Messages in this thread | | | Date | Thu, 24 Mar 2022 16:52:31 +0000 | Subject | Re: [REGRESSION] Recent swiotlb DMA_FROM_DEVICE fixes break ath9k-based AP | From | Robin Murphy <> |
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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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