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;
else
sglist[i].len = PAGE_SIZE;
totlen + = PAGE_SIZE;
}
/* Start the synchronous DMA engine */
my_start_dma(sglist);
kfree(sglist);
out_unmap:
/* Unpin user memory */
unmap_kiobuf(iobuf);
out:
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 : mailto:sp@scali.com Scali AS (http://www.scali.com)
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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This archive was generated by hypermail 2b29 : Mon Apr 30 2001 - 21:00:22 EST