S390_PCI_MMIO_WRITE(2) System Calls Manual S390_PCI_MMIO_WRITE(2)

s390_pci_mmio_write, s390_pci_mmio_read - transfer data to/from PCI MMIO memory page

#include <asm/unistd.h>
int s390_pci_mmio_write(unsigned long mmio_addr,
                        void *user_buffer, size_t length);
int s390_pci_mmio_read(unsigned long mmio_addr,
                        void *user_buffer, size_t length);

The s390_pci_mmio_write() system call writes length bytes of data from the user-space buffer user_buffer to the PCI MMIO memory location specified by mmio_addr. The s390_pci_mmio_read() system call reads length bytes of data from the PCI MMIO memory location specified by mmio_addr to the user-space buffer user_buffer.
These system calls must be used instead of the simple assignment or data-transfer operations that are used to access the PCI MMIO memory areas mapped to user space on the Linux System z platform. The address specified by mmio_addr must belong to a PCI MMIO memory page mapping in the caller's address space, and the data being written or read must not cross a page boundary. The length value cannot be greater than the system page size.

On success, s390_pci_mmio_write() and s390_pci_mmio_read() return 0. On error, -1 is returned and errno is set to one of the error codes listed below.

The address in mmio_addr is invalid.
user_buffer does not point to a valid location in the caller's address space.
Invalid length argument.
PCI support is not enabled.
Insufficient memory.

These system calls are available since Linux 3.19.

This Linux-specific system call is available only on the s390 architecture. The required PCI support is available beginning with System z EC12.

Glibc does not provide a wrapper for this system call, use syscall(2) to call it.

2017-09-15 Linux Programmer's Manual