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

[Christoph Hellwig <hch@infradead.org>]

On Mon, Jan 21, 2019 at 04:33:27PM +0100, Ard Biesheuvel wrote:
> On Mon, 21 Jan 2019 at 16:07, Christoph Hellwig <hch@infradead.org> wrote:
> >
> > > +#include <linux/dma-noncoherent.h>
> >
> > This header is not for usage in device drivers, but purely for
> > dma-mapping implementations!
> >
> 
> Is that documented anywhere?

I'll add big fat comments.  But the fact that nothing is exported
there should be a fairly big hint.

> > And even if something like this was valid to do, it would have to be
> > a core function with an arch hook, and not hidden in a random driver.
> 
> Why would it not be valid to do? Is it wrong for a driver to be aware
> of whether a device is cache coherent or not?
> 
> And in case it isn't, do you have an alternative suggestion on how to
> fix this mess?

For the write combine mappings we need a proper core API how instances
can advertise the support.  One thing I want to do fairly soon is
error checking of the attrs argument to dma_alloc_attrs - so if you
pass in something unsupported it will give you back an error.

It seems that isn't quite enough for the drm use case, so we might
also need a way to query these features, but that really has to go
through the usual dma layer abstraction as well and not hacked together
in a driver based on an eduacted guestimate.