Are linux IO ports software abstractions or real hardware ports

Question

I am learning linux device drivers. I learned about ports. I am confused that are ports in linux a software abstraction or are real hardware ports.

In the below device driver I wrote a code to allocate a port and to write or read.

The port is allocated even if there is no new hardware connected. What can be the reason?

Another problem is that when I write into the port using oub() and when I read from port using inb() then I always get the value 255 irrespective of what I wrote into the port.

#include<linux/init.h>
#include<linux/module.h>
#include<linux/kernel.h>
#include<linux/ioport.h>

struct resource *p;

static int start(void)
{
    printk("module registered\n");
    p=request_region(0x0062, 1, "my_port");
    if(p==NULL)
    {
            printk(KERN_ALERT "port allocation failed\n");
            return 0;
    }
    outb(12, 0x0062);
    return 0;
}

static void stop(void)    
{
    printk("module unregistered\n");
    unsigned a;
    a=inb(0x0062);
    printk("%d\n", a);
    release_region(0x0062, 1);
}

module_init(start);
module_exit(stop);
MODULE_LICENSE("GPL");

I allocate the port and write into it while inserting the module and read from it while unregistering the module.

Thanks in advance for any help

Answer 1

Unfortunately, in the course of the years, CPU manufacturers did not agree on a single mechanism for accessing devices' registers. There are therefore two modes, and a single hardware architecture can implement one mode or the other. These two modes are

• Port-mapped I/O (PMIO):
  • Separate address space for memory and I/O
  • Implemented by a few CPU manufacturers (x86 included)
  • Concept of I/O ports
• Memory-mapped I/O (MMIO):
  • Same address space for memory and I/O
  • Most CPU manufacturers (e.g., ARM)
  • The difference between registers and memory is transparent to software
  • Concept of I/O memory

The book Linux Device Drivers 3rd edition (freely available on-line) illustrates this difference and how to access each kind of I/O from a kernel driver.