[RFC PATCH] drm: disable WC optimization for cache coherent devices on non-x86

[Ard Biesheuvel]

On Thu, 24 Jan 2019 at 10:25, Koenig, Christian
<Christian.Koenig at amd.com> wrote:
>
> Am 24.01.19 um 10:13 schrieb Christoph Hellwig:
> > On Wed, Jan 23, 2019 at 05:52:50PM +0100, Ard Biesheuvel wrote:
> >> But my concern is that it seems likely that non-cache coherent
> >> implementations are relying on this hack as well. There must be a
> >> reason that this hack is only disabled for PowerPC platforms if they
> >> are cache coherent, for instance, and I suspect that that reason is
> >> that the hack is the only thing ensuring that the CPU mapping
> >> attributes match the device ones used for these buffers (the vmap()ed
> >> ones), whereas the rings and other consistent data structures are
> >> using the DMA API as intended, and thus getting uncached attributes in
> >> the correct way.
> > Dave, who added that commit is on Cc together with just about everyone
> > involved in the review chain.  Based on the previous explanation
> > that idea what we might want an uncached mapping for some non-coherent
> > architectures for this to work at all makes sense, but then again
> > the way to create those mappings is entirely architecture specific,
> > and also need a cache flushing before creating the mapping to work
> > properly.  So my working theory is that this code never properly
> > worked on architectures without DMA coherent for PCIe at all, but
> > I'd love to be corrected by concrete examples including an explanation
> > of how it actually ends up working.
>
> Cache coherency is mandatory for modern GPU operation.
>
> Otherwise you can't implement a bunch of the requirements of the
> userspace APIs.
>
> In other words the applications doesn't inform the driver that the GPU
> or the CPU is accessing data, it just does it and assumes that it works.
>

Wonderful!

In that case, do you have any objections to the patch proposed by
Christoph above?