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From:   Arnd Bergmann <arnd@kernel.org>
Date:   Mon, 19 Jul 2021 14:04:08 +0200
Message-ID: <CAK8P3a0udKQk_LLxUD0k2pWk=WSt=9AcaHsCEboYJX=Xj-7mwQ@mail.gmail.com>
Subject: Re: [PATCH 1/5] dt-bindings: virtio: mmio: Add support for device subnode
To:     Viresh Kumar <viresh.kumar@linaro.org>
Cc:     Rob Herring <robh+dt@kernel.org>, Jason Wang <jasowang@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Bill Mills <bill.mills@linaro.org>,
        =?UTF-8?B?QWxleCBCZW5uw6ll?= <alex.bennee@linaro.org>,
        "Enrico Weigelt, metux IT consult" <info@metux.net>,
        Jie Deng <jie.deng@intel.com>,
        DTML <devicetree@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "open list:DRM DRIVER FOR QEMU'S CIRRUS DEVICE" 
        <virtualization@lists.linux-foundation.org>
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On Mon, Jul 19, 2021 at 12:34 PM Viresh Kumar <viresh.kumar@linaro.org> wrote:
> On 14-07-21, 23:07, Arnd Bergmann wrote:
> > On Wed, Jul 14, 2021 at 5:43 PM Rob Herring <robh+dt@kernel.org> wrote:
> > > I guess it comes down to is 'virtio,mmio' providing a bus or is it
> > > just a device? I guess a bus (so 2 nodes) does make sense here.
> > > 'virtio,mmio' defines how you access/discover the virtio queues (the
> > > bus) and the functional device (i2c, gpio, iommu, etc.) is accessed
> > > via the virtio queues.
> >
> > It's not really a bus since there is only ever one device behind it.
> > A better analogy would be your 'serdev' framework: You could
> > have a 8250 or a pl011 uart, and behind that have a mouse, GPS
> > receiver or bluetooth dongle.
> >
> > In Documentation/devicetree/bindings/serial/serial.yaml, you also
> > have two nodes for a single device, so we could follow that
> > example.
>
> So two device nodes is final then ? Pretty much like how this patchset did it
> already ? I need to get rid of reg thing and use "virtio,DID" though.

Let's give Rob another day to reply here. I think two nodes is easier
to get working than one node, even when we continue supporting the
current iommu binding that relies on a single node, but we could make
it work either way if necessary.

       Arnd