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

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Date:   Thu, 3 Sep 2020 11:14:38 +0200
From:   Loic Poulain <loic.poulain@...aro.org>
To:     Jie Deng <jie.deng@...el.com>
Cc:     Jason Wang <jasowang@...hat.com>, linux-i2c@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        open list <linux-kernel@...r.kernel.org>, mst@...hat.com,
        wsa+renesas@...g-engineering.com, wsa@...nel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        jarkko.nikula@...ux.intel.com, jdelvare@...e.de,
        Sergey.Semin@...kalelectronics.ru, krzk@...nel.org,
        rppt@...nel.org, tali.perry1@...il.com,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        shuo.a.liu@...el.com, conghui.chen@...el.com, yu1.wang@...el.com
Subject: Re: [PATCH] i2c: virtio: add a virtio i2c frontend driver

On Thu, 3 Sep 2020 at 09:19, Jie Deng <jie.deng@...el.com> wrote:
>
>
> On 2020/9/3 14:12, Jason Wang wrote:
> >
> > On 2020/9/3 下午1:34, Jie Deng wrote:
> >> Add an I2C bus driver for virtio para-virtualization.
> >>
> >> The controller can be emulated by the backend driver in
> >> any device model software by following the virtio protocol.
> >>
> >> This driver communicates with the backend driver through a
> >> virtio I2C message structure which includes following parts:
> >>
> >> - Header: i2c_msg addr, flags, len.
> >> - Data buffer: the pointer to the i2c msg data.
> >> - Status: the processing result from the backend.
> >>
> >> People may implement different backend drivers to emulate
> >> different controllers according to their needs. A backend
> >> example can be found in the device model of the open source
> >> project ACRN. For more information, please refer to
> >> https://projectacrn.org.
> >
> >
> > May I know the reason why don't you use i2c or virtio directly?
> >
> We don't want to add virtio drivers for every I2C devices in the guests.
> This bus driver is designed to provide a way to flexibly expose the
> physical
> I2C slave devices to the guest without adding or changing the drivers of
> the
> I2C slave devices in the guest OS.

So AFAIU, what you're trying to do here is "I2C slave passthrough over
para-virtualized I2C bus"? While not totally crazy, that looks a bit weird since
a straightforward way would be to directly assign the I2C bus controller as a
passthrough device (vfio?), though I assume your goal is also having per slave
VM assignment control (and not exposing the whole bus)...

>
>
> >
> >>
> >> The virtio device ID 34 is used for this I2C adpter since IDs
> >> before 34 have been reserved by other virtio devices.
> >
> >
> > Is there a link to the spec patch?
> >
> > Thanks
> >
> I haven't submitted the patch to reserve the ID in spec yet.
> I write the ID here because I want to see your opinions first.
>
> Thanks
>
>

Regards,
Loic