On Tue, Jun 25, 2019 at 09:37:22AM -0700, Mark Greer wrote:

Yeah, the mpsc driver had lots of ugly cache related hacks because of
cache coherency bugs in the early version of the MV64x60 bridge chips
that it was embedded in.  That chip is pretty much dead now and I've
removed core support for it from the powerpc tree.  Removing the mpsc
driver is on my todo list but I've been busy and lazy.  So, to sum it
up, don't spend any more time worrying about it as it should be removed.

I'll post a patch to do that tonight and I'm sorry for any time you've
spent looking at it so far.

No problem.  And if future such broken chips show up we now have
support for per-device DMA coherency settings and could actually
handle it in a reaѕonably clean way.