linux-kernel - Re: linux-next: build failure after merge of the drm tree

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Date:   Mon, 28 Sep 2020 15:31:28 +0200
From:   Paul Cercueil <paul@...pouillou.net>
To:     Christoph Hellwig <hch@....de>
Cc:     Stephen Rothwell <sfr@...b.auug.org.au>,
        Dave Airlie <airlied@...ux.ie>,
        DRI <dri-devel@...ts.freedesktop.org>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: linux-next: build failure after merge of the drm tree



Le lun. 28 sept. 2020 à 14:10, Christoph Hellwig <hch@....de> a écrit 
:
> 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.

It's allocated with dma_alloc_wc, but then it's only accessed as 
non-coherent.

Anyway, for the time being I guess you could revert 37054fc81443. But I 
have patches on top of it in drm-misc-next so it's going to be a mess.

If we have time I can come up with a custom dumb_create() fonction, to 
make sure that the GEM buffers are allocated with 
dma_alloc_noncoherent(). Is there a dma_mmap_noncoherent() too?

-Paul