linux-kernel - Re: DMA direct mapping fix for 5.4 and earlier stable branches

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Message-ID: <20210209093642.GA1006@lst.de>
Date:   Tue, 9 Feb 2021 10:36:42 +0100
From:   Christoph Hellwig <hch@....de>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     obayashi.yoshimasa@...ionext.com, sumit.garg@...aro.org,
        hch@....de, m.szyprowski@...sung.com, robin.murphy@....com,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, daniel.thompson@...aro.org
Subject: Re: DMA direct mapping fix for 5.4 and earlier stable branches

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.