On Wed, Feb 23, 2022 at 08:18:08PM +0100, Heiko Carstens wrote:

quoted

The long term goal is to remove ZONE_DMA entirely at least for
architectures that only use the small 16MB ISA-style one.  It can
then be replaced with for example a CMA area and fall into a movable
zone.  I'd have to prototype this first and see how it applies to the
s390 case.  It might not be worth it and maybe we should replace
ZONE_DMA and ZONE_DMA32 with a ZONE_LIMITED for those use cases as
the amount covered tends to not be totally out of line for what we
built the zone infrastructure.

So probably I'm missing something; but for small systems where we
would only have ZONE_DMA, how would a CMA area within this zone
improve things?

It would not, but more importantly we would not need it at all.  The
thinking here is really about the nasty 16MB ISA-style zone DMA.
a 31-bit something rather different.