PCI: Add Kconfig options for MPS/MRRS strategy

Add Kconfig options for changing the default pcie_bus_config, i.e., the
strategy for configuration MPS and MRRS, in the same manner as the
CONFIG_PCIEASPM_XXXX choice.  The pci_bus_config setting may still be
overridden by kernel command-line parameters, e.g.,
"pci=pcie_bus_tune_off".

[bhelgaas: depend on EXPERT, tweak help texts]
Link: https://lore.kernel.org/r/20200928194651.5393-2-james.quinlan@broadcom.com
Signed-off-by: Jim Quinlan <james.quinlan@broadcom.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/Kconfig

@@ -187,6 +187,68 @@ config PCI_HYPERV
 	  The PCI device frontend driver allows the kernel to import arbitrary
 	  PCI devices from a PCI backend to support PCI driver domains.
 
+choice
+	prompt "PCI Express hierarchy optimization setting"
+	default PCIE_BUS_DEFAULT
+	depends on PCI && EXPERT
+	help
+	  MPS (Max Payload Size) and MRRS (Max Read Request Size) are PCIe
+	  device parameters that affect performance and the ability to
+	  support hotplug and peer-to-peer DMA.
+
+	  The following choices set the MPS and MRRS optimization strategy
+	  at compile-time.  The choices are the same as those offered for
+	  the kernel command-line parameter 'pci', i.e.,
+	  'pci=pcie_bus_tune_off', 'pci=pcie_bus_safe',
+	  'pci=pcie_bus_perf', and 'pci=pcie_bus_peer2peer'.
+
+	  This is a compile-time setting and can be overridden by the above
+	  command-line parameters.  If unsure, choose PCIE_BUS_DEFAULT.
+
+config PCIE_BUS_TUNE_OFF
+	bool "Tune Off"
+	depends on PCI
+	help
+	  Use the BIOS defaults; don't touch MPS at all.  This is the same
+	  as booting with 'pci=pcie_bus_tune_off'.
+
+config PCIE_BUS_DEFAULT
+	bool "Default"
+	depends on PCI
+	help
+	  Default choice; ensure that the MPS matches upstream bridge.
+
+config PCIE_BUS_SAFE
+	bool "Safe"
+	depends on PCI
+	help
+	  Use largest MPS that boot-time devices support.  If you have a
+	  closed system with no possibility of adding new devices, this
+	  will use the largest MPS that's supported by all devices.  This
+	  is the same as booting with 'pci=pcie_bus_safe'.
+
+config PCIE_BUS_PERFORMANCE
+	bool "Performance"
+	depends on PCI
+	help
+	  Use MPS and MRRS for best performance.  Ensure that a given
+	  device's MPS is no larger than its parent MPS, which allows us to
+	  keep all switches/bridges to the max MPS supported by their
+	  parent.  This is the same as booting with 'pci=pcie_bus_perf'.
+
+config PCIE_BUS_PEER2PEER
+	bool "Peer2peer"
+	depends on PCI
+	help
+	  Set MPS = 128 for all devices.  MPS configuration effected by the
+	  other options could cause the MPS on one root port to be
+	  different than that of the MPS on another, which may cause
+	  hot-added devices or peer-to-peer DMA to fail.  Set MPS to the
+	  smallest possible value (128B) system-wide to avoid these issues.
+	  This is the same as booting with 'pci=pcie_bus_peer2peer'.
+
+endchoice
+
 source "drivers/pci/hotplug/Kconfig"
 source "drivers/pci/controller/Kconfig"
 source "drivers/pci/endpoint/Kconfig"

drivers/pci/pci.c

@@ -101,7 +101,19 @@ unsigned long pci_hotplug_mmio_pref_size = DEFAULT_HOTPLUG_MMIO_PREF_SIZE;
 #define DEFAULT_HOTPLUG_BUS_SIZE	1
 unsigned long pci_hotplug_bus_size = DEFAULT_HOTPLUG_BUS_SIZE;
 
+
+/* PCIe MPS/MRRS strategy; can be overridden by kernel command-line param */
+#ifdef CONFIG_PCIE_BUS_TUNE_OFF
+enum pcie_bus_config_types pcie_bus_config = PCIE_BUS_TUNE_OFF;
+#elif defined CONFIG_PCIE_BUS_SAFE
+enum pcie_bus_config_types pcie_bus_config = PCIE_BUS_SAFE;
+#elif defined CONFIG_PCIE_BUS_PERFORMANCE
+enum pcie_bus_config_types pcie_bus_config = PCIE_BUS_PERFORMANCE;
+#elif defined CONFIG_PCIE_BUS_PEER2PEER
+enum pcie_bus_config_types pcie_bus_config = PCIE_BUS_PEER2PEER;
+#else
 enum pcie_bus_config_types pcie_bus_config = PCIE_BUS_DEFAULT;
+#endif
 
 /*
  * The default CLS is used if arch didn't set CLS explicitly and not