Access I/O in non-MMIO mode

configname: CONFIG_INDIRECT_PIO

Linux Kernel Configuration
└─>Library routines
└─>Access I/O in non-MMIO mode
In linux kernel since version 3.10 (release Date: 2013-06-30)  
On some platforms where no separate I/O space exists, there are I/O
hosts which can not be accessed in MMIO mode. Using the logical PIO
mechanism, the host-local I/O resource can be mapped into system
logic PIO space shared with MMIO hosts, such as PCI/PCIe, then the
system can access the I/O devices with the mapped-logic PIO through
I/O accessors.

This way has relatively little I/O performance cost. Please make
sure your devices really need this configure item enabled.

When in doubt, say N.