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From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date: Fri, 21 Sep 2012 08:22:44 +1000

> Hrm, that's ancient gunk, I'll have to dig. We potentially can support
> ISA devices DMA'ing from an ISA bridge... but via the iommu, which means
> isa_virt_to_bus is a non-starter.
> 
> But then, do we really care ? IE. Is there single device that actually
> requires ISA_DMA_API and that is expected to work on any currently
> supported powerpc hw ? :-)
> 
> We don't even support PReP anymore, so that leaves us with what ?

ISA_DMA_API implies a fixed window of addresses which are <= 32-bits
on the bus, which is a hardware requirement of these devices.

isa_virt_to_bus() goes to that physical address, and the expection is
that you use GFP_DMA and thus the physical addresses fit inside of
an unsigned int.

isa_virt_to_bus() basically amounts to a virt-->phys plus a cast.

> Anybody has an objection to turning ISA_DMA_API off ?

Then you can remove all of the DMA api stuff in powerpc's asm/dma.h
but some of it looks like it might be in use.