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 X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B0636C2B9F4 for ; Mon, 28 Jun 2021 09:04:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9773D61C2E for ; Mon, 28 Jun 2021 09:04:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232462AbhF1JGm (ORCPT ); Mon, 28 Jun 2021 05:06:42 -0400 Received: from mout.kundenserver.de ([212.227.126.133]:53691 "EHLO mout.kundenserver.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232466AbhF1JGj (ORCPT ); Mon, 28 Jun 2021 05:06:39 -0400 Received: from mail-wr1-f52.google.com ([209.85.221.52]) by mrelayeu.kundenserver.de (mreue010 [213.165.67.97]) with ESMTPSA (Nemesis) id 1Mr8vG-1lUWvC25NW-00oIvG; Mon, 28 Jun 2021 11:04:12 +0200 Received: by mail-wr1-f52.google.com with SMTP id d11so20363690wrm.0; Mon, 28 Jun 2021 02:04:12 -0700 (PDT) X-Gm-Message-State: AOAM532nngXd4SqZi5Pdm1ycas4L7lK0xVq2/g0U3xnQj3SL78Ptoe9y 8ZbgEM+OMIgsXiXKVn9nP1/tPQQbF5GclEuGN08= X-Google-Smtp-Source: ABdhPJyRl7QSXH5sB8nExkanlgljChSQUCIUP+JWsaWbuQfEbjisBPDii/DpjNvzI3LCvLAYvpEUSYGhq9xCpMZ1Phw= X-Received: by 2002:adf:e107:: with SMTP id t7mr25246408wrz.165.1624871051873; Mon, 28 Jun 2021 02:04:11 -0700 (PDT) MIME-Version: 1.0 References: <226a8d5663b7bb6f5d06ede7701eedb18d1bafa1.1616493817.git.jie.deng@intel.com> In-Reply-To: From: Arnd Bergmann Date: Mon, 28 Jun 2021 11:01:41 +0200 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v10] i2c: virtio: add a virtio i2c frontend driver To: Wolfram Sang , Jie Deng , Linux I2C , virtualization@lists.linux-foundation.org, Linux Kernel Mailing List , "Michael S. Tsirkin" , Jason Wang , Andy Shevchenko , conghui.chen@intel.com, Arnd Bergmann , kblaiech@mellanox.com, jarkko.nikula@linux.intel.com, Sergey Semin , Mike Rapoport , loic.poulain@linaro.org, Tali Perry , =?UTF-8?Q?Uwe_Kleine=2DK=C3=B6nig?= , Bjorn Andersson , yu1.wang@intel.com, shuo.a.liu@intel.com, Viresh Kumar , Stefan Hajnoczi , Paolo Bonzini Content-Type: text/plain; charset="UTF-8" X-Provags-ID: V03:K1:G+QIDcHqDNDPoQP79t/Wawoy5stDHoFoWqtxd2cTAhgtfoRy1wd YKU1PaYIKtLnyKcUqdf5sGAeI9zDpTJdsj0PiI1kf618Sq4czeCf6lxjhXxGMm4VQ9fmvbb ZJUcVeE6TodE6xI6CEyOl+xXgf0y8VzMUFkzQPNzCv8lGqySI4oS979f1YSD7Y3qiQ66Chk RN9WiFJfDQSDXG0tt30xQ== X-UI-Out-Filterresults: notjunk:1;V03:K0:JOUq9riS6IQ=:MzZgunBCeXCn+fHdLkpRfi ovg+rLECVAFtVplUD1GjjPs19ixFQi1eJRO19W+YAz3o7vkPZ4uWX15qtxH8YmW27qTKm+DsT Vkn3+Sl6GwYjjft4P/3ef2oN8wkCz84c0sifha2sTKqpFC3pzYS2y+dWXOizZy63sqy7dUxdn shepeUsn7+SvpvAA7X2P6zYAW7WEGQdHQJecusD61EualmI+KYgWNsZJSk2q4teOl2A7Gs7tc WpFnO9PhsjJNnBxaKOA85MRg/TasbeLYzG+t2iQ1urlMBJqQpzAiaQE5EFsDmXircvlXjbVSn v/ZciI7ltpZpNw49gSUgCBnttPRJa+pcYV5yzeUYQLlZo+ktjjmGrUDzF5+jDHVsrA1VigD3B Wq40TGt9ySP6USHVeVWXaeJMvhrfZCDEtmLxMnjHYLthfUgofA/28jXzRbMDTu89WpzvY2kf8 j86F8GK5ocN74olSjfuBYM3JN1UGZ/NATTlv/iF7vi1a8demBcU6Pt1JxpDs7aMv7aCiRTIOL 9Fm4ConDE639clhMIqzFMbzhxqyZnRFRypxx8zIkAMSGVwSY4DYKkPt7yQM1/nkjBa/dqyVT8 30lqFEoLwZ5X/iFw2OuUm0d0gH6IxOmiSIfg+vD8j3gKjT+WAE/gm1mZqDfdf5PL6uL8Mb/hP z0TY= Precedence: bulk List-ID: X-Mailing-List: linux-i2c@vger.kernel.org On Mon, Jun 28, 2021 at 10:39 AM Wolfram Sang wrote: > > sorry for the long delay. I am not familiar with VFIO, so I had to dive > into the topic a little first. I am still not seeing through it > completely, so I have very high-level questions first. You probably know this already, but just in case for clarification these are two different things: VFIO: kernel feature to make raw (usually PCI) devices available to user space drivers and virtual machines from a kernel running on bare metal. virtio: transport protocol for implementing arbitrary paravirtualized drivers in (usually) a virtual machine guest without giving the guest access to hardware registers. Both can be used for letting a KVM guest talk to the outside world, but usually you have one or the other, not both. > > The device specification can be found on > > https://lists.oasis-open.org/archives/virtio-comment/202101/msg00008.html. > > I think we need to start here: > > === > > If ``length of \field{read_buf}''=0 and ``length of \field{write_buf}''>0, > the request is called write request. > > If ``length of \field{read_buf}''>0 and ``length of \field{write_buf}''=0, > the request is called read request. > > If ``length of \field{read_buf}''>0 and ``length of \field{write_buf}''>0, > the request is called write-read request. It means an I2C write segment followed > by a read segment. Usually, the write segment provides the number of an I2C > controlled device register to be read. > > === > > I2C transactions can have an arbitrary number of messages which can > arbitrarily be read or write. As I understand the above, only one read, > write or read-write transaction is supported. If that is the case, it > would be not very much I2C but more SMBus. If my assumptions are true, > we first need to decide if you want to go the I2C way or SMBus subset. This has come up in previous reviews already. I think it comes down to the requirement that the virtio i2c protocol should allow passthrough access to any client devices connected to a physical i2c bus on the host, and this should ideally be independent of whether the host driver exposes I2C_RDWR or I2C_SMBUS ioctl interface, or both. This can be done either by having both interface types in the transport, or picking one of the two, and translating to the host interface type in software. As far as I understand me (please clarify), implementing only the smbus subset would mean that we cannot communicate with all client devices, while implementing both would add more complexity than the lower-level protocol. > === > > The case when ``length of \field{write_buf}''=0, and at the same time, > ``length of \field{read_buf}''=0 doesn't make any sense. > > === > > Oh, it does. That's a legal transfer, both in SMBus and I2C. It is used > to e.g. discover devices. I think it should be supported, even though > not all bus master drivers on the host can support it. Is it possible? > > Also, as I read it, a whole bus is para-virtualized to the guest, or? > Wouldn't it be better to allow just specific devices on a bus? Again, I > am kinda new to this, so I may have overlooked things. Do you mean just allowing a single device per bus (as opposed to having multiple devices as on a real bus), or just allowing a particular set of client devices that can be identified using virtio specific configuration (as opposed to relying on device tree or similar for probing). Both of these are valid questions that have been discussed before, but that could be revisited. Arnd