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>

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 69f97a8..dd80599 100644 (file)
--- a/Documentation/kernel-parameters.txt
+++ b/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.