mmap() driver entry point

["swayampakulaa, sudhindra" <swayampakulaa_sudhindra@emc.com>]

> iam using this driver to fetch me some memory
> in my user space application by calling mmap with
> the file descriptor got by opening the device file 
> for this driver. 
> mmap(0, length, flags, flags, fd, offset) 
> and the mmap entry point in the driver is implemented as below: 
> In the implementation, it tries to write an index into the first 
> four bytes, which i use to store in my internal structures
> ( to later reference it through the index), 
> before zeroing out the buffer. But the index is always -1. 
> I dont understand the part where its getting the 
> physical address to map. 
> whats the high_memory and whats the PAGE_OFFSET. 
> Can someone help me in understanding what exactly 
> is the mmap() entry point doing here and if its doing it right.
> 
> himem_buf_allocated = 0;
> 
> int xxx_mmap(struct file *filp,
> 		  struct vm_area_struct *vma)
> {
>   unsigned long size;
>   char * virt_addr;
>   int		index;
> 
>   size = vma->vm_end - vma->vm_start;
>   if ((size % PAGE_SIZE) != 0){
>     size = (size / PAGE_SIZE) * PAGE_SIZE + PAGE_SIZE;
>   }
> 
>   /* himem_buf_size is 0x80000000 */
>   if (size + himem_buf_allocated >= himem_buf_size){
>     
>     return -ENOMEM;
>   }
>   
>   /* himem_buf is calculated as high_memory - PAGE_OFFSET */
>   umem_addr = himem_buf + himem_buf_allocated;
>   if (umem_addr == 0){
>     return -ENOMEM;
>   }
>   himem_buf_allocated += size;
>   
> 
>   virt_addr = ioremap((unsigned long)umem_addr, PAGE_SIZE);  
>   if (virt_addr == 0){
>     return -ENOMEM;
>   }
>   /* write the index into the first 4 bytes */
>   writel(index, (uint32_t *)virt_addr);
> 
>     /* the values of index and *(virt_addr) do not match */
>     /*    *(virt_addr) is always -1                */
>     /* Is something wrong here   */
>     dbg_printf(0,"index is %d, *(virt_addr) is %d\n", index,
> (int)readl(virt_addr));
>   iounmap(virt_addr);
> 
>    
> 
>   remap_page_range(vma->vm_start, (ulong)umem_addr, 
> 		   vma->vm_end - vma->vm_start, vma->vm_page_prot);
> 
>   return 0;
>