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Date:   Mon, 28 Sep 2020 14:10:02 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Paul Cercueil <paul@crapouillou.net>
Cc:     Christoph Hellwig <hch@lst.de>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Dave Airlie <airlied@linux.ie>,
        DRI <dri-devel@lists.freedesktop.org>,
        Linux Next Mailing List <linux-next@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: linux-next: build failure after merge of the drm tree
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On Mon, Sep 28, 2020 at 01:46:55PM +0200, Paul Cercueil wrote:
>> dma_mmap_attrs can only be used on allocations from dma_mmap_attrs with
>> the same attrs.  As there is no allocation using DMA_ATTR_NON_CONSISTENT
>> in the drm core, something looks very fishy here.
>
> Is that a fact? I don't see why you couldn't change the cache settings 
> after allocation. In practice it works just fine.

Accessing the same physical address using different caching attributes
is undefined behavior and fairly dangerous on most architectures, and
thus not supported by the DMA API.