[PATCH 2/5] ARM: make dma_addr_t 64-bit on LPAE

[Russell King - ARM Linux]

On Mon, Aug 05, 2013 at 08:59:18PM -0500, Rob Herring wrote:
> On Mon, Aug 5, 2013 at 7:15 PM, Russell King - ARM Linux
> <linux at arm.linux.org.uk> wrote:
> > On Mon, Aug 05, 2013 at 05:50:06PM -0500, Rob Herring wrote:
> >> From: Rob Herring <rob.herring at calxeda.com>
> >>
> >> LPAE systems may have 64-bit capable DMA, so dma_addr_t needs to be
> >> 64-bit.
> >
> > The question I put to you here is: any can your DMA controllers produce
> > more than 32-bits of address?
> 
> Yes. SATA is 64-bit. And the ones that are 32-bit only have an IOMMU
> in front of them.
> 
> > Yes, physical memory may be mapped at addresses greater than 4GB phys.
> > That doesn't mean that dma_addr_t also becomes 64-bit.
> >
> > dma_addr_t is the bus address - the address you program into the DMA
> > controller.  If you're only ever writing it into a 32-bit address
> > register then you can only address 32-bits of memory, even though that
> > may be in the physical address range of 4GB-8GB.  In such cases where
> > that applies to all DMA controllers in the system, dma_addr_t should
> > still be 32-bit.
> >
> > This is where it's broken to think that DMA addresses are the same as
> > physical addresses.  They aren't.
> 
> Understood, but it's not that clear once IOMMUs are in the mix. While
> IOMMU drivers will return 32-bit addresses at the DMA API level, I'd
> think internally they will need 64-bit addresses.
> 
> I'm fine with just selecting this in ARCH_HIGHBANK, but the machine
> kconfig selects are getting rather long. We'll be cleaning those up at
> some point if we keep selecting everything at the machine level.

Given that this breaks PL08x, I think you need to drop this change.  We
need the root problem fixed, as I have done in my dma-masks patches,
rather than endlessly plastering around this.

If we don't solve the root problem first, this issue is just going to get
really crappy with workaround on top of more workarounds, and it's going
to be impossible to undo all that crap later.