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

Arnd Bergmann
2014-06-04T14:01:15Z

On Wednesday 04 June 2014 14:56:01 Will Deacon wrote:
> On Wed, Jun 04, 2014 at 02:44:03PM +0100, Thierry Reding wrote:
> > On Fri, May 30, 2014 at 09:54:37PM +0200, Arnd Bergmann wrote:
> > > On Friday 30 May 2014 22:29:13 Hiroshi Doyu wrote:
> > > > The disadvantage of this is that this limits the max number of streamIDs
> > > > to support. If # of streamID is increased later more than 64, this
> > > > format cannot cover any more. You have to predict the max # of streamIDs
> > > > in advance if steamID is statically assigned.
> > > >
> > >
> > > Well, the iommu specific binding could allow a variable #address-cells.
> > > That way, you just need to know the number of stream IDs for that instance
> > > of the iommu.
> >
> > That sounds fairly complicated to me. I don't see what that buys us over
> > the clarity and simplicity that the above explicit notation gives us. Is
> > it not more common for a device to have a single master rather than a
> > whole bunch of them?
>
> I've never seen a device upstream of an ARM SMMU with a single stream-id;
> they always seem to have a whole bunch of them. Calxeda's SATA controller
> had 10 IDs, for example, and a PL330 DMA controller tends to have at least
> 3.

What are those good for? Would we just always use the first one?

Arnd