On Tue, 2008-01-29 at 15:36 -0600, Olof Johansson wrote: > On Wed, Jan 30, 2008 at 08:18:15AM +1100, Benjamin Herrenschmidt wrote: > > > > On Wed, 2008-01-30 at 02:13 +1100, Michael Ellerman wrote: > > > On Tue, 2008-01-29 at 09:15 -0600, Olof Johansson wrote: > > > > On Wed, Jan 30, 2008 at 01:14:03AM +1100, Michael Ellerman wrote: > > > > > > > > > For example a machine with 4GB of memory would end up with the normal > > > > > IOMMU window from 0-2GB and the fixed mapping window from 2GB to 6GB. > > > > > In > > > > > this case a 64-bit device wishing to DMA to 1GB would be told to DMA > > > > > to > > > > > 3GB, plus any offset required by firmware. The firmware offset is > > > > > encoded > > > > > in the "dma-ranges" property. > > > > > > > > Shouldn't the fixed mapping be between 4G and 8G (and the offset for 1G > > > > is at 5G), to account for the MMIO range at 2-4G? > > > > > > I don't think so, ie. it works setup like that, but I'm not entirely > > > sure why. Presumably the 2-4GB for MMIO is only for cycles heading out > > > of the CPU. > > > > No no no... it's because on the PCI segment, it's all offset up > > remember ? > > > > Basically, the PCI host bridge on these has 2 interesting windows for > > us: > > > > 0....2G -> This goes up to memory @0 (via a couple of > > layers) > > > > 0x80*....0xF* -> This goes untranslated to the PLB5 which > > drops the top bits and does some other > > manipulations, which allows to access, among > > others the full 32GB of the cell inbound > > range. > > > > The MMIO region of 2...4G is on the PCI (outbound from the Cell is yet > > another range of addresses with different constraints but that ends up > > generating cycles between 2 and 4G on the PCI segment). > > > > If we had set the direct mapped region so that it uses 2G...N on PCI, we > > would indeed be toast. But instead, the addresses for direct DMA that we > > hand out to devices are in the 0x80* region and go hit the cell > > directly, they never match MMIO. > > Yeah, ok. That makes more sense. Thanks for the clarification.
Right, that's the firmware offset I mentioned in the changelog - 2am is not a good time to think about these things. > Michael, btw, I wonder if it would make sense to duplicate the patch > description at the top of the file as well, since it'll be lost in the > change log for people who don't go back and read history, and having > the intentions documented in the file could be a good idea. Yeah that probably makes sense. I've got most of the fixed mapping code in a block, so I'll put a comment above that section. New patch coming. cheers -- Michael Ellerman OzLabs, IBM Australia Development Lab wwweb: http://michael.ellerman.id.au phone: +61 2 6212 1183 (tie line 70 21183) We do not inherit the earth from our ancestors, we borrow it from our children. - S.M.A.R.T Person
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