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

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Message-ID: <CAAd53p5PAhX6OO0xzaF5TKJ4qT6=nMjQqv5vZM=7rFKtgr-H=A@mail.gmail.com>
Date:   Fri, 11 Aug 2023 16:34:52 +0800
From:   Kai-Heng Feng <kai.heng.feng@...onical.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     "Limonciello, Mario" <mario.limonciello@....com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        linux-pci@...r.kernel.org,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        linux-kernel@...r.kernel.org, Vidya Sagar <vidyas@...dia.com>,
        Michael Bottini <michael.a.bottini@...ux.intel.com>,
        intel-wired-lan@...osl.org, bhelgaas@...gle.com,
        Mika Westerberg <mika.westerberg@...ux.intel.com>
Subject: Re: [Intel-wired-lan] [PATCH] PCI/ASPM: Enable ASPM on external PCIe devices

On Wed, Jul 19, 2023 at 3:24 AM Bjorn Helgaas <helgaas@...nel.org> wrote:
>
> On Mon, Jul 17, 2023 at 11:51:32AM -0500, Limonciello, Mario wrote:
> > On 7/16/2023 10:34 PM, Kai-Heng Feng wrote:
> > > On Sat, Jul 15, 2023 at 12:37 AM Mario Limonciello <mario.limonciello@....com> wrote:
> > > > On 7/14/23 03:17, Kai-Heng Feng wrote:
>
> > > > > 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.
>
> I think using the BIOS default as a limit is problematic.  I think it
> would be perfectly reasonable for a BIOS to (a) configure only devices
> it needs for console and boot, leaving others at power-on defaults,
> and (b) configure devices in the safest configuration possible on the
> assumption that an OS can decide the runtime policy itself.

This is not using BIOS as a "limit". OS is still capable of changing
the ASPM policy at boot time or runtime.
The main point is to find a "sane" setting for devices where BIOS
can't program ASPM.

>
> Obviously I'm not a BIOS writer (though I sure wish I could talk to
> some!), so maybe these are bad assumptions.
>
> > > > 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 suppose this means that the OEM image contains drivers that aren't
> in the Microsoft media, and those drivers may set constraints on ASPM
> usage?
>
> If you boot the Microsoft media that lacks those drivers, maybe it
> doesn't bother to configure ASPM for those devices?  Linux currently
> configures ASPM for everything at enumeration-time, so we do it even
> if there's no driver.

This can be another topic to explore. But sounds like it can break things.

>
> > > I wonder if there's any particular modification should be improved for
> > > this patch?
> >
> > Knowing this information I personally think the original patch that started
> > this thread makes a lot of sense.
>
> I'm still opposed to using dev_is_removable() as a predicate because I
> don't think it has any technical connection to ASPM configuration.

OK. So what should we do instead? Checking if the device is connected
to TBT switch?

Kai-Heng


>
> Bjorn