linux-kernel - Re: [PATCH] PCI: Only put >= 2015 root ports into D3 on Intel

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Date:   Wed, 17 May 2023 07:33:17 -0500
From:   "Limonciello, Mario" <mario.limonciello@....com>
To:     Mika Westerberg <mika.westerberg@...ux.intel.com>,
        "Limonciello, Mario" <mlimonci@....com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        S-k Shyam-sundar <Shyam-sundar.S-k@....com>,
        Natikar Basavaraj <Basavaraj.Natikar@....com>,
        Deucher Alexander <Alexander.Deucher@....com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Iain Lane <iain@...ngesquash.org.uk>
Subject: Re: [PATCH] PCI: Only put >= 2015 root ports into D3 on Intel


>> AFAICT the actual issue is entirely a wakeup platform firmware sequencing
>> issue
>> while in a hardware sleep state and not PMEs.
>>
>> It's only exposed by putting the root ports into D3 over s2idle.
> But there are two ways to enter s2idle (well or the S0ix whatever is the
> AMD term for that). Either through system sleep or simply waiting that
> all the needed devices runtime suspend. There should be no difference
> from device perspective AFAICT.
On AMD all devices in runtime suspend and SoC entering system
suspend aren't the same state.
>> As an experiment on an unpatched kernel if I avoid letting amd-pmc bind then
>> the
>> hardware will never enter a hardware sleep state over Linux s2idle and this
>> issue
>> doesn't occur.
>>
>> That shows that PMEs *do* work from D3cold.
>>
>> With all of this I have to wonder if the Windows behavior of what to do with
>> the root
>> ports is tied to the uPEP requirements specified in the firmware.
>>
>> Linux doesn't do any enforcement or adjustments from what uPEP indicates.
>>
>> The uPEP constraints for the root port in question in an affected AMD system
>> has:
>>
>>                      Package (0x04)
>>                      {
>>                          Zero,
>>                          "\\_SB.PCI0.GP19",
>>                          Zero,
>>                          Zero
>>                      },
>>
>> AMD's parsing is through 'lpi_device_get_constraints_amd' so that structure
>> shows
>> as not enabled and doesn't specify any D-state requirements.
> AFAIK this object does not exist in ChromeOS so Linux cannot use it
> there.
OK that means that if we came up with a solution that utilized
uPEP that it would have to remain optional.
>
>> What do they specify for Intel on a matching root port?
> I think the corresponding entry in ADL-P system for TBT PCIe root port 0
> looks like this:
>
> 	Package (0x03)
> 	{
> 	    "\\_SB.PC00.TRP0",
> 	    Zero,
> 	    Package (0x02)
> 	    {
> 		Zero,
> 		Package (0x02)
> 		{
> 		    0xFF,
> 		    0x03
> 		}
> 	    }
> 	},
>
> I'm not entirely sure what does it tell? ;-)

It's parsed using `lpi_device_get_constraints`.

So should I follow it right this means for ACPI device
\\_SB.PC00.TRP0 the constraint is disabled.

It's described as
Version 0, UID 0xFF has a minimum D-state of 3.

That means my idea to try to only change D-states at
suspend for enabled constraints won't help.