Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752684AbdG1SeE (ORCPT ); Fri, 28 Jul 2017 14:34:04 -0400 Received: from mo4-p00-ob.smtp.rzone.de ([81.169.146.162]:22306 "EHLO mo4-p00-ob.smtp.rzone.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752608AbdG1SeC (ORCPT ); Fri, 28 Jul 2017 14:34:02 -0400 X-RZG-AUTH: :P2MHfkW8eP4Mre39l357AZT/I7AY/7nT2yrT1q0ngWNsKR9DbcHksQLwp93IWrV39LZt X-RZG-CLASS-ID: mo00 Subject: Re: [PATCH v2 2/4] can: fixed-transceiver: Add documentation for CAN fixed transceiver bindings To: Franklin S Cooper Jr , Andrew Lunn , linux-kernel@vger.kernel.org, devicetree@vger.kernel.org, netdev@vger.kernel.org, linux-can@vger.kernel.org, wg@grandegger.com, mkl@pengutronix.de, robh+dt@kernel.org, quentin.schulz@free-electrons.com, sergei.shtylyov@cogentembedded.com, dev.kurt@vandijck-laurijssen.be References: <20170728130231.GA10680@airbook.vandijck-laurijssen.be> From: Oliver Hartkopp Message-ID: Date: Fri, 28 Jul 2017 20:33:28 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: <20170728130231.GA10680@airbook.vandijck-laurijssen.be> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2672 Lines: 75 Hi Kurt, On 07/28/2017 03:02 PM, Kurt Van Dijck wrote: >>> The word 'max-arbitration-bitrate' makes the difference very clear. >> >> I think you are mixing up ISO layer 1 and ISO layer 2. > > In order to provide higher data throughput without putting extra limits > on transceiver & wire, the requirement for the round-trip delay to be > within 1 bittime has been eliminated, but only for the data phase when > arbitration is over. > So layer 2 (CAN FD) has been adapted to circumvent the layer 1 > (transceiver + wire) limitations. > > In fact, the round-trip delay requirement never actually did matter for > plain CAN during data bits either. CAN FD just makes use of that, > but is therefore incompatible on the wire. > > I forgot the precise wording, but this is the principle that Bosch > explained on the CAN conference in Nurnberg several years ago, or at > least this is how I remembered it :-) I just checked an example for a CAN FD qualified transceiver http://www.nxp.com/products/automotive-products/energy-power-management/can-transceivers/high-speed-can-transceiver-with-standby-mode:TJA1044 where it states: The TJA1044T is specified for data rates up to 1 Mbit/s. Pending the release of ISO11898-2:2016 including CAN FD and SAE-J2284-4/5, additional timing parameters defining loop delay symmetry are specified for the TJA1044GT and TJA1044GTK. This implementation enables reliable communication in the CAN FD fast phase at data rates up to 5 Mbit/s. and TJA1044GT/TJA1044GTK - Timing guaranteed for data rates up to 5 Mbit/s - Improved TXD to RXD propagation delay of 210 ns > I haven't followed the developments of transceivers, but with the above > principle in mind, it's obvious that any transceiver allows higher > bitrates during the data segment because the TX-to-RX line delay must > not scale with the bitrate. > In reality, maybe not all transceivers will mention this in their > datasheet. > > So whether you call it 'max-arbitration-bitrate' & 'max-data-bitrate' > or 'max-bitrate' & 'max-data-bitrate' does not really matter (I prefer > 1st) but you will one day need 2 bitrates. The question to me is whether it is right option to specify two bitrates OR to specify one maximum bitrate and provide a property that a CAN FD capable propagation delay is available. E.g. max-bitrate max-data-bitrate or max-bitrate canfd-capable // CAN FD capable propagation delay available I assume the optimized propagation delay is 'always on' as the transceiver is not able to detect which kind of bits it is processing. That's why I think providing two bitrates leads to a wrong view on the transceiver. Regards, Oliver