PCI: Document MPS parameters pci=pcie_bus_safe, pci=pcie_bus_perf, etc

Document PCIe bus MPS parameters pcie_bus_tune_off, pcie_bus_safe,
pcie_bus_peer2peer, pcie_bus_perf.

These parameters were introduced by Jon Mason <jdmason@kudzu.us> at commit
5f39e6705 and commit b03e7495a8.

[bhelgaas: mention hot-add for pcie_bus_peer2peer]
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

Documentation/kernel-parameters.txt

@@ -2227,6 +2227,21 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 				This sorting is done to get a device
 				order compatible with older (<= 2.4) kernels.
 		nobfsort	Don't sort PCI devices into breadth-first order.
+		pcie_bus_tune_off	Disable PCIe MPS (Max Payload Size)
+				tuning and use the BIOS-configured MPS defaults.
+		pcie_bus_safe	Set every device's MPS to the largest value
+				supported by all devices below the root complex.
+		pcie_bus_perf	Set device MPS to the largest allowable MPS
+				based on its parent bus. Also set MRRS (Max
+				Read Request Size) to the largest supported
+				value (no larger than the MPS that the device
+				or bus can support) for best performance.
+		pcie_bus_peer2peer	Set every device's MPS to 128B, which
+				every device is guaranteed to support. This
+				configuration allows peer-to-peer DMA between
+				any pair of devices, possibly at the cost of
+				reduced performance.  This also guarantees
+				that hot-added devices will work.
 		cbiosize=nn[KMG]	The fixed amount of bus space which is
 				reserved for the CardBus bridge's IO window.
 				The default value is 256 bytes.