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

[Will Deacon]

On Wed, Jan 07, 2015 at 07:29:15PM +0000, Arnd Bergmann wrote:
> On Wednesday 07 January 2015 18:57:05 Will Deacon wrote:
> > Sorry for the delay on this, I had to do a bit of digging.
> > 
> > On Mon, Dec 22, 2014 at 01:36:01PM +0000, Arnd Bergmann wrote:
> > > Do you think it's possible that we might have to deal with a single PCI host
> > > that is connected two different SMMU instances for the purposes of extending
> > > the StreamID space from 15 to 16 bits? I think we would have trouble
> > > expressing this with the current syntax.
> > 
> > Unfortunately, this sounds like something we may well have to support.
> > Whilst the SMMUv2 architecture did grow a late extension to support 16-bit
> > StreamIDs, that may have been too late for all silicon vendors and, as such,
> > I'm not confident that such systems will present a single software interface
> > for their SMMU.
> 
> So it's technically possible to connect two SMMU instances to a single
> PCIe root complex? I knew that there is at least one vendor (AMD) that
> can only do 128 bus numbers on PCIe, which seems a much simpler workaround.

I think you'd need to add some additional logic around the host controller,
but I don't see why it wouldn't be possible to send the first 32k SIDs off
to one SMMU and the other 32k off somewhere else -- those IDs are carried on
the bus, after all.

That said, something being `technically possible' isn't always a good reason
for us to support it :)

Will