How to do it...

Let's see how to do it by following these steps:

1. Again, as in previous sections, the first step is adding our new mmap() method to the driver's struct file_operations:

static const struct file_operations chrdev_fops = {
    .owner          = THIS_MODULE,
    .mmap           = chrdev_mmap,
    .unlocked_ioctl = chrdev_ioctl,
    .llseek         = chrdev_llseek,
    .read           = chrdev_read,
    .write          = chrdev_write,
    .open           = chrdev_open,
    .release        = chrdev_release
};

2. Then, we add the chrdev_mmap() implementation, as explained in the previous section and reported in the following:

static int chrdev_mmap(struct file *filp, struct vm_area_struct *vma)
{
    struct chrdev_device *chrdev = filp->private_data;
    size_t size = vma->vm_end - vma->vm_start;
    phys_addr_t offset = (phys_addr_t) vma->vm_pgoff << PAGE_SHIFT;
    unsigned long pfn;

    /* Does it even fit in phys_addr_t? */
    if (offset >> PAGE_SHIFT != vma->vm_pgoff)
        return -EINVAL;

    /* We cannot mmap too big areas */
    if ((offset > BUF_LEN) || (size > BUF_LEN - offset))
        return -EINVAL;

3. Then, we must get the physical address of the buf buffer:

    /* Get the physical address belong the virtual kernel address */
    pfn = virt_to_phys(chrdev->buf) >> PAGE_SHIFT;

Note that this step won't be needed if we simply wanted to remap the physical address on which our peripheral is mapped.

4. Finally, we can do the remap:

    /* Remap-pfn-range will mark the range VM_IO */
    if (remap_pfn_range(vma, vma->vm_start,
                pfn, size,
                vma->vm_page_prot))
        return -EAGAIN;

    return 0;
}