On 2013/8/27 5:33, Bjorn Helgaas wrote: > On Sun, Aug 25, 2013 at 9:42 PM, Yijing Wang <wangyijing@xxxxxxxxxx> wrote: >>> I think the strategy of updating the device MPS when possible makes >>> sense, but I don't think we should do it in PCIE_BUS_TUNE_OFF mode. >>> That mode is documented as "Disable PCIe MPS tuning and use the >>> BIOS-configured MPS defaults." This patch changes that to something >>> like "Disable PCIe MPS tuning, except for hot-added devices" and there >>> is no longer a way to tell Linux to never touch MPS. >> >> Hi Bjorn, >> Thanks for your review and comments! >> >> As you mentioned, PCIE_BUS_TUNE_OFF means "Disable PCIe MPS tuning and use the >> BIOS-configured MPS defaults.", But hotplug action make the BIOS default mps setting >> changed(power off, all registers reset). So If we only touch the newly inserted device mps, >> I think maybe it's reasonable. > > I agree, it might be reasonable. But I think it's too hard to > document that behavior. I think it's better to have behavior that is > easy to understand and explain, even if it is slightly suboptimal. > > The current Linux default is PCIE_BUS_TUNE_OFF, and given that I don't > want to touch any MPS settings in that mode, I don't see a way to > safely fix https://bugzilla.kernel.org/show_bug.cgi?id=60671 (the > problem with hot-added devices not working because MPS is incorrect). > In the long term, I hope we can fix it by making the default > PCIE_BUS_SAFE, but that doesn't help right now. I also think we should consider to change default mode to pcie_bus_safe. Jon mentioned that there are number of issues discovered on some x86 chipsets. However, no further details. But if we use PCIE_BUS_TUNE_OFF all the time, we never have chance to fix these issues. > > That leaves us with only the workaround of booting the Huawei rh5885 > box with "pci=pcie_bus_safe". > > I'm willing to accept that because I think we can argue that this is > really a BIOS defect. The BIOS *can* program MPS to values that will > be safe for hotplug even if the OS does nothing, i.e., it can set > MPS=128 in all paths that lead to a hotpluggable slot. I think that's > probably what this BIOS *should* do, since it has no way of knowing > whether the OS will support hotplug or whether the OS will reprogram > any MPS values. > Yes, we temporarily make BIOS program all MPS to 128 to avoid this problem now. Thanks! Yijing. -- 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
