Re: [PATCH] devicetree: Add generic IOMMU device tree bindings

[Thierry Reding <thierry.reding-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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On Wed, May 21, 2014 at 10:50:38AM +0200, Arnd Bergmann wrote:
> On Wednesday 21 May 2014 10:26:11 Thierry Reding wrote:
> > On Tue, May 20, 2014 at 10:26:12PM +0200, Arnd Bergmann wrote:
> > > On Tuesday 20 May 2014 16:24:59 Dave Martin wrote:
> > > > On Tue, May 20, 2014 at 02:41:18PM +0200, Arnd Bergmann wrote:
> > > > > On Tuesday 20 May 2014 14:02:43 Thierry Reding wrote:
> > [...]
> > > > > > Multiple-master IOMMU:
> > > > > > ----------------------
> > > > > > 
> > > > > > 	iommu {
> > > > > > 		/* the specifier represents the ID of the master */
> > > > > > 		#address-cells = <1>;
> > > > > > 		#size-cells = <0>;
> > > > 
> > > > How do we know the size of the input address to the IOMMU?  Do we
> > > > get cases for example where the IOMMU only accepts a 32-bit input
> > > > address, but some 64-bit capable masters are connected through it?
> > > 
> > > I was stuck on this question for a while before, but then I realized
> > > that it doesn't matter at all: It's the IOMMU driver itself that
> > > manages the address space, and it doesn't matter if a slave can
> > > address a larger range than the IOMMU can accept. If the IOMMU
> > > needs to deal with the opposite case (64-bit input addresses
> > > but a 32-bit master), that limitation can be put into the specifier.
> > 
> > Isn't this what DMA masks are for? Couldn't the IOMMU simply use the
> > master device's DMA mask to do the right thing here?
> 
> Ah, yes. I guess that's the right way to do it.
> 
> > > > For determining dma masks, it is the output address that it
> > > > important.  Santosh's code can probably be taught to handle this,
> > > > if given an additional traversal rule for following "iommus"
> > > > properties.  However, deploying an IOMMU whose output address size
> > > > is smaller than the 
> > > 
> > > Something seems to be missing here. I don't think we want to handle
> > > the case where the IOMMU output cannot the entire memory address
> > > space. If necessary, that would mean using both an IOMMU driver
> > > and swiotlb, but I think it's a reasonable assumption that hardware
> > > isn't /that/ crazy.
> > 
> > Similarily, should the IOMMU not be treated like any other device here?
> > Its DMA mask should determine what address range it can access.
> 
> Right. But for that we need a dma-ranges property in the parent of the
> iommu, just so the mask can be set correctly and we don't have to
> rely on the 32-bit fallback case.

Shouldn't the IOMMU driver be the one to set the DMA mask for the device
in exactly the same way that other drivers override the 32-bit default?

Thierry

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