On Thu, 2012-09-20 at 18:53 -0400, David Miller wrote:

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

quoted

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.

Right, but on ppc, GFP_DMA is a nop (no separate ZONE_DMA, or rather all
of memory is ZONE_DMA). It's always been like that afaik.

We could support ISA device limited addressability using the iommu but
that would involve a map/unmap type API (which I remember we did support
in the old days by passing NULL to pci_map_*, but we dropped that along
the way).

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

quoted

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.

I will dig a bit. I think there might be some users of the ISA DMA
engine for legacy floppies and parport on some early pSeries and CHRP
machines (including Pegasos).

Cheers,
Ben.