Query about UIO

[Thayumanavar S]

On Thu, Jul 7, 2011 at 1:02 PM, bill <bill_carson at 126.com> wrote:
> Hi, Greg
>
> I've been reading the UIO code, one place puzzled me a lot.
> May I ask one question about it?
>
> -------------------------------------------------------------------------------
> drivers/uio/uio.c
> static int uio_find_mem_index(struct vm_area_struct *vma)
> {
>     int mi;
>     struct uio_device *idev = vma->vm_private_data;
>
>     for (mi = 0; mi < MAX_UIO_MAPS; mi++) {
>         if (idev->info->mem[mi].size == 0)
>             return -1;
>         if (vma->vm_pgoff == mi)
>             return mi;
>     }
>     return -1;
> }
>
> I don't get it why use vma->vm_pgoff as the index for struct uio_mem
> mem[MAX_UIO_MAPS],
> Suppose there are twohardware memory space, if user intends to map the 2nd
> mem, then
> the last parameter of mmap should be 1,
>
> mapaddr = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED,
>                fd, 1);
>

the offset need in page units, vm_pgoff is the offset in page.
to map nth mem, you need to do (n-1)<<PAGE_SHIFT(n-1* page size
specific to arch). in mmap call, ultimately in the kernel vm_pgoff =
offset >>PAGE_SHIFT which is index into uio_mem array.

thanks,
thayumanavar s.