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

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Message-ID: <20210209155058.GA8912@lst.de>
Date:   Tue, 9 Feb 2021 16:50:58 +0100
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Sumit Garg <sumit.garg@...aro.org>, Christoph Hellwig <hch@....de>,
        Greg KH <gregkh@...uxfoundation.org>,
        obayashi.yoshimasa@...ionext.com, m.szyprowski@...sung.com,
        iommu@...ts.linux-foundation.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        Daniel Thompson <daniel.thompson@...aro.org>
Subject: Re: DMA direct mapping fix for 5.4 and earlier stable branches

On Tue, Feb 09, 2021 at 12:45:11PM +0000, Robin Murphy wrote:
> It's not a bug, it's a fundamental design failure. dma_get_sgtable() has 
> only ever sort-of-worked for DMA buffers that come from CMA or regular page 
> allocations. In particular, a "no-map" DMA pool is not backed by kernel 
> memory, so does not have any corresponding page structs, so it's impossible 
> to generate a *valid* scatterlist to represent memory from that pool, 
> regardless of what you might get away with provided you don't poke too hard 
> at it.
>
> It is not a good API...

Yes, I don't think anyone should add new users of the API.

That being said the commit he is trying to backport fixes a bug in
the implementation that at least in theory could also affect in-tree
drivers.