help on driver's nopage mmap method
From: John Wang
Date: Wed Dec 22 2004 - 00:24:30 EST
Hi, I have been struggling for a couple of days about
a problem of using nopage method to mmap memory from
driver to user application.
What I try to do is very simple - just to get the
simplest case to work, but I always get some junk in
user app (to be more specific, always 0xFF in the mmap
the memory area). I don't know what is wrong here - it
appears perfectly normal (compared to the examples in
the book Linux Device Driver). The kernel is 2.4.2
running on Intel x86 platform.
The program is very simple so I attached here (header
includes are ignored here to shorten the length).
Please CC me directly if anyone has answer since I am
not subscribed to the mailing list. Thanks in advance.
<test user app>
int
main(int argc, char *argv[])
{
int fd;
char *addr;
fd = open("/dev/my_mmap", O_RDONLY);
if(fd < 0) {
perror("open");
exit(1);
}
addr = mmap(NULL, 4096, PROT_READ, MAP_SHARED,
fd, 0);
if(addr == MAP_FAILED) {
perror("mmap");
exit(1);
}
write(1, addr, 4096);
return 0;
}
<end of test user app>
<kernel driver>
#define SOCK_MMAP_MAJOR 254
#define MY_NAME "mmap_sock"
void
mmap_sock_vma_open(struct vm_area_struct *vma)
{
printk("<1>%s called\n", __FUNCTION__);
return;
}
void
mmap_sock_vma_close(struct vm_area_struct *vma)
{
printk("<1>%s called\n", __FUNCTION__);
return;
}
void *k_vaddr;
struct page *
mmap_sock_vma_nopage(struct vm_area_struct *vma,
unsigned long address, int write_access)
{
struct page *pageptr;
pageptr = virt_to_page(k_vaddr);
get_page(pageptr);
return pageptr;
}
static struct vm_operations_struct mmap_sock_vm_ops =
{
open: mmap_sock_vma_open,
close: mmap_sock_vma_close,
nopage: mmap_sock_vma_nopage,
};
int
mmap_sock_mmap(struct file* fp, struct vm_area_struct
*vma)
{
size_t vma_size, pd_size;
printk("<1>%s called\n", __FUNCTION__);
if(vma->vm_pgoff != 0) {
// this device has to be mapped from
offset 0
return -EINVAL;
}
vma->vm_flags |= VM_RESERVED;
vma->vm_ops = &mmap_sock_vm_ops;
if(k_vaddr == NULL) {
k_vaddr = __get_free_pages(GFP_KERNEL,
0);
memset(k_vaddr, 0x30, PAGE_SIZE);
}
return 0;
}
int
mmap_sock_open(struct inode *inode, struct file *fp)
{
if ( (fp->f_flags & O_ACCMODE) != O_RDONLY) {
// only open for read
return -EACCES;
}
return 0;
}
int
mmap_sock_release(struct inode *inode, struct file
*fp)
{
return 0;
}
static struct file_operations mmap_sock_fops = {
open: mmap_sock_open,
release: mmap_sock_release,
mmap: mmap_sock_mmap,
owner: THIS_MODULE,
};
int
init_module(void)
{
int ret = 0;
ret = register_chrdev(SOCK_MMAP_MAJOR,
MY_NAME, &mmap_sock_fops);
if (ret < 0) {
printk("<1>%s: can't get major %d\n",
MY_NAME, SOCK_MMAP_MAJOR); return ret;
}
return 0;
}
void
cleanup_module(void)
{
int ret;
ret = unregister_chrdev(SOCK_MMAP_MAJOR,
MY_NAME);
if(ret < 0) {
printk("<1>%s: can't unregister dev,
err: %d\n", MY_NAME,
ret);
}
}
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