Re: [PATCH v2] devicetree: Add generic IOMMU device tree bindings

Arnd Bergmann
2014-05-30T19:50:59Z

On Friday 30 May 2014 14:31:55 Rob Herring wrote:
> On Fri, May 30, 2014 at 2:06 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> > On Friday 30 May 2014 08:16:05 Rob Herring wrote:
> >> On Fri, May 23, 2014 at 3:33 PM, Thierry Reding
> >> <thierry.reding@gmail.com> wrote:
> >> > From: Thierry Reding <treding@nvidia.com>
> >> > +IOMMU master node:
> >> > +==================
> >> > +
> >> > +Devices that access memory through an IOMMU are called masters. A device can
> >> > +have multiple master interfaces (to one or more IOMMU devices).
> >> > +
> >> > +Required properties:
> >> > +--------------------
> >> > +- iommus: A list of phandle and IOMMU specifier pairs that describe the IOMMU
> >> > + master interfaces of the device. One entry in the list describes one master
> >> > + interface of the device.
> >> > +
> >> > +When an "iommus" property is specified in a device tree node, the IOMMU will
> >> > +be used for address translation. If a "dma-ranges" property exists in the
> >> > +device's parent node it will be ignored. An exception to this rule is if the
> >> > +referenced IOMMU is disabled, in which case the "dma-ranges" property of the
> >> > +parent shall take effect.
> >>
> >> Just thinking out loud, could you have dma-ranges in the iommu node
> >> for the case when the iommu is enabled rather than putting the DMA
> >> window information into the iommus property?
> >>
> >> This would probably mean that you need both #iommu-cells and #address-cells.
> >
> > The reason for doing like this was that you may need a different window
> > for each device, while there can only be one dma-ranges property in
> > an iommu node.
>
> My suggestion was that you also put the IDs in the dma-ranges as the
> first cell much as ranges for PCI encodes other information in the
> first cell. Then you can have an entry for each ID. The downside is
> another special case like PCI.
>
> The argument for using #address-cells and #size-cells seems to be to
> align with how ranges work. If that's the route we want to go, then I
> say we should not stop there and actually use dma-ranges as well.
> Otherwise, I don't see the advantage over #iommu-cells.

I can see how dma-ranges in bus nodes work, it just doesn't seem to
have any reasonable meaning in the iommu node itself.

> > I don't understand the problem. If you have stream IDs 0 through 7,
> > you would have
> >
> > master@a {
> > ...
> > iommus = <&smmu 0>;
> > };
> >
> > master@b {
> > ...
> > iommus = <&smmu 1;
> > };
> >
> > ...
> >
> > master@12 {
> > ...
> > iommus = <&smmu 7;
> > };
> >
> > and you don't need a window at all. Why would you need a mask of
> > some sort?
>
> 1 master with 7 IDs like this:
>
> master@a {
> ...
> iommus = <&smmu 0> <&smmu 1> <&smmu 2> <&smmu 3>
> <&smmu 4> <&smmu 5> <&smmu 6> <&smmu 7>;
> };
>
> If there was any sort of window, then it is almost certainly the same
> window for each ID.

Ok, I see. In that case you'd probably want to have #address-cells = <1>
and #size-cells = <1> and give a range of IDs like

iommus = <&smmu 0 8>;

Do you think that ranges can have a meaningful definition with the ARM
SMMU stream IDs?

Arnd