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

On Wednesday 04 June 2014 14:56:01 Will Deacon wrote:

On Wed, Jun 04, 2014 at 02:44:03PM +0100, Thierry Reding wrote:

quoted

On Fri, May 30, 2014 at 09:54:37PM +0200, Arnd Bergmann wrote:

quoted

On Friday 30 May 2014 22:29:13 Hiroshi Doyu wrote:

quoted

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