On 19/04/17 12:32 PM, Jason Gunthorpe wrote: > On Wed, Apr 19, 2017 at 12:01:39PM -0600, Logan Gunthorpe wrote: > Not entirely, it would have to call through the whole process > including the arch_p2p_cross_segment().. Hmm, yes. Though it's still not clear what, if anything, arch_p2p_cross_segment would be doing. In my experience, if you are going between host bridges, the CPU address (or PCI address -- I'm not sure which seeing they are the same on my system) would still work fine -- it just _may_ be a bad idea because of performance. Similarly if you are crossing via a QPI bus or similar, I'd expect the CPU address to work fine. But here the performance is even less likely to be any good. > // Try the arch specific helper > const struct dma_map_ops *comp_ops = get_dma_ops(completer); > const struct dma_map_ops *init_ops = get_dma_ops(initiator); So, in this case, what device does the completer point to? The PCI device or a more specific GPU device? If it's the former, who's responsible for setting the new dma_ops? Typically the dma_ops are arch specific but now you'd be adding ones that are tied to hmm or the gpu. >> I'm not sure I like the name pci_p2p_same_segment. It reads as though >> it's only checking if the devices are not the same segment. > > Well, that is exactly what it is doing. If it succeeds then the caller > knows the DMA will not flow outside the segment and no iommu setup/etc > is required. It appears to me like it's calculating the DMA address, and the check is just a side requirement. It reads as though it's only doing the check. Logan -- To unsubscribe from this list: send the line "unsubscribe linux-rdma" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
