Re: [PATCH] powerpc/dma: Fix invalid DMA mmap behavior

[Christoph Hellwig <hch@lst.de>]

On Thu, Jul 18, 2019 at 01:45:16PM +1000, Oliver O'Halloran wrote:
> > Other than m68k, mips, and arm64, everybody else that doesn't have
> > ARCH_NO_COHERENT_DMA_MMAP set uses this default implementation, so
> > I assume this behavior is acceptable on those architectures.
> 
> It might be acceptable, but there's no reason to use pgport_noncached
> if the platform supports cache-coherent DMA.
> 
> Christoph (+cc) made the change so maybe he saw something we're missing.

I always found the forcing of noncached access even for coherent
devices a little odd, but this was inherited from the previous
implementation, which surprised me a bit as the different attributes
are usually problematic even on x86.  Let me dig into the history a
bit more, but I suspect the righ fix is to default to cached mappings
for coherent devices.