Re: dma_mask semantic problems

["David S. Miller" <davem@redhat.com>]

On Mon, 01 Mar 2004 11:37:36 +1100
Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:

>  - dma_set_dma_mask : Get passed the mask of supported addresses the
> driven HW supports (that is output of dma_map_*). Failure means the
> arch cannot address those restrictions (doesn't provide a zoned
> allocator on non-iommu archs for addressing such a zone or the iommu
> code cannot enforce such a restriction).

OK, I'm fine with this of course.

>  - either the above is modified to _return_ the modified mask (as
> described in step 2) or we add a separate function
> dma_adjust_dma_mask(mask *). This is where the arch will actually
> provide a mask that tells the driver what kind of addresses will
> be supported on _input_ of dma_map_*. This is usually to be passed
> upstream to control bouncing etc... For example, an iommu arch would
> usually return the full 64 bits in there to indicate that any
> memory page can be mapped for DMA.

Hmmm, Ok I guess.

>  - dma_get_arch_limit() (or find a better name) is a _hinting_
> function that returns what kind of addresses will be produced by
> dma_map_* for this device. It can be used by a driver like some
> SCSI ones to disabled 64 bits addressing and thus win more queue
> space on iommu archs that will never generate a more than 32
> (or even 31) bits address.

This needs work.  What we want to say for what SCSI is doing is:

1) A way to say "I can map all of physical space into the 32-bit
   PCI BUS DMA addressing space, ie. iommu platform.

2) A way to say "I can map all of physical space using 64-bit
   addressing too"

I think the first thing is a seperate question.  Purely, the IOMMU exists
precisely to serve this purpose, map physical memory space (however large
bitness wise it is) into the 32-bit SAC PCI address space.

I would go so far as to propose a:

	int dma_can_use_iommu(dev);
	u64 dma_iommu_mask(dev);

so scsi could say:

	if (dma_can_use_iommu(dev) &&
            !(dma_iommu_mask(dev) & ~(u64)0xffffffff)) {
		/* do the 32-bit space saving trick... */
		dma_set_blah_mask(0xffffffff);
		... etc. ...
	}

Sound ok?