On Thu, Jul 6, 2023 at 12:07 PM Mario Limonciello <mario.limonciello@xxxxxxx> wrote: > > On 7/5/23 15:06, Bjorn Helgaas wrote: > > On Wed, Jun 28, 2023 at 01:09:49PM +0800, Kai-Heng Feng wrote: > >> On Wed, Jun 28, 2023 at 4:54 AM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote: > >>> On Tue, Jun 27, 2023 at 04:35:25PM +0800, Kai-Heng Feng wrote: > >>>> On Fri, Jun 23, 2023 at 7:06 AM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote: > >>>>> On Tue, Jun 20, 2023 at 01:36:59PM -0500, Limonciello, Mario wrote: > > > >>> It's perfectly fine for the IP to support PCI features that are not > >>> and can not be enabled in a system design. But I expect that > >>> strapping or firmware would disable those features so they are not > >>> advertised in config space. > >>> > >>> If BIOS leaves features disabled because they cannot work, but at the > >>> same time leaves them advertised in config space, I'd say that's a > >>> BIOS defect. In that case, we should have a DMI quirk or something to > >>> work around the defect. > >> > >> That means most if not all BIOS are defected. > >> BIOS vendors and ODM never bothered (and probably will not) to change > >> the capabilities advertised by config space because "it already works > >> under Windows". > > > > This is what seems strange to me. Are you saying that Windows never > > enables these power-saving features? Or that Windows includes quirks > > for all these broken BIOSes? Neither idea seems very convincing. > > > > I see your point. I was looking through Microsoft documentation for > hints and came across this: > > https://learn.microsoft.com/en-us/windows-hardware/customize/power-settings/pci-express-settings-link-state-power-management > > They have a policy knob to globally set L0 or L1 for PCIe links. > > They don't explicitly say it, but surely it's based on what the devices > advertise in the capabilities registers. So essentially it can be achieved via boot time kernel parameter and/or sysfs knob. The main point is OS should stick to the BIOS default, which is the only ASPM setting tested before putting hardware to the market. Kai-Heng > > >>>> So the logic is to ignore the capability and trust the default set > >>>> by BIOS. > >>> > >>> I think limiting ASPM support to whatever BIOS configured at boot-time > >>> is problematic. I don't think we can assume that all platforms have > >>> firmware that configures ASPM as aggressively as possible, and > >>> obviously firmware won't configure hot-added devices at all (in > >>> general; I know ACPI _HPX can do some of that). > >> > >> Totally agree. I was not suggesting to limiting the setting at all. > >> A boot-time parameter to flip ASPM setting is very useful. If none has > >> been set, default to BIOS setting. > > > > A boot-time parameter for debugging and workarounds is fine. IMO, > > needing a boot-time parameter in the course of normal operation is > > not OK. > > > > Bjorn >
