How to Reserve RAM

Determine total memory available.

cat /proc/meminfo dumps the memory information

krmohan@krmohan:~$ cat /proc/meminfo
MemTotal:        4040084 kB  (~ 3.8 G)
MemFree:         3125056 kB  (~ 2.9 G) free

Locate grub file on your system.
1) Ubuntu
Add or append an entry to /etc/default/grub
GRUB_CMDLINE_LINUX="mem=2000m"

Rebuild grub.cfg with your changes
krmohan@krmohan:~$ sudo update-grub


or

memmap=64M$1024M in grub file reserve 64MB at offset 1G


Reboot your system

 krmohan@krmohan:~$ cat /proc/meminfo
MemTotal:        2001116 kB  (~1.8 G)  2 G gone after your changes
MemFree:         1318796 kB

Driver Changes
============
In your init or probe function

{
      /* Tell the kernel you have reserved this resource, thus preventing other driver to do the same */
       request_mem_region(2000*1024*1024, size, "who r u");
       /* get the virtual address of the physical address */
     virt_addr =  ioremap_nocache(2000*1024*1024, size);
}

Make sure, you release the resource in _exit or release
{
       /* Tell kernel that you are done with the resource */
       release_mem_region(2000*1024*1024, size);
      /* unmap io memory */
       iounmap(virt_addr);
}

read_entry_point()
{
           printk(KERN_INFO "ioread32 %x",  ioread32(virt_addr + i));
}

In Userspace
=========
#define RAM_START       (2000 * 1024 * 1024)
#define MAP_SIZE        (unsigned)48*1024*1024
#define MAP_MASK        (MAP_SIZE - 1)

volatile        char *hw = (volatile char *)RAM_START;
memfd = open("/dev/mem", O_RDWR|O_SYNC);
membase = mmap(0,  size, ROT_READ|PROT_WRITE, MAP_SHARED, memfd, (unsigned) hw & ~MAP_MASK);
(unsigned int *)membase[i] = rand();

fd = open("my_driver", O_RDONLY);
read(fd, x, 1);   // verify with kernel message, what u wrote was seen by driver.
close(fd)

In kernel
======
 #define OFFSET 0x40000000   //1G



mmap()
{
       unsigned long vsize = vma->vm_end - vma->vm_start;
     /* mmap physical memory @ 1G */
        if (remap_pfn_range(vma, vma->vm_start,
                        //virt_to_phys(virt_addr) >> PAGE_SHIFT,
                        OFFSET >> PAGE_SHIFT,
                vsize, vma->vm_page_prot)) {
                printk(KERN_INFO "Failed ...\n");
                return -EAGAIN;
        }
        printk(KERN_INFO "Size mmap is %ld\n", vsize);
        return (0);
}