linux-kernel - Re: [PATCH 1/5] dt-bindings: virtio: mmio: Add support for device subnode

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Date:   Tue, 13 Jul 2021 13:34:05 -0600
From:   Rob Herring <robh+dt@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Jason Wang <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Arnd Bergmann <arnd@...nel.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Bill Mills <bill.mills@...aro.org>,
        Alex Bennée <alex.bennee@...aro.org>,
        "Enrico Weigelt, metux IT consult" <info@...ux.net>,
        Jie Deng <jie.deng@...el.com>, devicetree@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list:DRM DRIVER FOR QEMU'S CIRRUS DEVICE" 
        <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH 1/5] dt-bindings: virtio: mmio: Add support for device subnode

On Tue, Jul 13, 2021 at 9:19 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> On 13-07-21, 08:43, Rob Herring wrote:
> > On Tue, Jul 13, 2021 at 4:50 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
> > >
> > > Allow virtio,mmio nodes to contain device specific subnodes. Since each
> > > virtio,mmio node can represent a single virtio device, each virtio node
> > > is allowed to contain a maximum of one device specific subnode.
> >
> > Doesn't sound like we need 2 nodes here. Just add I2C devices as child
> > nodes. You could add a more specific compatible string, but the
> > protocol is discoverable, so that shouldn't be necessary.
>
> I am not sure if it will be a problem, but you can clarify it better.
>
> The parent node (virtio,mmio) is used to create a platform device,
> virtio-mmio, (and so assigned as its of_node) and we create the
> virtio-device from probe() of this virtio-mmio device.
>
> Is it going to be a problem if two devices in kernel use the same
> of_node ?

There shouldn't be. We have nodes be multiple providers (e.g clocks
and resets) already.

> Are there cases where we would need to get the device
> pointer from the of_node ? Then we will have two here.

Rarely...

In any case, should these potential kernel issues be dictating the DT
binding design? No!

>
> > BTW, what's the usecase for these protocols? A standard interface to
> > firmware controlled I2C, GPIO, etc.?
>
> Right now we are looking to control devices in the host machine from
> guests. That's what Linaro's project stratos is doing. There are other
> people who want to use this for other kind of remote control stuff,
> maybe from firmware.

Project stratos means nothing to me.

Direct userspace access to I2C, GPIO, etc. has its issues, we're going
to repeat that with guests?

> > > diff --git a/include/dt-bindings/virtio/virtio_ids.h b/include/dt-bindings/virtio/virtio_ids.h
> > > new file mode 120000
> > > index 000000000000..6e59ba332216
> > > --- /dev/null
> > > +++ b/include/dt-bindings/virtio/virtio_ids.h
> > > @@ -0,0 +1 @@
> > > +../../uapi/linux/virtio_ids.h
> >
> > This will break the devicetree-rebasing tree I think. DT files
> > shouldn't reference kernel files.
>
> We already do this for linux-event-codes.h and so I thought it is the
> right way of doing it :)

Humm, maybe it's okay. Please double check then...

> Else we can create a new copy, which will be a mess, or use hardcoded
> values.

Though you may not need the header based on what Arnd and I have suggested.

Rob