[RFC] Describing arbitrary bus mastering relationships in DT

[swarren@wwwdotorg.org (Stephen Warren)]

On 05/02/2014 09:19 AM, Arnd Bergmann wrote:
> On Friday 02 May 2014 15:23:29 Thierry Reding wrote:
...
>> To some degree this also depends on how we want to handle IOMMUs. If
>> they should remain transparently handled via dma_map_ops, then it makes
>> sense to set this up at device instantiation time. But how can we handle
>> this in situations where one device needs to master on two IOMMUs at the
>> same time? Or if the device needs physically contiguous memory for
>> purposes other than device I/O. Using dma_map_ops we can't control which
>> allocations get mapped via the IOMMU and which don't.
> 
> I still hope we can handle this in common code by selecting the right
> dma_map_ops when the devices are instantiated, at least for 99% of the
> cases. I'm not convinced we really need to handle the 'multiple IOMMUs
> on one device' case in a generic way. If there are no common use cases
> for that, we can probably get away with having multiple device nodes
> and an ugly driver for the exception, instead of making life complicated
> for everybody.

By "multiple device nodes", I assume you mean device tree nodes? I'm not
sure I like the sound of that.

I believe that DT should represent the structure of the HW in terms of
HW modules or blocks. If there's a single cohesive HW module that
happens to talk to multiple MMUs, or indeed has any kind of unusual case
at all, I don't think that should force the DT representation to be
broken up into multiple nodes. We should have a DT node for that HW
module, and it should be up to the device driver to make the internal SW
representation work correctly.