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

Will Deacon
2014-06-04T13:56:04Z

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.

Will