Man page 9 : pci_request_region_exclusive

pci_request_region_exclusive - Reserved PCI I/O and memory resource

int pci_request_region_exclusive(struct pci_dev *  pdev , int  bar , const char *  res_name );

pdev
    PCI device whose resources are to be reserved

bar
    BAR to be reserved

res_name
    Name to be associated with resource.

Mark the PCI region associated with PCI device pdev BR bar as being reserved by owner res_name. Do not access any address inside the PCI regions unless this call returns successfully.

Returns 0 on success, or EBUSY on error. A warning message is also printed on failure.

The key difference that _exclusive makes it that userspace is explicitly not allowed to map the resource via /dev/mem or sysfs.