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Date: Fri, 24 Sep 2010 15:04:19 -0700 From: "Ira W. Snyder" <iws@...o.caltech.edu> To: Dan Williams <dan.j.williams@...el.com> Cc: "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: Re: [PATCH RFCv1 1/2] dmaengine: add support for scatterlist to scatterlist transfers On Fri, Sep 24, 2010 at 02:53:14PM -0700, Dan Williams wrote: > On Fri, 2010-09-24 at 14:24 -0700, Ira W. Snyder wrote: > > On Fri, Sep 24, 2010 at 01:40:56PM -0700, Dan Williams wrote: > > > I don't think any dma channels gracefully handle descriptors that were > > > prepped but not submitted. You would probably need to submit the > > > backlog, poll for completion, and then return the error. > > > Alternatively, the expectation is that descriptor allocations are > > > transient, i.e. once previously submitted transactions are completed > > > the descriptors will return to the available pool. So you could do > > > what async_tx routines do and just poll for a descriptor. > > > > > > > Can you give me an example? Even some pseudocode would help. > > Here is one from do_async_gen_syndrome() in crypto/async_tx/async_pq.c: > > /* Since we have clobbered the src_list we are committed > * to doing this asynchronously. Drivers force forward > * progress in case they can not provide a descriptor > */ > for (;;) { > tx = dma->device_prep_dma_pq(chan, dma_dest, > &dma_src[src_off], > pq_src_cnt, > &coefs[src_off], len, > dma_flags); > if (likely(tx)) > break; > async_tx_quiesce(&submit->depend_tx); > dma_async_issue_pending(chan); > } > > > The other DMAEngine functions (dma_async_memcpy_*()) don't do anything > > with the descriptor if submit fails. Take for example > > dma_async_memcpy_buf_to_buf(). If tx->tx_submit(tx); fails, any code > > using it has no way to return the descriptor to the free pool. > > > > Does tx_submit() implicitly return descriptors to the free pool if it > > fails? > > No, submit() failures are a hold over from when the ioatdma driver used > to perform additional descriptor allocation at ->submit() time. After > prep() the expectation is that the engine is just waiting to be told > "go" and can't fail. The only reason ->submit() retains a return code > is to support the "cookie" based method for polling for operation > completion. A dma driver should handle all descriptor submission > failure scenarios at prep time. > Ok, that's more like what I expected. So we still need the try forever code similar to the above. I can add that for the next version. > > Ok, I thought the list was clearer, but this is equally easy. How about > > the following change that does away with the list completely. Then > > things should work on ioatdma as well. > > > > From d59569ff48a89ef5411af3cf2995af7b742c5cd3 Mon Sep 17 00:00:00 2001 > > From: Ira W. Snyder <iws@...o.caltech.edu> > > Date: Fri, 24 Sep 2010 14:18:09 -0700 > > Subject: [PATCH] dma: improve scatterlist to scatterlist transfer > > > > This is an improved algorithm to improve support on the Intel I/OAT > > driver. > > > > Signed-off-by: Ira W. Snyder <iws@...o.caltech.edu> > > --- > > drivers/dma/dmaengine.c | 52 +++++++++++++++++++++----------------------- > > include/linux/dmaengine.h | 3 -- > > 2 files changed, 25 insertions(+), 30 deletions(-) > > > > diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c > > index 57ec1e5..cde775c 100644 > > --- a/drivers/dma/dmaengine.c > > +++ b/drivers/dma/dmaengine.c > > @@ -983,10 +983,13 @@ dma_async_memcpy_sg_to_sg(struct dma_chan *chan, > > struct dma_async_tx_descriptor *tx; > > dma_cookie_t cookie = -ENOMEM; > > size_t dst_avail, src_avail; > > - struct list_head tx_list; > > + struct scatterlist *sg; > > size_t transferred = 0; > > + size_t dst_total = 0; > > + size_t src_total = 0; > > dma_addr_t dst, src; > > size_t len; > > + int i; > > > > if (dst_nents == 0 || src_nents == 0) > > return -EINVAL; > > @@ -994,12 +997,17 @@ dma_async_memcpy_sg_to_sg(struct dma_chan *chan, > > if (dst_sg == NULL || src_sg == NULL) > > return -EINVAL; > > > > + /* get the total count of bytes in each scatterlist */ > > + for_each_sg(dst_sg, sg, dst_nents, i) > > + dst_total += sg_dma_len(sg); > > + > > + for_each_sg(src_sg, sg, src_nents, i) > > + src_total += sg_dma_len(sg); > > + > > What about overrun or underrun do we not care if src_total != dst_total? > > Otherwise looks ok. > I don't know if we should care about that. The algorithm handles that case just fine. It copies the maximum amount it can, which is exactly min(src_total, dst_total). Whichever scatterlist runs out of entries first is the shortest. As a real world example, my driver verifies that both scatterlists have exactly the right number of bytes available before trying to program the hardware. Ira -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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