Re: [PATCH v4] devicetree: Add generic IOMMU device tree bindings

[Will Deacon]

Hi Thierry,

On Mon, Jul 14, 2014 at 07:44:53AM +0100, Thierry Reding wrote:
> On Sun, Jul 13, 2014 at 10:43:41AM +0100, Will Deacon wrote:
> > My plan for the ARM SMMU driver is:
> > 
> >   (1) Change ->probe() to walk the device-tree looking for all masters with
> >       phandles back to the SMMU instance being probed
> 
> You and Rob mentioned this several times and I don't understand why the
> SMMU needs to know all masters up front. Is this necessary because it
> needs to program all registers at .probe() time and they can't be
> reprogrammed subsequently? Or is this just some kind of optimization?

It's an optimization to reduce resource usage in the IOMMU, but one that
is *required* by certain platforms (e.g. Olav mentioned his 43-ID master
and Calxeda had something similar).

Basically, in order to perform an ->attach() for a master to a domain, we
need complete knowledge of the system so that we can avoid accidentally
attaching other masters to the same domain. The programming is done using
a form of StreamID wildcarding, so at this point we would need to have
parsed the entire DT to ensure our wildcard doesn't match other masters.

> >   (2) For each master, extract the Stream IDs and add them to the internal
> >       SMMU driver data structures (an rbtree per SMMU instance). For
> >       hotpluggable buses, we'll need a way for the bus controller to
> >       reserve a range of IDs -- this will likely be a later extension to
> >       the binding.
> > 
> >   (3) When we get an ->add() call, warn if it's a device we haven't seen
> >       and reject the addition.
> 
> It seems to me like this would be the logical place to parse stream IDs.

We could do that only if we were guaranteed to have an ->add() call for
*every* master before an ->attach() call for *any* master. I don't think
that is necessarily true.

> You could for example have a case where some device tree contains a node
> for which no driver will ever be loaded (for example because it hasn't
> been built-in, or the device is never used and the module is therefore
> never loaded). That's a situation that you cannot determine by simply
> walking the device tree in the IOMMU's .probe().

Why not? If we're simply searching for phandles to the IOMMU, why does it
matter whether a driver is bound to the master?

> I've always thought about IOMMU masters much in the same way as other
> types of resources, such as memory or interrupts. In the rest of the
> kernel we do carefully try to postpone allocation of these resources
> until they are required, specifically so we don't waste resources when
> they're unused.

See above, they are all required the moment anybody tries an ->attach().

> That's also one of the reasons why I think associating an IOMMU with the
> bus type is bad. Currently if an IOMMU driver thinks it should enable
> translation for a given device, then there's no way for that device's
> driver to opt out again. There may be reasons (performance, hardware
> bugs, ...) for the driver to decide against using the IOMMU for
> translation, but there's currently no way to do that if the IOMMU driver
> disagrees.

Yes, we need a way to associate an IOMMU with a bus instance, but I think
that's a separate topic, no?

Will
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