On Tue, 17 Jul 2018 11:02:04 -0600
Logan Gunthorpe [off-list ref] wrote:

In order to support P2P traffic on a segment of the PCI hierarchy,
we must be able to 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,
a kernel command line parameter is created 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 it's 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 sets the bits or not seeing some BIOS firmware is known
to set the bits on boot.

If the user tries to disable the ACS redirct for a device without the
ACS capability, a warning is printed to dmesg.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Stephen Bates <redacted>
Acked-by: Christian König <christian.koenig@amd.com>
---
 Documentation/admin-guide/kernel-parameters.txt |  9 +++
 drivers/pci/pci.c                               | 76 ++++++++++++++++++++++++-
 2 files changed, 83 insertions(+), 2 deletions(-)

Thanks for the re-spins!

Reviewed-by: Alex Williamson <redacted>
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