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Re: [REGRESSION] Recent swiotlb DMA_FROM_DEVICE fixes break ath9k-based AP

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On 2022-03-24 16:31, Christoph Hellwig wrote:
On Thu, Mar 24, 2022 at 05:29:12PM +0100, Maxime Bizon wrote:
I'm looking into this; but in the interest of a speedy resolution of
the regression I would be in favour of merging that partial revert
and reinstating it if/when we identify (and fix) any bugs in ath9k :)

This looks fishy:

ath9k/recv.c

                 /* We will now give hardware our shiny new allocated skb */
                 new_buf_addr = dma_map_single(sc->dev, requeue_skb->data,
                                               common->rx_bufsize, dma_type);
                 if (unlikely(dma_mapping_error(sc->dev, new_buf_addr))) {
                         dev_kfree_skb_any(requeue_skb);
                         goto requeue_drop_frag;
                 }

                 /* Unmap the frame */
                 dma_unmap_single(sc->dev, bf->bf_buf_addr,
                                  common->rx_bufsize, dma_type);

                 bf->bf_mpdu = requeue_skb;
                 bf->bf_buf_addr = new_buf_addr;

Creating a new mapping for the same buffer before unmapping the
previous one does looks rather bogus.  But it does not fit the
pattern where revering the sync_single changes make the driver
work again.

OK, you made me look :)

Now that it's obvious what to look for, I can only conclude that during the stanza in ath_edma_get_buffers(), the device is still writing to the buffer while ownership has been transferred to the CPU, and whatever got written while ath9k_hw_process_rxdesc_edma() was running then gets wiped out by the subsequent sync_for_device, which currently resets the SWIOTLB slot to the state that sync_for_cpu copied out. By the letter of the DMA API that's not allowed, but on the other hand I'm not sure if we even have a good idiom for "I can't tell if the device has finished with this buffer or not unless I look at it" :/

Robin.



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