in kernel/pci-sysfs.c [350:372]
int pci_legacy_write(struct pci_bus *bus, loff_t port, u32 val, size_t size)
{
struct pci_controller *hose = bus->sysdata;
port += hose->io_space->start;
switch(size) {
case 1:
outb(port, val);
return 1;
case 2:
if (port & 1)
return -EINVAL;
outw(port, val);
return 2;
case 4:
if (port & 3)
return -EINVAL;
outl(port, val);
return 4;
}
return -EINVAL;
}