PCI_P2PDMA_MAP_BUS_ADDR and PCIe ATS?

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

Hi!

I was looking at the way pci_p2pdma_distance() returns
PCI_P2PDMA_MAP_BUS_ADDR if devices sit beneath a local switch with ACS
disabled or not present.

The client then programs the bus address directly to perform p2p dma,
as handed out by the dma layer.

However if the device (p2pdma client) has IOMMU turned on and ATS
enabled, the device may or may not send a translation request to the
IOMMU. Wouldn't that fail returnint an IOMMU fault if the device is
programmed with the bus address rather than the IOVA?

So if ATS is enabled, shouldn't the device always be handed the IOVA to
avoid this? Granted, untranslated transactions will then take the
round-trip to the host bridge, but pre-translated transactions will
still benefit from the direct routing through the switch, although that
is not directly visible to the kernel p2pdma layer?

Does pci_p2pdma_distance() need to be client ATS-enabled aware?

Any input appreciated,
Thanks,
Thomas