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

[Paul Cercueil]

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