ongoing writecombine on ppc

[Benjamin Herrenschmidt]

On Fri, 2015-10-02 at 14:45 +1000, Benjamin Herrenschmidt wrote:
> On Fri, 2015-10-02 at 14:42 +1000, Dave Airlie wrote:
> > I don't think we resolved this the last time we talked about it,
> > 
> > but radeon writecombine maps fail hard on ppc, I think all the
> > fixes
> > either did something bad to AGP systems or weren't liked.
> > 
> > My patch attached just fixes radeon, which is where I'm still
> > seeing
> > the issue.
> 
> Yes, you MUST NOT set the flags of system memory to anything other
> than
> fully cachable on any cache coherent powerpc machine. This should be
> bolted into the DRM core imho.
> 
> _wc is only suitable for MMIO.

Note that I wouldn't be surprised if we weren't the only arch like
that.

Playing with caching attributes of main memory smells from a system
design and cache coherency protocol standpoint. x86 supports it but
I wouldn't be surprised if some ARMs puke in interesting way as well.

Cheers,
Ben.