On Wed, Jun 15, 2011 at 3:30 PM, Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx> wrote: > On Wed, 2011-06-15 at 17:23 -0500, Jon Mason wrote: >> I could add the simpler way of doing it as an alternative and have a >> kernel boot parameter to switch between the two. For PPC, the kernel >> parameter could default to always use the alternative way of doing it. >> Would this be acceptable? > > That would work for me. What are the failure modes here? I think the proposal is to have two modes: 1) clamp MPS aggressively to (minimum MPS of all devices in the system), or 2) clamp MPS to the minimum of (device MPS, upstream bridge MPS). The first mode should be safe and allow peer-to-peer DMA transfers. The second (if I understand correctly) is unsafe in that peer-to-peer transfers may cause bus errors. Do we have any clue about whether drivers are attempting peer-to-peer DMA? It would be kind of mysterious to users if a new driver or new way of using a device made the system unstable. It'd be nice if there were some way to tell the user that "PCI hierarchy under X is configured for speed, but is unsafe for peer-to-peer." Or maybe this just something we debug if/when peer-to-peer failures occur. Bjorn -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
