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SubjectKiobufs and userspace memory
Hi all,

I'm writing a device driver for a shared memory adapter from which I plan to
support DMA directly from userspace memory (zero copy). I have already
implemented a version which I think works, but I'm not sure if I get the IO
addresses calculated correctly. The case is as follows :

The userspace application has allocated some memory with malloc() and use write
to the device's /dev entry.

The drivers write() function looks something like this :

ssize_t my_write(struct file* file, char* userbuf, size_t len, loff_t* poff)
struct kiobuf * iobuf;
size_t size;
struct my_sglist *sglist = NULL;
int i, err;

/* Pin user memory */
err = alloc_kiovec(1, &iobuf);
if (err)
return err;

err = map_user_kiobuf(WRITE, userbuf, len);
if (err)
goto out;

/* Traverse the iobuf to get the IO address for each page,
* building up the SG table for my DMA machine */

sglist = kmalloc(sizeof(struct my_sglist) * iobuf->nr_pages, GFP_ATOMIC)
if (!sglist)
goto out_unmap;

totlen = 0;
for (i = 0; i < iobuf->nr_pages; ++i)
struct page *page = iobuf->maplist[i];
void *vaddr = page_address(page) + iobuf->offset;
unsigned long ioaddr = virt_to_bus(vaddr);

sglist[i].start = ioaddr;

if ((totlen + PAGE_SIZE) > len)
sglist[i].len = len - totlen;
sglist[i].len = PAGE_SIZE;

totlen + = PAGE_SIZE;

/* Start the synchronous DMA engine */

/* Unpin user memory */

free_kiovec(1, &iobuf);

Is this use of kiobufs sensible to you ? If not, what should I really be doing
in order to acheive zero copy DMA ?

I also have a question regarding a DMA read to userspace memory :

If the application didn't initialize the buffer by memset or anything, all the
pages maps to the same page (the zero page) right ? So if a map_user_kiobuf()
call is made on this buffer, will it sort this out and map the pages to real
ones ?

Any response greatly appreciated,
Steffen Persvold Systems Engineer
Email : Scali AS (
Norway : Tel : (+47) 2262 8950 Olaf Helsets vei 6
Fax : (+47) 2262 8951 N-0621 Oslo, Norway

USA : Tel : (+1) 713 706 0544 10500 Richmond Avenue, Suite 190
Houston, Texas 77042, USA
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