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Linux 2.6 - man page for s390_pci_mmio_read (linux section 2)

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 map- ping 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.
EFAULT The address in mmio_addr is invalid. EFAULT user_buffer does not point to a valid location in the caller's address space. EINVAL Invalid length argument. ENODEV PCI support is not enabled. ENOMEM 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.
syscall(2) Linux Programmer's Manual 2015-01-15 S390_PCI_MMIO_WRITE(2)