> 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. > > Eventually, I think the default mode should change to PCIE_BUS_SAFE, > where Linux changes MPS settings at boot-time and at hotplug-time to > make sure every device works. (This mode assumes no peer-to-peer > DMA.) I know this was tried in the past, and we tripped over all > sorts of issues, but it's not clear how many were problems with the > Linux code and how many were unsolvable BIOS or platform issues. Agree. > > Then we'd have these choices: > > PCIE_BUS_TUNE_OFF Never touch MPS > PCIE_BUS_PEER2PEER Set all MPS to 128, so peer-to-peer DMA works > PCIE_BUS_SAFE Configure each device with largest safe MPS > (assumes no peer-to-peer DMA) > PCIE_BUS_PERFORMANCE Use MRRS in addition to MPS > (assumes no peer-to-peer DMA) > > The hot-add issue [1] could be regarded as a BIOS bug -- the BIOS > programmed a hotplug bridge with MPS=256. A hot-added device powers > up with MPS=128, so it's only safe for BIOS to set MPS=256 if the OS > is smart enough to change the bridge MPS, the device MPS, or both, at > hot-add time. That doesn't seem like a good assumption for a BIOS to > make. > > I think we should always *warn* about potential MPS issues, even in > PCIE_BUS_TUNE_OFF mode. That would help diagnose the hot-add issue as > well as issues like the ones Joe Jin reported [2] and [3]. OK, I will add a new patch to provide "warn" info if necessary like Joe Jin reported. But because hotplug issue [1] and Joe reported [2] and [3] only encountered in PCIE_BUS_TUNE_OFF mode. > > I think what we should do is *always* call pcie_bus_configure_set(), > no matter what mode we're in, but make pcie_bus_configure_set() smart > enough to do different things (print warnings, adjust settings, do the > stuff you added in pcie_bus_update_set(), etc.) depending on what mode > we're in. OK, I will try to rework this patch. Thanks! Yijing. >> + } >> >> /* FIXME - Peer to peer DMA is possible, though the endpoint would need >> * to be aware to the MPS of the destination. To work around this, >> -- >> 1.7.1 >> >> > > [1] https://bugzilla.kernel.org/show_bug.cgi?id=60671 > [2] http://lkml.kernel.org/r/4FFA9B96.6040901@xxxxxxxxxx > [3] http://lkml.kernel.org/r/509B5038.8090304@xxxxxxxxxx > > . > -- 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
