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

[Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>]

On Wed, Jun 18, 2014 at 12:37:16AM +0100, Thierry Reding wrote:
> On Tue, Jun 17, 2014 at 01:18:11PM +0100, Will Deacon wrote:
> > On Tue, Jun 17, 2014 at 12:58:30PM +0100, Thierry Reding wrote:
> > > On Mon, Jun 16, 2014 at 01:57:04PM +0100, Will Deacon wrote:
> > > > On Wed, Jun 04, 2014 at 10:12:38PM +0100, Thierry Reding wrote:
> > > > > It can easily be argued that if the algorithm used to remap the ID
> > > > > varies, the compatibility of the device changes. Therefore I would
> > > > > expect any variant of the GICv3 that deviates from the "standard"
> > > > > mapping (if there is such a thing) to have its own compatible string.
> > > > 
> > > > There is no standard mapping; it's a property defined at system integration
> > > > time. I fully expect different SoCs to do different things here.
> > > 
> > > My point was that the mapping itself seems to be fundamental enough to
> > > make devices with different mappings "incompatible". Therefore I think
> > > this could probably be handled by using different compatible values,
> > > something along the lines of this:
> > > 
> > > 	compatible = "vendor,soc-gicv3", "arm,gicv3";
> > > 
> > > Then the mapping can be described in code, which should be a whole lot
> > > easier and more flexible than a more or less generic notation in device
> > > tree.
> > 
> > I don't think that scales well beyond a handful of unique mappings, and I
> > really anticipate everybody doing something different based on their
> > integration constraints.
> > 
> > You'd very quickly end up with sets of tables for each SoC, describing the
> > topology and associated IDs in the kernel source, which feels like a giant
> > step backwards from where we are today with device tree.
> 
> Well, today we don't have a generic binding at all, so anything will
> really be a giant step forward in my opinion.

Oh, I'm not disputing that at all. I just think it's worth considering how
we can extend the binding in future to describe some of the ID routing and
remapping that we've discussed.

> But seriously, from what you said earlier I got the impression that some
> of the mappings may not be easy or possible to represent in DT, which is
> why I proposed to encode it into the compatible property so that it can
> be handled in code instead.

For any old arbitrary mappings, we're going to have a rough time, but that
doesn't mean we need to support that until we're forced to (and then we can
consider our options). What I *do* think we need to describe is:

  - Each master has a set of fixed StreamIDs
  - StreamIDs can be remastered by adding a constant offset (this could also
    be used to describe RequesterID -> StreamID mapping)

I'd hope this would be sufficient for most people. Dynamic ID assignment can
be worked out later (I'm not even sure it belongs in this binding) and any
mappings other than `add a constant offset' can be treated on a case-by-case
basis. We don't want to throw the kitchen sink at a language for describing
arbitrary transformations!

> We've had similar discussions before (power sequences anyone?) where we
> tried to come up with a generic way to describe something in device tree
> that just didn't work out too well. Some things are better done in code,
> so I think we should at least consider that possibility rather than
> blindly try and force everything into device tree.

If we can support 90% of SoCs with a simple DT-based description, we can
address the corner cases as they arise. I'm not ruling our hardcoding
topology if we have no choice, but I don't think that's a healthy place to
start from.

Will