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 268D0C636CC for ; Tue, 7 Feb 2023 07:10:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230161AbjBGHKU (ORCPT ); Tue, 7 Feb 2023 02:10:20 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43854 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231324AbjBGHKN (ORCPT ); Tue, 7 Feb 2023 02:10:13 -0500 Received: from soltyk.jannau.net (soltyk.jannau.net [144.76.91.90]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4E225227BD; Mon, 6 Feb 2023 23:10:11 -0800 (PST) Received: by soltyk.jannau.net (Postfix, from userid 1000) id 5DDC326F72B; Tue, 7 Feb 2023 08:10:09 +0100 (CET) Date: Tue, 7 Feb 2023 08:10:09 +0100 From: Janne Grunau To: Andrew Lunn Cc: Rob Herring , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Krzysztof Kozlowski , Mailing List , Kalle Valo , van Spriel , =?iso-8859-1?B?Suly9G1l?= Pouiller , netdev@vger.kernel.org, devicetree@vger.kernel.org, linux-kernel@vger.kernel.org, linux-wireless@vger.kernel.org Subject: Re: [PATCH RFC 0/3] dt-bindings: net: Add network-class.yaml schema Message-ID: <20230207071009.GB9004@jannau.net> References: <20230203-dt-bindings-network-class-v1-0-452e0375200d@jannau.net> <20230206163154.GA9004@jannau.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On 2023-02-07 02:34:41 +0100, Andrew Lunn wrote: > > > > I've ignored "max-frame-size" since the description in > > > > ethernet-controller.yaml claims there is a contradiction in the > > > > Devicetree specification. I suppose it is describing the property > > > > "max-frame-size" with "Specifies maximum packet length ...". > > > > > > Please include it and we'll fix the spec. It is clearly wrong. 2 nios > > > boards use 1518 and the consumer for them says it is MTU. Everything > > > else clearly uses mtu with 1500 or 9000. > > > > Ok, the example in the pdf is 'max-frame-size = <1518>;'. I'll include > > it with the description of ethernet-controller.yaml which specifies it > > as MTU. > > You need to be careful here. Frame and MTU are different things. yes, we are aware. The description in of the property in Documentation/devicetree/bindings/net/ethernet-controller.yaml is: | Maximum transfer unit (IEEE defined MTU), rather than the | maximum frame size (there\'s contradiction in the Devicetree | Specification). The description for the property in the Devicetree is: | Specifies maximum packet length in bytes that the physical interface | can send and receive. While the "packet length" in the description is a little confusing this seems to refer to the ethernet frame size. > The IEEE 802.3 standard says nothing about MTU. I believe MTU is an IP > concept. It is the size of the SDU an Ethernet PDU can carry. This is > typically 1500. > > Historically, the max Ethernet frame size was 1518. But with 802.1Q > which added the VLAN header, all modern hardware actual uses 1522 to > accommodate the extra 4 bytes VLAN header. So i would not actually put > max-frame-size = <1518> anywhere, because it will get copy/pasted and > break VLAN setups. > > It looks like the ibm,emac.txt makes this error, max-frame-size = > <5dc>; 0x5dc is 1500. And there are a few powerpc .dtc using > 1500/0x5dc, which are probably broken. I would not say it is an error. The specification/name and use of "max-frame-size" has clearly diverged. All 4 in-tree users of this property interpret it as MTU. With the exception of the 2 nios2 boards Rob found all device trees use either 1500, 3800 or 9000 as 'max-frame-size'. I think Rob's plan to deal with this conflict between specification and actual use is to accept the use and update the description in the specification. This results in a "max-frame-size" property which describes the maximal payload / MTU. The upside of this is that we can leave all devicetrees and drivers unchanged and avoid breaking out-of-tree users. I'll fix the 2 nios2 boards since those currently end up with a MTU of 1518 in altera_tse_main.c. Janne