I took a look at the code, and found one suspicious smoking gun.

This probably isn't the problem you're trying to track down, but it
looks like _isa_bus_dmamap_sync() in arch/arm32/isa/isa_machdep.c,
(which does bounce-buffering for ISA DMA) should be, but isn't,
calling _bus_dmamap_sync() (in arch/arm32/arm32/bus.c) at an
appropriate time to do cache flushes and the like.

See bus_dmamap_sync(9) for information on this kernel internal
interface..

				- Bill