PCI: Set MPS to match upstream bridge

Firmware typically configures the PCIe fabric with a consistent Max Payload
Size setting based on the devices present at boot.  A hot-added device
typically has the power-on default MPS setting (128 bytes), which may not
match the fabric.

The previous Linux default, in the absence of any "pci=pcie_bus_*" options,
was PCIE_BUS_TUNE_OFF, in which we never touch MPS, even for hot-added
devices.

Add a new default setting, PCIE_BUS_DEFAULT, in which we make sure every
device's MPS setting matches the upstream bridge.  This makes it more
likely that a hot-added device will work in a system with optimized MPS
configuration.

Note that if we hot-add a device that only supports 128-byte MPS, it still
likely won't work because we don't reconfigure the rest of the fabric.
Booting with "pci=pcie_bus_peer2peer" is a workaround for this because it
sets MPS to 128 for everything.

[bhelgaas: changelog, new default, rework for pci_configure_device() path]
Tested-by: Keith Busch <keith.busch@intel.com>
Tested-by: Jordan Hargrave <jharg93@gmail.com>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Yinghai Lu <yinghai@kernel.org>

1 files changed, 5 insertions, 4 deletions

diff --git a/include/linux/pci.h b/include/linux/pci.h
index 4d4f9d2..0007942 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -738,10 +738,11 @@ struct pci_driver {
 void pcie_bus_configure_settings(struct pci_bus *bus);
 
 enum pcie_bus_config_types {
-	PCIE_BUS_TUNE_OFF,
-	PCIE_BUS_SAFE,
-	PCIE_BUS_PERFORMANCE,
-	PCIE_BUS_PEER2PEER,
+	PCIE_BUS_TUNE_OFF,	/* don't touch MPS at all */
+	PCIE_BUS_DEFAULT,	/* ensure MPS matches upstream bridge */
+	PCIE_BUS_SAFE,		/* use largest MPS boot-time devices support */
+	PCIE_BUS_PERFORMANCE,	/* use MPS and MRRS for best performance */
+	PCIE_BUS_PEER2PEER,	/* set MPS = 128 for all devices */
 };
 
 extern enum pcie_bus_config_types pcie_bus_config;