On Saturday, September 07, 2013 07:59:39 AM Bjorn Helgaas wrote: > On Fri, Sep 6, 2013 at 6:01 PM, Rafael J. Wysocki <rjw@xxxxxxx> wrote: > > On Friday, September 06, 2013 11:13:24 AM Bjorn Helgaas wrote: > >> This is a long series of mostly minor changes with the goal of > >> simplifying the _OSC-related code and making the negotiation between > >> Linux and the platform more understandable. > >> > >> My intent is that this doesn't change any behavior except for the text > >> in dmesg. > >> > >> However, this restructuring does raise some questions in my mind, such > >> as the fact that we do not call pcie_no_aspm() to disable ASPM in two > >> cases where it seems like we might want to: > >> > >> 1) When pcie_ports_disabled, e.g., when booting with > >> "pcie_ports=compat". > >> > >> 2) When Linux doesn't support all the required services, e.g., if > >> compiled with ASPM support but not MSI support. > > > > We tried that (i.e. 2)), but then it caused energy usage to increase on some > > systems with power-hungry graphics adapters and Phoronix made some fuss about > > that. :-) > > The _OSC section of the spec is pretty obtuse, but my reading of Table > 4-6 (PCI Firmware r3.0) is that the OS is prohibited from writing to > the PCIe Capability (for ASPM, VC, AER, or other control) unless it > has been granted "PCI Express Capability Structure control". That's > why I'm dubious about the fact that we use ASPM in those two cases > where we don't even *ask* for that control. > > For the Phoronix case, I assume they made the argument that Windows > does use ASPM without having PCIe Capability control? No, this was regarded as a kernel regression that needed to be addressed. > Do you have any pointers to that discussion? This is the Phoronix link: http://www.phoronix.com/scan.php?page=article&item=linux_aspm_solution&num=1 and I think you can get to some more info from there. > I don't propose changing this now, but it does raise my eyebrows. > Maybe some sort of dmesg note would be appropriate. > > Thanks for looking at all this stuff; I know from experience that it's > pretty hard to wade through :) No problem. :-) Thanks, Rafael -- 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
