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

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Date:   Tue, 13 Jul 2021 20:49:17 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Rob Herring <robh+dt@...nel.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 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 ? Are there cases where we would need to get the device
pointer from the of_node ? Then we will have two here.

> 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.

> > 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 :)

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

-- 
viresh