After a bit of googling, here's what I've come up with:
The Max Payload Size and TLP are functions related to the PCI-Express bus
The PCI Express
bus uses three "layers" of communication (similar to the layers in the OSI Model (
http://en.wikipedia.org/wiki/Osi_model)) -- physical, data link, and transaction.
The transaction layer is where all "real" communication takes place (the physical layer is the wires, and the data link layer controls/maintains the link between PCI Express devices)
The smallest chunk of data on the transaction layer is known as a "Transaction Layer Packet" or TLP (similarly, the smallest chunk of data on the data link layer is known as a "Data Link Layer Packet" or DLLP)
A TLP consists of both a header (containing packet information), a payload (containing the actual data to be transmitted), and possibly an ECRC (containing error checking info)
The Max Payload Size sets the maximum size of the payload that any TLP can contain. Devices must be able to recieve payloads that large, and must not send payloads any larger.
Essentially what I can gather is that the Max Payload Size sets just how much actual data can be transfered within a single packet along the PCI-Express bus. Increasing the size would increase the theoretical throughput of the link (as more data would be payload rather than header/crc/etc), though does not respond as well to errors (if a single bit is incorrect, all 4096 bytes will be thrown out for retransmission). Personally I'd leave it at the highest value (should be 4096 from what I can tell) since I doubt many packets will be getting corrupted unless you're in a really electrically noisy environment.
JigPu
