On Thu, Jul 7, 2011 at 1:02 PM, bill <bill_carson@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.