Enabling peer to peer device transactions for PCIe devices

[Christian König]

Am 24.11.2016 um 00:25 schrieb Jason Gunthorpe:
> There is certainly nothing about the hardware that cares
> about ZONE_DEVICE vs System memory.
Well that is clearly not so simple. When your ZONE_DEVICE pages describe 
a PCI BAR and another PCI device initiates a DMA to this address the DMA 
subsystem must be able to check if the interconnection really works.

E.g. it can happen that PCI device A exports it's BAR using ZONE_DEVICE. 
Not PCI device B (a SATA device) can directly read/write to it because 
it is on the same bus segment, but PCI device C (a network card for 
example) can't because it is on a different bus segment and the bridge 
can't handle P2P transactions.

We need to be able to handle such cases and fall back to bouncing 
buffers, but I don't see that in the DMA subsystem right now.

Regards,
Christian.