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Root Complex Integrated Endpoints (RCiEPs) do not have an upstream bridge,
so pci_configure_mps() previously ignored them, which may result in reduced
performance.

Instead, program the Max_Payload_Size of RCiEPs to the maximum supported
value (unless it is limited for the PCIE_BUS_PEER2PEER case).  This also
affects the subsequent programming of Max_Read_Request_Size because Linux
programs MRRS based on the MPS value.

Fixes: 9dae3a97 ("PCI: Move MPS configuration check to pci_configure_device()")
Link: https://lore.kernel.org/r/1585343775-4019-1-git-send-email-ashok.raj@intel.com

Tested-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Ashok Raj <ashok.raj@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: stable@vger.kernel.org