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SubjectRe: mapping user space buffer to kernel address space
Linus Torvalds wrote:
> Anyway, I didn't realize you were talking about the sound drivers use of
> remap_page_range(). That's not the original reason for remap_page_range()
> at all, and in fact it's the _ugly_ way to do things. It's simple and it
> works, but it's not pretty.
> Quite frankly, the way I'd conceptually prefer people do these kinds of
> DMA buffers etc is to just have a "nopage()" function, and let that
> nopage() function just fill in the page from the DMA buffer directly. This
> would work fine, and would get rid of all the playing with PG_reserved
> etc.

Well coolio. Would somebody be up for sanity checking my audio mmap
code (attached)? It doesn't look too hard at all to get the audio
drivers doing the correct thing.

Converting to this was easy... my driver hardcodes the buffer size at
PAGE_SIZE. Since Via audio uses scatter-gather DMA, all I need to do in
'nopage' is get the virtual page number for the vma, and use that as a
direct index into the scatter-gather array.

Since this code works in my local tests, my two concerns at this point
are correct vma->vm_pgoff treatment, and correct vm_flags/vm_file usage.


Jeff Garzik | The difference between laziness and
Building 1024 | prioritization is the end result.
MandrakeSoft |
static struct page * via_mm_nopage (struct vm_area_struct * vma,
unsigned long address, int write_access)
struct via_info *card = vma->vm_private_data;
struct via_channel *chan = &card->ch_out;
unsigned long pgoff;
int rd, wr;

DPRINTK ("ENTER, start %lXh, ofs %lXh, pgoff %ld, addr %lXh, wr %d\n",
address - vma->vm_start,
(address - vma->vm_start) >> PAGE_SHIFT,


if (address > vma->vm_end) {
return NOPAGE_SIGBUS; /* Disallow mremap */
if (!card) {
DPRINTK ("EXIT, returning NOPAGE_OOM\n");
return NOPAGE_OOM; /* Nothing allocated */

/* XXX what about vma->vm_pgoff? */
pgoff = (address - vma->vm_start) >> PAGE_SHIFT;
rd = card->ch_in.is_mapped;
wr = card->ch_out.is_mapped;

#ifndef VIA_NDEBUG
unsigned long max_bufs = VIA_DMA_BUFFERS;
if (rd && wr) max_bufs *= 2;
/* via_dsp_mmap() should ensure this */
assert (pgoff < max_bufs);

/* if full-duplex (read+write) and we have two sets of bufs,
* then the playback buffers come first, sez soundcard.c */
if (pgoff >= VIA_DMA_BUFFERS) {
chan = &card->ch_in;
} else if (!wr)
chan = &card->ch_in;

assert ((((unsigned long)chan->sgbuf[pgoff].cpuaddr) % PAGE_SIZE) == 0);
DPRINTK ("EXIT, returning page %p for cpuaddr %lXh\n",
virt_to_page (chan->sgbuf[pgoff].cpuaddr),
(unsigned long) chan->sgbuf[pgoff].cpuaddr);
return virt_to_page (chan->sgbuf[pgoff].cpuaddr);

static int via_mm_swapout (struct page *page, struct file *filp)
return 0;

struct vm_operations_struct via_mm_ops = {
nopage: via_mm_nopage,
swapout: via_mm_swapout,

static int via_dsp_mmap(struct file *file, struct vm_area_struct *vma)
struct via_info *card;
int nonblock = (file->f_flags & O_NONBLOCK);
int rc = -EINVAL, rd=0, wr=0;
unsigned long max_size, size, start, offset;

assert (file != NULL);
assert (vma != NULL);
card = file->private_data;
assert (card != NULL);

DPRINTK ("ENTER, start %lXh, size %ld, pgoff %ld\n",
vma->vm_end - vma->vm_start,


max_size = 0;
if (file->f_mode & FMODE_READ) {
rd = 1;
if (file->f_mode & FMODE_WRITE) {
wr = 1;

start = vma->vm_start;
offset = (vma->vm_pgoff << PAGE_SHIFT);
size = vma->vm_end - vma->vm_start;

if (size > max_size)
goto out;
if ((offset >= max_size) || (offset >= size))
goto out;

rc = via_syscall_down (card, nonblock);
if (rc) goto out;

vma->vm_ops = &via_mm_ops;
vma->vm_private_data = card;
vma->vm_flags |= VM_LOCKED | VM_SHM; /* Don't swap */
vma->vm_file = file;

if (rd)
card->ch_in.is_mapped = 1;
if (wr)
card->ch_out.is_mapped = 1;

up (&card->syscall_sem);
rc = 0;

DPRINTK("EXIT, returning %d\n", rc);
return rc;
#endif /* VIA_SUPPORT_MMAP */
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