Read and write functions in src/kernel/include/linux/io.h utilize a static memory region check (of address 0-0xFFFF) to determine when to use I/O port functions in() and out(). For addresses beyond this region, functions utilize memory address operations with the appropriate memory barriers.
This however much not elegant, works fine for x86 platform, however for non-x86 platform it could be problematic. For example, if on some architectures the PCI device bar memory regions with address 0-0xFFFF come up as MEM (or memory mapped regions) instead of I/O port regions, then the read and write functions will still use the static 0xFFFF region threshold check to direct the operations to in() and out() functions. These functions correspond to x86 in and out assembler operations, which perhaps won't work on other architectures, depends on how QNX has implemented the in() and out() functions.
Nevertheless, a better implementation would be to track the PCI bar I/O and MEM address blocks and use a more specific I/O memory threshold in read and write functions. This will still give us a simple and fast check, more over still a correct check. When run on x86 platform, this more specific threshold can be more precise and on non-x86 platform the threshold would be 0 and thus all operations will be done as memory operations.
A further benefit of such an implementation would be the behaviour of the read and write functions will be determined by the src/pci.c functions that are responsible for managing the PCI interface for QNX implementation.
…or non-x86 architectures.