May I know what "inbound" and "outbound" mean? do we have any training material can share with me?
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Apologies for the late reply.
For max payload outbound being 128 byte, I think this is due to the MPS (Max_Payload_Size) field in the capability register for PCIe for AM5716 is 128 bytes, while MRRS (Max_Read_Request_Size) is 256 bytes. The specifics for these registers should be in the standard PCI_Express_2.0_Base_Specification documents that could be searched online if you would like to find out more.
For differences in inbound vs outbound, I think this application note would be beneficial to read, especially section "3 PCIe Address Translation": https://www.ti.com/lit/an/sprabk8/sprabk8.pdf. The application note is for a legacy Keystone device architecture, but the general flow and usage of an ATU (Address Translation Unit) for inbound vs outbound PCIe traffic should be applicable to AM57x as well. Of course, the device specific register address locations are different than AM57x, but the general usage of ATU and the usage of the word "outbound" vs "inbound" should be similar.