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 A8CD9C04A68 for ; Wed, 27 Jul 2022 20:12:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234134AbiG0UMb (ORCPT ); Wed, 27 Jul 2022 16:12:31 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51094 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233755AbiG0UMa (ORCPT ); Wed, 27 Jul 2022 16:12:30 -0400 Received: from mo4-p01-ob.smtp.rzone.de (mo4-p01-ob.smtp.rzone.de [85.215.255.53]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1135D564C9 for ; Wed, 27 Jul 2022 13:12:28 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; t=1658952734; s=strato-dkim-0002; d=hartkopp.net; h=In-Reply-To:From:References:Cc:To:Subject:Date:Message-ID:Cc:Date: From:Subject:Sender; bh=zf3hu7M3LvvEmzfKZIjNzdc/JfMNFzqSTpMxpu54JFk=; b=NWM1ZklkAQkxY8sVcrnmQ6XqisOoSiNw3AKr/n8DDwn3BbXMnSTJ6IBpYBZKjtOI8e CRTt13uKFhHTSE0iSIO7yEOUnGKPn0slpRIlnanx9Dz8ITIvRnbJB0BLegWv2d97cWP5 ImUbDxO3TlK/KLBxMhqe5OqF/ceUxqh/V+5rMNvkYGEScLj71qwsS+ZczsdC2PFPynkd 2IJ9dTHCtUP9WFRH/E+eWhYmV7p0QPQE6iS3QpUzpGNYr3f7xuXy/rxbrMTo+aGoYeaq 7ZxiJWh9If1eQ2Iy/6QGSPL9vKbkgYn3VISp3K6QkKLaXtFURBkGXoS1lwwOdXXkaCdX Fc4w== Authentication-Results: strato.com; dkim=none X-RZG-AUTH: ":P2MHfkW8eP4Mre39l357AZT/I7AY/7nT2yrDxb8mjG14FZxedJy6qgO1qCHSa1GLptZHusx3hdIrpKytJSr63tDxrw==" X-RZG-CLASS-ID: mo00 Received: from [IPV6:2a00:6020:1cfd:d100::b82] by smtp.strato.de (RZmta 47.47.0 AUTH) with ESMTPSA id Icb1b0y6RKCD5ow (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256 bits)) (Client did not present a certificate); Wed, 27 Jul 2022 22:12:13 +0200 (CEST) Message-ID: Date: Wed, 27 Jul 2022 22:12:13 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 Subject: Re: [RFC PATCH v3 8/9] can: slcan: add support to set bit time register (btr) Content-Language: en-US To: Marc Kleine-Budde , Max Staudt Cc: Dario Binacchi , linux-kernel@vger.kernel.org, linux-can@vger.kernel.org, michael@amarulasolutions.com, Amarula patchwork , Jeroen Hofstee , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Wolfgang Grandegger , netdev@vger.kernel.org References: <20220726210217.3368497-1-dario.binacchi@amarulasolutions.com> <20220726210217.3368497-9-dario.binacchi@amarulasolutions.com> <20220727113054.ffcckzlcipcxer2c@pengutronix.de> <20220727192839.707a3453.max@enpas.org> <20220727182414.3mysdeam7mtnqyfx@pengutronix.de> From: Oliver Hartkopp In-Reply-To: <20220727182414.3mysdeam7mtnqyfx@pengutronix.de> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-can@vger.kernel.org On 27.07.22 20:24, Marc Kleine-Budde wrote: > On 27.07.2022 19:28:39, Max Staudt wrote: >> On Wed, 27 Jul 2022 13:30:54 +0200 >> Marc Kleine-Budde wrote: >> >>> As far as I understand, setting the btr is an alternative way to set the >>> bitrate, right? I don't like the idea of poking arbitrary values into a >>> hardware from user space. >> >> I agree with Marc here. >> >> This is a modification across the whole stack, specific to a single >> device, when there are ways around. >> >> If I understand correctly, the CAN232 "S" command sets one of the fixed >> bitrates, whereas "s" sets the two BTR registers. Now the question is, >> what do BTR0/BTR1 mean, and what are they? If they are merely a divider >> in a CAN controller's master clock, like in ELM327, then you could >> >> a) Calculate the BTR values from the bitrate userspace requests, or > > Most of the other CAN drivers write the BTR values into the register of > the hardware. How are these BTR values transported into the driver? > > There are 2 ways: > > 1) - user space configures a bitrate > - the kernel calculates with the "struct can_bittiming_const" [1] given > by driver and the CAN clock rate the low level timing parameters. > > [1] https://elixir.bootlin.com/linux/v5.18/source/include/uapi/linux/can/netlink.h#L47 > > 2) - user space configures low level bit timing parameter > (Sample point in one-tenth of a percent, Time quanta (TQ) in > nanoseconds, Propagation segment in TQs, Phase buffer segment 1 in > TQs, Phase buffer segment 2 in TQs, Synchronisation jump width in > TQs) > - the kernel calculates the Bit-rate prescaler from the given TQ and > CAN clock rate > > Both ways result in a fully calculated "struct can_bittiming" [2]. The > driver translates this into the hardware specific BTR values and writes > the into the registers. > > If you know the CAN clock and the bit timing const parameters of the > slcan's BTR register you can make use of the automatic BTR calculation, > too. Maybe the framework needs some tweaking if the driver supports both > fixed CAN bit rate _and_ "struct can_bittiming_const". > > [2] https://elixir.bootlin.com/linux/v5.18/source/include/uapi/linux/can/netlink.h#L31 > >> b) pre-calculate a huge table of possible bitrates and present them >> all to userspace. Sounds horrible, but that's what I did in can327, >> haha. Maybe I should have reigned them in a little, to the most >> useful values. > > If your adapter only supports fixed values, then that's the only way to > go. > >> c) just limit the bitrates to whatever seems most useful (like the >> "S" command's table), and let users complain if they really need >> something else. In the meantime, they are still free to slcand or >> minicom to their heart's content before attaching slcan, thanks to >> your backwards compatibility efforts. > > In the early stages of the non-mainline CAN framework we had tables for > BTR values for some fixed bit rates, but that turned out to be not > scaleable. The Microchip CAN BUS Analyzer Tool is another example for fixed bitrates: https://elixir.bootlin.com/linux/v5.18.14/source/drivers/net/can/usb/mcba_usb.c#L156 So this might be the way to go here too ... Best regards, Oliver > >> In short, to me, this isn't a deal breaker for your patch series. > > Marc >