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

[Dave Martin <Dave.Martin-5wv7dgnIgG8@public.gmane.org>]

On Fri, May 30, 2014 at 09:49:59PM +0200, Arnd Bergmann wrote:
> On Friday 30 May 2014 14:31:55 Rob Herring wrote:
> > On Fri, May 30, 2014 at 2:06 PM, Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org> 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-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> > >> > From: Thierry Reding <treding-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
> > >> > +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.

dma-ranges defines a static mapping for mastering through the bus node.

The whole point of an IOMMU is that it maps dynamically, so I agree:
I'm unclear on what dma-ranges should mean in the IOMMU node itself
(if anything).

> 
> > > 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.

Do we have real examples of using a window *and* an ID?  I thought the
windowed-IOMMU concept was partly a way of encoding the ID in some
real address bits on the bus.  If you're doing that, it seems less likely
that there is a true "ID" as such (though it is possible).

> 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?

In the strictest sense, no.

But for a large set of sane configurations, this probably works.

Small sets of randomly-assigned IDs can just be enumerated one by one.

We wouldn't be able to describe folding and bit shuffling, but we
probably don't want to encourage that anyway.

Cheers
---Dave