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

[Mario Limonciello]

On 7/14/23 03:17, Kai-Heng Feng wrote:
> 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.

Unfortunately; I don't think you can jump to this conclusion.

A big difference in the Windows world to Linux world is that OEMs ship 
with a factory Windows image that may set policies like this.  OEM 
"platform" drivers can set registry keys too.

I think the next ASPM issue that comes up what we (collectively) need to 
do is compare ASPM policy and PCI registers for:
1) A "clean" Windows install from Microsoft media before all the OEM 
drivers are installed.
2) A Windows install that the drivers have been installed.
3) A up to date mainline Linux kernel.

Actually as this thread started for determining policy for external PCIe 
devices, maybe that would be good to check with those.

> 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