netdev - Question on IOAT

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Date:	Mon, 5 Feb 2007 17:59:57 +0100
From:	Olaf Kirch <olaf.kirch@...cle.com>
To:	netdev@...r.kernel.org
Subject: Question on IOAT

Hi,

I looked into the IOAT code today as I'm trying to find out how to add support
for it to NFS. I ran into this piece of code, which waits for the DMA
operation to complete:

                while (dma_async_memcpy_complete(tp->ucopy.dma_chan,
                                                 tp->ucopy.dma_cookie, &done,
                                                 &used) == DMA_IN_PROGRESS) {
                        /* do partial cleanup of sk_async_wait_queue */
                        while ((skb = skb_peek(&sk->sk_async_wait_queue)) &&
                               (dma_async_is_complete(skb->dma_cookie, done,
                                                      used) == DMA_SUCCESS)) {
                                __skb_dequeue(&sk->sk_async_wait_queue);
                                kfree_skb(skb);
                        }
                }

Nowhere in the dma_async_*complete functions can I see any code
that would sleep if the DMA is not yet complete. Am I missing something,
or are we really busy-waiting on the DMA engine? Wouldn't this kind of
defeat the purpose of freeing up the CPU from the chores of memcpying?

I also checked the code in ioatdma.c - I would have expected there to
be some kind of interrupt handler that kicks the upper layers when a
DMA operation completes. But the interrupt handler seems to be for
error reporting exclusively... 

Olaf
-- 
Olaf Kirch  |  --- o --- Nous sommes du soleil we love when we play
okir@....de |    / | \   sol.dhoop.naytheet.ah kin.ir.samse.qurax
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