To support peer-to-peer traffic on a segment of the PCI hierarchy, we must
disable the ACS redirect bits for select PCI bridges.  The bridges must be
selected before the devices are discovered by the kernel and the IOMMU
groups created.  Therefore, add a kernel command line parameter to specify
devices which must have their ACS bits disabled.

The new parameter takes a list of devices separated by a semicolon.  Each
device specified will have its ACS redirect bits disabled.  This is
similar to the existing 'resource_alignment' parameter.

The ACS Request P2P Request Redirect, P2P Completion Redirect and P2P
Egress Control bits are disabled, which is sufficient to always allow
passing P2P traffic uninterrupted.  The bits are set after the kernel
(optionally) enables the ACS bits itself.  It is also done regardless of
whether the kernel or platform firmware sets the bits.

If the user tries to disable the ACS redirect for a device without the ACS
capability, print a warning to dmesg.
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
[bhelgaas: reorder to add the generic code first and move the
device-specific quirk to subsequent patches]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Stephen Bates <sbates@raithlin.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Acked-by: Christian König <christian.koenig@amd.com>