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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