linux-kernel - Re: [PATCH v10] i2c: virtio: add a virtio i2c frontend driver

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Message-ID: <20210629041017.dsvzldikvsaade37@vireshk-i7>
Date:   Tue, 29 Jun 2021 09:40:17 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Wolfram Sang <wsa@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Jie Deng <jie.deng@...el.com>,
        Linux I2C <linux-i2c@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        conghui.chen@...el.com, kblaiech@...lanox.com,
        jarkko.nikula@...ux.intel.com,
        Sergey Semin <Sergey.Semin@...kalelectronics.ru>,
        Mike Rapoport <rppt@...nel.org>, loic.poulain@...aro.org,
        Tali Perry <tali.perry1@...il.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        yu1.wang@...el.com, shuo.a.liu@...el.com,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v10] i2c: virtio: add a virtio i2c frontend driver

I will be replying here instead of replying to each and every msg :)

On 28-06-21, 16:58, Wolfram Sang wrote:
> 
> > You can fine Viresh's vhost-user implementation at
> > https://lore.kernel.org/qemu-devel/cover.1617278395.git.viresh.kumar@linaro.org/t/#m3b5044bad9769b170f505e63bd081eb27cef8db2
> 
> It looks OK so far; yet, it is not complete. But it might be bearable
> in the end.

While we are at it, this has been replaced by a Rust counterpart [1]
(as that makes it hypervisor agnostic, which is the goal of my work
here) and I need someone with I2C knowledge to help review it. It
should be okay even if you don't understand Rust a lot, just review
this file[2] which is where most of i2c specific stuff lies.

> > As you say, it does get a bit clumsy, but I think there is also a good argument
> > to be made that the clumsiness is based on the host Linux user interface
> > more than the on the requirements of the physical interface,
> > and that should not have to be reflected in the virtio specification.
> 
> Makes sense to me.
> 
> > Right, this one has come up before as well: the preliminary result
> > was to assume that this probably won't be needed, but would be easy
> > enough to add later if necessary.
> 
> If adding support incrementally works for such an interface, this makes
> sense as well.

Yes, we don't support few of SMBUS transaction (the block ones) as you
specified.

> So, where are we?

The virtio specification is already merged and here is the latest
version [3].

> As I understand, this v10 does not support I2C transactions (or
> I2C_RDWR as you said).

I am not sure why you say I2C_RDWR isn't supported. The spec and Linux
driver (+ my Rust/qemu backend), they all support I2C_RDWR as well as
SMBUS. To clarify on an earlier point, every virtio transfer may
contain one or more struct i2c_msg instances, all processed together
(as expected).

If you see virtio_i2c_send_reqs() in this patch, you will see that it
converts a stream of i2c_req messages to their virtio counterparts and
send them together, consider it a single transaction.

> But you want to support all clients. So, this doesn't match, or?

-- 
viresh

[1] https://github.com/rust-vmm/vhost-device/pull/1
[2] https://github.com/rust-vmm/vhost-device/blob/5aa22c92faac84ab07b6b15a214513556e8b1d01/src/i2c/src/i2c.rs
[3] https://github.com/oasis-tcs/virtio-spec/blob/master/virtio-i2c.tex