Specifications

This is the Title of the Book, eMatter Edition

426

Chapter 15: Memory Mapping and DMA

derived from drivers/char/mem.c and shows how this task is performed in a typical mod-

ule called simple (Simple Implementation Mapping Pages with Little Enthusiasm):

static int simple_remap_mmap(struct file *filp, struct vm_area_struct *vma)

{

if (remap_pfn_range(vma, vma->vm_start, vm->vm_pgoff,

vma->vm_end - vma->vm_start,

vma->vm_page_prot))

return -EAGAIN;

vma->vm_ops = &simple_remap_vm_ops;

simple_vma_open(vma);

return 0;

}

As you can see, remapping memory just a matter of calling remap_pfn_range to cre-

ate the necessary page tables.

Adding VMA Operations

As we have seen, the vm_area_struct structure contains a set of operations that may

be applied to the VMA. Now we look at providing those operations in a simple way.

In particular, we provide open and close operations for our VMA. These operations

are called whenever a process opens or closes the VMA; in particular, the open

method is invoked anytime a process forks and creates a new reference to the VMA.

The open and close VMA methods are called in addition to the processing performed

by the kernel, so they need not reimplement any of the work done there. They exist

as a way for drivers to do any additional processing that they may require.

As it turns out, a simple driver such as simple need not do any extra processing in

particular. So we have created open and close methods, which print a message to the

system log informing the world that they have been called. Not particularly useful,

but it does allow us to show how these methods can be provided, and see when they

are invoked.

To this end, we override the default

vma->vm_ops with operations that call printk:

void simple_vma_open(struct vm_area_struct *vma)

{

printk(KERN_NOTICE "Simple VMA open, virt %lx, phys %lx\n",

vma->vm_start, vma->vm_pgoff << PAGE_SHIFT);

}

void simple_vma_close(struct vm_area_struct *vma)

{

printk(KERN_NOTICE "Simple VMA close.\n");

}

static struct vm_operations_struct simple_remap_vm_ops = {

.open = simple_vma_open,

.close = simple_vma_close,

};

,ch15.13676 Page 426 Friday, January 21, 2005 11:04 AM