx3aFTRfx+W7+ZT9URZnolio rdvxiV4rfaQVB4ISVYv/iuthulsjxDhJH3SrSqohMvKj6qwxjd0LAicq3OjeHg35EOYnlx n46ipD/l/baalEMoG1wmOkd/dNPzNzBChI9l0XUYPajQTBeBpIpPdUb3AhJZpwaMbDJqQa h3H3Vpkf1+E6oKnbD4wWfESDyqbNcBgFMRC22ANt+DxSQlDY99MkW97sHrc7qQ== Date: Tue, 17 May 2022 09:01:56 +0200 From: Maxime Chevallier To: Vladimir Oltean Cc: "davem@davemloft.net" , Rob Herring , "netdev@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "devicetree@vger.kernel.org" , "thomas.petazzoni@bootlin.com" , Andrew Lunn , Florian Fainelli , Heiner Kallweit , Russell King , "linux-arm-kernel@lists.infradead.org" , Luka Perkov , Robert Marko Subject: Re: [PATCH net-next v2 2/5] net: dsa: add out-of-band tagging protocol Message-ID: <20220517090156.3fde5a8f@pc-20.home> In-Reply-To: <20220514224002.vvmd43lnjkbsw2g3@skbuf> References: <20220514150656.122108-1-maxime.chevallier@bootlin.com> <20220514150656.122108-3-maxime.chevallier@bootlin.com> <20220514224002.vvmd43lnjkbsw2g3@skbuf> Organization: Bootlin X-Mailer: Claws Mail 4.1.0 (GTK 3.24.33; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: devicetree@vger.kernel.org Hi Vlad, On Sat, 14 May 2022 22:40:03 +0000 Vladimir Oltean wrote: > On Sat, May 14, 2022 at 05:06:53PM +0200, Maxime Chevallier wrote: > > This tagging protocol is designed for the situation where the link > > between the MAC and the Switch is designed such that the Destination > > Port, which is usually embedded in some part of the Ethernet > > Header, is sent out-of-band, and isn't present at all in the > > Ethernet frame. > > > > This can happen when the MAC and Switch are tightly integrated on an > > SoC, as is the case with the Qualcomm IPQ4019 for example, where > > the DSA tag is inserted directly into the DMA descriptors. In that > > case, the MAC driver is responsible for sending the tag to the > > switch using the out-of-band medium. To do so, the MAC driver needs > > to have the information of the destination port for that skb. > > > > This out-of-band tagging protocol is using the very beggining of > > the skb headroom to store the tag. The drawback of this approch is > > that the headroom isn't initialized upon allocating it, therefore > > we have a chance that the garbage data that lies there at > > allocation time actually ressembles a valid oob tag. This is only > > problematic if we are sending/receiving traffic on the master port, > > which isn't a valid DSA use-case from the beggining. When dealing > > from traffic to/from a slave port, then the oob tag will be > > initialized properly by the tagger or the mac driver through the > > use of the dsa_oob_tag_push() call. > > > > Signed-off-by: Maxime Chevallier > > --- > > Why put the DSA pseudo-header at skb->head rather than push it using > skb_push()? I thought you were going to check for the presence of a > DSA header using something like skb->mac_len == ETH_HLEN + tag len, > but right now it sounds like treating garbage in the headroom as a > valid DSA tag is indeed a potential problem. If you can't sort that > out using information from the header offsets alone, maybe an skb > extension is required? Indeed, I thought of that, the main reason is that pushing/poping in itself is not enough, you also have to move the whole mac_header to leave room for the tag, and then re-set it in it's original location. There's nothing wrong with this, but it looked a bit cumbersome just to insert a dummy tag that gets removed rightaway. Does that make sense ? But yes I would really like to get a way to know wether the tag is there or not, I'll dig a bit more to see if I can find a way to get this info from the various skb offsets in a reliable way. Thanks, Maxime