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

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Message-ID: <20210629085213.7a7eqcgkmtk5y7nh@vireshk-i7>
Date:   Tue, 29 Jun 2021 14:22:13 +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

On 29-06-21, 10:27, Wolfram Sang wrote:
> > While we are at it, this has been replaced by a Rust counterpart [1]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.
> 
> Can't promise I can do this before my holidays, but I will try.

Thanks.

> > I am not sure why you say I2C_RDWR isn't supported. The spec and Linux
> 
> This is how I interpreted Arnd's response. I said mulitple times that I
> might be missing something so I double check.
> 
> > SMBUS. To clarify on an earlier point, every virtio transfer may
> > contain one or more struct i2c_msg instances, all processed together
> > (as expected).
> 
> That was the information missing for me so far becasue...
> 
> > 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.
> 
> ... when I checked virtio_i2c_send_reqs(), I also saw
> virtqueue_add_sgs() but I had no idea if this will end up as REP_START
> on the physical bus or not. But it definately should.

Just think of virtqueue_add_sgs() as something that setups the
structures for transfer. The actual stuff at the other end (host)
happens only after virtqueue_kick() is called at the guest (this
notifies the host that data is present now), in response the backend
running at host will re-create the struct i2c_msg and issue:

    struct i2c_rdwr_ioctl_data data;
    data.nmsgs = count;
    data.msgs = msgs;

    return ioctl(adapter->fd, I2C_RDWR, &data);

So we will end up recreating the exact situation as when
virtio_i2c_xfer() is called.

-- 
viresh