Re: [PATCH v2] documentation: iommu: add description of ARM System MMU binding

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

On Mon, May 13, 2013 at 10:50:20AM +0100, Andreas Herrmann wrote:
> Hi Will,

Hi Andreas,

> so far, I thought, that this proposal is fine. After I have tried to
> make use of the binding I have some points that might need further
> disucssion.

Sure, although I've been using the binding without issues so it would be
interesting to understand your use-case and why it's raising problems.

> Olav already asked (in another thread) how to model the mapping of
> stream IDs to contexts for master devices that support multiple
> contexts.  I doubt that this is fully covered yet.

Yeah, I've been meaning to reply to that. Given the way that the IOMMU API
is structure in Linux, I don't think having multiple stream IDs per (struct)
device, where each StreamID points at a different context (and therefore
address space) makes much sense. It also doesn't solve the more general
problem where StreamIDs for a device might have different SMMUs downstream.

To solve this, I think it is better to treat the device as having multiple
struct device instances, and managing the mappings in the device driver.

For most devices, I'd expect a single context to be enough. This is
certainly the case for device virtualisation at stage-2 and DMA at stage-1.

> I also think that it is more useful to move the stream-id property to
> the device node of a master device. (It's a characteristic of the
> master device not of the SMMU.) Currently with multiple stream IDs per
> master device you have repeated entries in the mmu-master property.

The problem with that approach is how to handle StreamID remastering. This
can and will happen, so the StreamID for a device is actually a property of
both the device *and* a particular point in the bus topology. Putting this
information in the device nodes will drag topology information all over the
place and I don't think it will make things clearer to read or easier to parse.

> But all that is needed is to point (once) to each mmu-master in the
> SMMU device node. Then you should be able to look up the corresponding
> stream IDs in the device node for each mmu-master.

Again, you also need to tie in topology information if you go down this
route.

Will