[PATCH v5 02/18] PCI/P2PDMA: Document the TLP attribute assumptions

[Leon Romanovsky <leon@kernel.org>]

From: Leon Romanovsky <leonro@nvidia.com>

P2PDMA selects a mapping without receiving the Request's ordering or
Address Type attributes. Its ACS handles only strictly ordered Requests
carrying an Untranslated address.

Document that the result is not defined for Relaxed Ordering or
ATS-translated Requests because those TLP attributes can select different
routes through the fabric.

Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
---
 Documentation/driver-api/pci/p2pdma.rst | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/driver-api/pci/p2pdma.rst b/Documentation/driver-api/pci/p2pdma.rst
index 63cff9e4d2c9..80f8fec9b0e9 100644
--- a/Documentation/driver-api/pci/p2pdma.rst
+++ b/Documentation/driver-api/pci/p2pdma.rst
@@ -15,6 +15,13 @@ then based on the ACS settings the transaction can route entirely within
 the PCIe hierarchy and never reach the root port. The kernel will evaluate
 the PCIe topology and always permit P2P in these well-defined cases.
 
+This evaluation assumes clients issue strictly ordered Requests carrying an
+Untranslated address. Its result is not defined when clients use Relaxed
+Ordering or issue ATS-translated Requests because those TLP attributes can
+select different routes through the fabric. Unless ACS Translation Blocking
+is enabled, a Port with ACS Direct Translated P2P enabled routes a
+Translated Request directly to the peer regardless of the redirect controls.
+
 However, if the P2P transaction reaches the host bridge then it might have to
 hairpin back out the same root port, be routed inside the CPU SOC to another
 PCIe root port, or routed internally to the SOC.

-- 
2.55.0