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

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Message-ID: <e077b229-c92b-c9a6-3581-61329c4b4a4b@arm.com>
Date:   Fri, 25 Mar 2022 18:42:19 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Toke Høiland-Jørgensen <toke@...e.dk>,
        Christoph Hellwig <hch@....de>,
        Halil Pasic <pasic@...ux.ibm.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        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 2022-03-25 18:15, Toke Høiland-Jørgensen wrote:
> Christoph Hellwig <hch@....de> writes:
> 
>> On Thu, Mar 24, 2022 at 07:02:16PM +0100, Halil Pasic wrote:
>>>> If
>>>> ddbd89deb7d3 alone turns out to work OK then I'd be inclined to try a
>>>> partial revert of just that one hunk.
>>>>
>>>
>>> I'm not against being pragmatic and doing the partial revert. But as
>>> explained above, I do believe for correctness of swiotlb we ultimately
>>> do need that change. So if the revert is the short term solution,
>>> what should be our mid-term road-map?
>>
>> Unless I'm misunderstanding this thread we found the bug in ath9k
>> and have a fix for that now?
> 
> According to Maxim's comment on the other subthread, that ath9k patch
> wouldn't work on all platforms (and constitutes a bit of a violation of
> the DMA API ownership abstraction). So not quite, I think?

Indeed, it would potentially stand to pose the same problem as the 
SWIOTLB change, but on the scale of individual cache lines touched by 
ath9k_hw_process_rxdesc_edma() rather than the whole buffer. However, 
that might represent a less severe impact on a smaller number of users 
(maybe the MIPS systems? I'm not sure...) so perhaps it's an acceptable 
tourniquet? Note that the current code is already a violation of the DMA 
API (because the device keeps writing even when it doesn't have 
ownership), so there's not a very strong argument in that regard.

Thanks,
Robin.