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

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Date:   Wed, 14 Jul 2021 10:20:21 +0200
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Jason Wang <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        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>,
        DTML <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 10:34:03PM +0200, Arnd Bergmann wrote:
> > > 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.
> 
> I think this would be a little different, but it can still work. There is in
> fact already some precedent of doing this, with Jean-Philippe's virtio-iommu
> binding, which is documented in both
> 
> Documentation/devicetree/bindings/virtio/iommu.txt
> Documentation/devicetree/bindings/virtio/mmio.txt
> 
> Unfortunately, those are still slightly different from where I think we should
> be going here, but it's probably close enough to fit into the general
> system.
> 
> What we have with virtio-iommu is two special hacks:
>  - on virtio-mmio, a node with 'compatible="virtio,mmio"' may optionally
>    have an '#iommu-cells=<1>', in which case we assume it's an iommu.
>  - for virtio-pci, the node has the standard PCI 'reg' property but a special
>    'compatible="virtio,pci-iommu"' property that I think is different from any
>    other PCI node.

Yes in retrospect I don't think the compatible property on the PCI
endpoint node is necessary nor useful, we could deprecate it. The OS gets
the IOMMU topology information early from 'iommus', 'iommu-map' and
'#iommu-cells' properties, which is the only reason we need this PCI
endpoint explicitly described in DT. The rest is discovered while probing
just like virtio-mmio.

Thanks,
Jean