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From:   Christoph Hellwig <hch@lst.de>
To:     Maxime Bizon <mbizon@freebox.fr>
Cc:     Toke =?iso-8859-1?Q?H=F8iland-J=F8rgensen?= <toke@toke.dk>,
        Robin Murphy <robin.murphy@arm.com>,
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Subject: Re: [REGRESSION] Recent swiotlb DMA_FROM_DEVICE fixes break
 ath9k-based AP
Message-ID: <20220324163132.GB26098@lst.de>
References: <1812355.tdWV9SEqCh@natalenko.name> <f88ca616-96d1-82dc-1bc8-b17480e937dd@arm.com> <20220324055732.GB12078@lst.de> <4386660.LvFx2qVVIh@natalenko.name> <81ffc753-72aa-6327-b87b-3f11915f2549@arm.com> <878rsza0ih.fsf@toke.dk> <4be26f5d8725cdb016c6fdd9d05cfeb69cdd9e09.camel@freebox.fr>
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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.