linux-kernel - Re: [REGRESSION] Recent swiotlb DMA_FROM

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Date:   Thu, 24 Mar 2022 17:31:32 +0100
From:   Christoph Hellwig <hch@....de>
To:     Maxime Bizon <mbizon@...ebox.fr>
Cc:     Toke Høiland-Jørgensen <toke@...e.dk>,
        Robin Murphy <robin.murphy@....com>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Christoph Hellwig <hch@....de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Halil Pasic <pasic@...ux.ibm.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Kalle Valo <kvalo@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Olha Cherevyk <olha.cherevyk@...il.com>,
        iommu <iommu@...ts.linux-foundation.org>,
        linux-wireless <linux-wireless@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [REGRESSION] Recent swiotlb DMA_FROM_DEVICE fixes break
 ath9k-based AP

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.