Re: [PATCH v2 1/5] dt-bindings: virtio: Specify #iommu-cells value for a virtio-iommu

[Robin Murphy <robin.murphy-5wv7dgnIgG8@public.gmane.org>]

On 27/06/18 18:46, Rob Herring wrote:
> On Tue, Jun 26, 2018 at 11:59 AM Jean-Philippe Brucker
> <jean-philippe.brucker-5wv7dgnIgG8@public.gmane.org> wrote:
>>
>> On 25/06/18 20:27, Rob Herring wrote:
>>> On Thu, Jun 21, 2018 at 08:06:51PM +0100, Jean-Philippe Brucker wrote:
>>>> A virtio-mmio node may represent a virtio-iommu device. This is discovered
>>>> by the virtio driver at probe time, but the DMA topology isn't
>>>> discoverable and must be described by firmware. For DT the standard IOMMU
>>>> description is used, as specified in bindings/iommu/iommu.txt and
>>>> bindings/pci/pci-iommu.txt. Like many other IOMMUs, virtio-iommu
>>>> distinguishes masters by their endpoint IDs, which requires one IOMMU cell
>>>> in the "iommus" property.
>>>>
>>>> Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker-5wv7dgnIgG8@public.gmane.org>
>>>> ---
>>>>   Documentation/devicetree/bindings/virtio/mmio.txt | 8 ++++++++
>>>>   1 file changed, 8 insertions(+)
>>>>
>>>> diff --git a/Documentation/devicetree/bindings/virtio/mmio.txt b/Documentation/devicetree/bindings/virtio/mmio.txt
>>>> index 5069c1b8e193..337da0e3a87f 100644
>>>> --- a/Documentation/devicetree/bindings/virtio/mmio.txt
>>>> +++ b/Documentation/devicetree/bindings/virtio/mmio.txt
>>>> @@ -8,6 +8,14 @@ Required properties:
>>>>   - reg:              control registers base address and size including configuration space
>>>>   - interrupts:       interrupt generated by the device
>>>>
>>>> +Required properties for virtio-iommu:
>>>> +
>>>> +- #iommu-cells:     When the node describes a virtio-iommu device, it is
>>>> +            linked to DMA masters using the "iommus" property as
>>>> +            described in devicetree/bindings/iommu/iommu.txt. For
>>>> +            virtio-iommu #iommu-cells must be 1, each cell describing
>>>> +            a single endpoint ID.
>>>
>>> The iommus property should also be documented for the client side.
>>
>> Isn't section "IOMMU master node" of iommu.txt sufficient? Since the
>> iommus property applies to any DMA master, not only virtio-mmio devices,
>> the canonical description in iommu.txt seems the best place for it, and
>> I'm not sure what to add in this file. Maybe a short example below the
>> virtio_block one?
> 
> No, because somewhere we have to capture if 'iommus' is valid for
> 'virtio-mmio' or not. Hopefully soon we'll actually be able to
> validate that.

Indeed, it's rather unusual to have a single compatible which may either 
be an IOMMU or an IOMMU client (but not both at once, I hope!), so 
nailing down the exact semantics as clearly as possible would definitely 
be desirable.

Robin.