Re: [Intel-wired-lan] [PATCH] PCI/ASPM: Enable ASPM on external PCIe devices

[Mario Limonciello]

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 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