Robin Murphy robin.murphy@arm.com writes:
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" :/
Right, but is that sync_for_device call really needed? AFAICT, that ath9k_hw_process_rxdesc_edma() invocation doesn't actually modify any of the data when it returns EINPROGRESS, so could we just skip it? Like the patch below? Or am I misunderstanding the semantics here?
-Toke
diff --git a/drivers/net/wireless/ath/ath9k/recv.c b/drivers/net/wireless/ath/ath9k/recv.c index 0c0624a3b40d..19244d4c0ada 100644 --- a/drivers/net/wireless/ath/ath9k/recv.c +++ b/drivers/net/wireless/ath/ath9k/recv.c @@ -647,12 +647,8 @@ static bool ath_edma_get_buffers(struct ath_softc *sc, common->rx_bufsize, DMA_FROM_DEVICE);
ret = ath9k_hw_process_rxdesc_edma(ah, rs, skb->data); - if (ret == -EINPROGRESS) { - /*let device gain the buffer again*/ - dma_sync_single_for_device(sc->dev, bf->bf_buf_addr, - common->rx_bufsize, DMA_FROM_DEVICE); + if (ret == -EINPROGRESS) return false; - }
__skb_unlink(skb, &rx_edma->rx_fifo); if (ret == -EINVAL) {