On Sat, Jul 12, 2014 at 02:22:01PM +0200, Arnd Bergmann wrote:

On Saturday 12 July 2014, Rob Clark wrote:

quoted

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Was there actually a good reason for having the device link to the
iommu rather than the other way around?  How much would people hate it
if I just ignore the generic bindings and use something that works for
me instead.  I mean, it isn't exactly like there is going to be .dts
re-use across different SoC's..  and at least with current IOMMU API
some sort of of_get_named_iommu() API doesn't really make sense.

The thing is, if you end up ignoring the generic binding then we have two
IOMMUs using the same (ARM SMMU) binding and it begs the question as to
which is the more generic! I know we're keen to get this merged, but merging
something that people won't use and calling it generic doesn't seem ideal
either. We do, however, desperately need a generic binding.

yeah, ignoring the generic binding is not my first choice.  I'd rather
have something that works well for everyone.  But I wasn't really sure
if the current proposal was arbitrary, or if there are some
conflicting requirements between different platforms.

The common case that needs to be simple is attaching one (master) device
to an IOMMU using the shared global context for the purposes of implementing
the dma-mapping API.

The way that Thierry's binding does that is the obvious solution to this,
and it mirrors what we do in practically every other subsystem.

That wasn't really the intention, though. We shouldn't be designing
bindings to work well in one use-case or another. My motivation for
doing it this way was that I think it naturally models the flow of
master IDs. They originate within the masters and flow towards the
IOMMU device. In other words, they are a property of the masters so
quite literally should be described in the device tree nodes of the
masters.

Thierry