Messages in this thread | | | Date | Sun, 29 Apr 2001 12:29:53 -0500 | From | Steffen Persvold <> | Subject | Kiobufs and userspace memory |
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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
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