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

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Date:   Fri, 14 Jul 2023 11:37:31 -0500
From:   Mario Limonciello <mario.limonciello@....com>
To:     Kai-Heng Feng <kai.heng.feng@...onical.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        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 7/14/23 03:17, Kai-Heng Feng wrote:
> On Thu, Jul 6, 2023 at 12:07 PM Mario Limonciello
> <mario.limonciello@....com> 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@...nel.org> 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@...nel.org> 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
>>