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Date:   Wed, 31 Oct 2018 14:24:13 +0000 (GMT)
From:   "Maciej W. Rozycki" <macro@linux-mips.org>
To:     Christoph Hellwig <hch@lst.de>
cc:     iommu@lists.linux-foundation.org,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Paul Burton <paul.burton@mips.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-mips@linux-mips.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 4/6] dma-mapping: merge direct and noncoherent ops
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Hi Christoph,

On Fri, 14 Sep 2018, Christoph Hellwig wrote:

> All the cache maintainance is already stubbed out when not enabled,
> but merging the two allows us to nicely handle the case where
> cache maintainance is required for some devices, but not others.

 FYI, you commit bc3ec75de545 ("dma-mapping: merge direct and noncoherent 
ops") has caused:

fddi0: DMA command request failed!
fddi0: Adapter open failed!

with the `defxx' driver on my R4400SC TURBOchannel DECstation (but not the 
R3400 one) and consequently the interface does not work anymore.  Both are 
non-coherent cache systems, however the R3400 implements the write-through 
policy while the policy of the R4400SC is write-back (it also has 1MiB of 
secondary cache), which I suspect is the reason of the difference.

 I'll be experimenting with this commit to figure out what the real cause 
is, but it'll take a while as my resources are limited, so I'm sending 
this early heads-up in case you or someone else has an idea what might be 
going wrong here.

  Maciej