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Date:   Tue, 9 Feb 2021 10:36:42 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     obayashi.yoshimasa@socionext.com, sumit.garg@linaro.org,
        hch@lst.de, m.szyprowski@samsung.com, robin.murphy@arm.com,
        iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org, daniel.thompson@linaro.org
Subject: Re: DMA direct mapping fix for 5.4 and earlier stable branches
Message-ID: <20210209093642.GA1006@lst.de>
References: <CAFA6WYNazCmYN20irLdNV+2vcv5dqR+grvaY-FA7q2WOBMs__g@mail.gmail.com> <YCIym62vHfbG+dWf@kroah.com> <CAFA6WYM+xJ0YDKenWFPMHrTz4gLWatnog84wyk31Xy2dTiT2RA@mail.gmail.com> <YCJCDZGa1Dhqv6Ni@kroah.com> <27bbe35deacb4ca49f31307f4ed551b5@SOC-EX02V.e01.socionext.com> <YCJUgKDNVjJ4dUqM@kroah.com>
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On Tue, Feb 09, 2021 at 10:23:12AM +0100, Greg KH wrote:
> >   From the view point of ZeroCopy using DMABUF, is 5.4 not 
> > mature enough, and is 5.10 enough mature ?
> >   This is the most important point for judging migration.
> 
> How do you judge "mature"?
> 
> And again, if a feature isn't present in a specific kernel version, why
> would you think that it would be a viable solution for you to use?

I'm pretty sure dma_get_sgtable has been around much longer and was
supposed to work, but only really did work properly for arm32, and
for platforms with coherent DMA.  I bet he is using non-coherent arm64,
and it would be broken for other drivers there as well if people did
test them, which they apparently so far did not.