On Thu, Aug 23, 2018 at 10:01:45AM +1000, Benjamin Herrenschmidt wrote:

quoted

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.