Re: [PATCH v6 6/8] dma-mapping: detect and configure IOMMU in of_dma_configure

[Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>]

On Wednesday 17 December 2014 15:01:58 Will Deacon wrote:
> On Wed, Dec 17, 2014 at 02:27:30PM +0000, Robin Murphy wrote:
> > On 17/12/14 12:09, Will Deacon wrote:
> > > I think that's a slightly different case. The `grouping' in the DT, is on a
> > > per-master basis where a master may have a set of StreamIDs, which can be
> > > expressed in a more efficient (per-IOMMU) manner that cannot easily be
> > > determined at runtime.
> > 
> > I'm pretty convinced that automatic SMR allocation is solvable; I was 
> > planning to take a look at it soon, once I've finished warming up the 
> > dormant mathematician in me on implementing the OF group allocator. So 
> > far I'm lucky that stream indexing is sufficient for the bits of our 
> > hardware that actually work.
> 
> Hmm, a few of us have looked at that and failed, so I'm all ears if you
> think you can do it. The main issue is that you need *complete* knowledge
> of the allocated StreamID space before you can safely create any SMR entry
> unless you restrict yourself to consecutive, power-of-2 sized ID regions
> for each master (which doesn't match reality, unfortunately).

Right, scanning the entire device tree in the SMMU driver is
definitely something to avoid.

> > > The problem with all of this is how we distinguish the different ID formats
> > > in the `iommus' device-tree property. For the ARM SMMU, we could have:
> > >
> > >    (1) [easy case] A device has a list of StreamIDs
> > >
> > >    (2) A device has a list of SMR mask/value pairs
> > >
> > >    (3) A (bus) device has a range translation for a downstream bus (e.g.
> > >        a PCI host controller which needs to convert RequesterID to StreamID).
> > >
> > >  From the SMMU driver's perspective, they will all look like of_xlate calls
> > > unless we augment the generic IOMMU bindings with extra properties to
> > > identify the format. It also makes it difficult to pick a sensible value for
> > > #iommu-cells, as it depends on the format.
> > 
> > I thoroughly dislike the idea, but one /could/ simply abuse the generic 
> > bindings well within the current framework, e.g.
> > 
> > #1: iommus = <&smmu 0 streamID 0 0>, <&smmu 0 streamID 0 0>, ...;
> > #2: iommus = <&smmu 1 mask value 0>;
> > #3: iommus = <&smmu 2 start end offset>;
> > 
> > and have a big complicated of_xlate that knows the secret. I don't think 
> > it would be unreasonable to constrain all masters on the same SMMU to 
> > use the same format either - that way you can lose the type indicator 
> > and the padding and the driver simply inspects #iommu-cells in 
> > of_iommu_init and sets up that instance to take the right path in its 
> > of_xlate calls later. Having a handful of single-stream-ID masters 
> > behind the same SMMU as a whacking great bus controller with a massive 
> > range seems like the kind of system-designer-insanity we should 
> > emphatically NOT be looking to accommodate if we can possibly help it.
> > 
> > That said, for the time being I see definite use-cases for #1, which 
> > already works, and #3, for which I really think we should be looking to 
> > add a parallel generic "iommu-ranges" or "iommu-bus-range" binding for 
> > bus controllers, rather than trying to overload "iommus" beyond 
> > describing actual bus masters.
> 
> Yeah, let's avoid overloading the current bindings too much. Adding the
> ranges property makes perfect sense, I'm just trying to to straighten
> out how the "iommus" property should differ depending on the presence of
> SMR groups.

Is a range something that is defined in the hardware spec, or is that
just what you expect the PCI host controllers to use? 

	Arnd