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

Will Deacon
2014-06-04T16:41:31Z

On Wed, Jun 04, 2014 at 03:35:10PM +0100, Thierry Reding wrote:
> On Mon, Jun 02, 2014 at 11:41:04AM +0100, Dave Martin wrote:
> > 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.
>
> I'm having some difficulty understanding this. You make it sound like
> there's a fairly arbitrary number of IDs that the SMMU can handle. So
> how is the mapping to devices defined? If you say encourage that does
> make it sound like the assignment of IDs is purely defined by some
> mechanism in software rather than in hardware. Or they are more or less
> randomly picked by someone. If that's the case, is that not something
> that should be dynamically allocated by the kernel rather than put into
> the device tree?

The set of StreamIDs that can be generated by a master is fixed in the
hardware. The SMMU can then be programmed to map these incoming IDs onto
a context ID (or a set of context IDs), which are the IDs used internally
by the SMMU to find the page tables etc.

The StreamID -> ContextID mapping is dynamic and controlled by software.
The Master -> StreamIDs mapping is fixed in the hardware.

Will