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

[Ard Biesheuvel]

On Mon, 21 Jan 2019 at 18:55, Michel DÃnzer <michel@xxxxxxxxxxx> wrote:
>
> On 2019-01-21 5:30 p.m., Ard Biesheuvel wrote:
> > On Mon, 21 Jan 2019 at 17:22, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> >
> >> Until that happens we should just change the driver ifdefs to default
> >> the hacks to off and only enable them on setups where we 100%
> >> positively know that they actually work.  And document that fact
> >> in big fat comments.
> >
> > Well, as I mentioned in my commit log as well, if we default to off
> > unless CONFIG_X86, we may break working setups on MIPS and Power where
> > the device is in fact non-cache coherent, and relies on this
> > 'optimization' to get things working.
>
> FWIW, the amdgpu driver doesn't rely on non-snooped transfers for
> correct basic operation (the scenario Christian brought up is a very
> specialized use-case), so that shouldn't be an issue.
>

The point is that this is only true for x86.

On other architectures, the use of non-cached mappings on the CPU side
means that you /do/ rely on non-snooped transfers, since if those
transfers turn out not to snoop inadvertently, the accesses are
incoherent with the CPU's view of memory.