Sorry, didn't mean to drop the lists here. re-adding. On Wed, Mar 28, 2018 at 4:05 PM, Alex Deucher <alexdeucher at gmail.com> wrote: > On Wed, Mar 28, 2018 at 3:53 PM, Logan Gunthorpe <logang at deltatee.com> wrote: >> >> >> On 28/03/18 01:44 PM, Christian König wrote: >>> Well, isn't that exactly what dma_map_resource() is good for? As far as >>> I can see it makes sure IOMMU is aware of the access route and >>> translates a CPU address into a PCI Bus address. >> >>> I'm using that with the AMD IOMMU driver and at least there it works >>> perfectly fine. >> >> Yes, it would be nice, but no arch has implemented this yet. We are just >> lucky in the x86 case because that arch is simple and doesn't need to do >> anything for P2P (partially due to the Bus and CPU addresses being the >> same). But in the general case, you can't rely on it. > > Could we do something for the arches where it works? I feel like peer > to peer has dragged out for years because everyone is trying to boil > the ocean for all arches. There are a huge number of use cases for > peer to peer on these "simple" architectures which actually represent > a good deal of the users that want this. > > Alex > >> >>>>> Yeah, but not for ours. See if you want to do real peer 2 peer you need >>>>> to keep both the operation as well as the direction into account. >>>> Not sure what you are saying here... I'm pretty sure we are doing "real" >>>> peer 2 peer... >>>> >>>>> For example when you can do writes between A and B that doesn't mean >>>>> that writes between B and A work. And reads are generally less likely to >>>>> work than writes. etc... >>>> If both devices are behind a switch then the PCI spec guarantees that A >>>> can both read and write B and vice versa. >>> >>> Sorry to say that, but I know a whole bunch of PCI devices which >>> horrible ignores that. >> >> Can you elaborate? As far as the device is concerned it shouldn't know >> whether a request comes from a peer or from the host. If it does do >> crazy stuff like that it's well out of spec. It's up to the switch (or >> root complex if good support exists) to route the request to the device >> and it's the root complex that tends to be what drops the load requests >> which causes the asymmetries. >> >> Logan >> _______________________________________________ >> amd-gfx mailing list >> amd-gfx at lists.freedesktop.org >> https://lists.freedesktop.org/mailman/listinfo/amd-gfx
