Re: [PATCH 02/20] kernel/dma/direct: refine dma_direct_alloc zone selection

[Christoph Hellwig <hch@xxxxxx>]

On Thu, Aug 23, 2018 at 10:01:45AM +1000, Benjamin Herrenschmidt wrote:
> > The general scheme that architectures should implement is:
> > 
> > ZONE_DMA:	Any memory below a magic threshold that is lower than
> > 		32-bit.  Only enabled if actually required (usually
> > 		either 24-bit for ISA, or some other weird architecture
> > 		specific value like 32-bit for S/390)
> 
> It should have been ZONE_ISA_DMA :-)

For most of these use cases it should have been indeed, and that
would avoid a lot of confusion where people use GFP_DMA just because
they do DMA.

Anyway, switching powerpc to this scheme would be great, but I don't
think it is required - GFP_KERNEL will silently fall back to ZONE_DMA,
so except for an additional GFP_DMA fallback allocation when the
GFP_KERNEL one fails the code should just work.