linux-kernel - Re: [PATCH v7 2/2] PCI: Don't put non-power manageable PCIe root ports into D3

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Date:   Wed, 2 Aug 2023 09:35:35 -0500
From:   Mario Limonciello <mario.limonciello@....com>
To:     Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Bjorn Helgaas <helgaas@...nel.org>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, Len Brown <lenb@...nel.org>,
        linux-acpi@...r.kernel.org, Iain Lane <iain@...ngesquash.org.uk>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH v7 2/2] PCI: Don't put non-power manageable PCIe root
 ports into D3



On 8/2/23 09:31, Mika Westerberg wrote:
> Hi,
> 
> On Wed, Aug 02, 2023 at 09:10:38AM -0500, Mario Limonciello wrote:
>>
>>
>> On 8/2/23 00:26, Mika Westerberg wrote:
>>> Hi Mario,
>>>
>>> On Tue, Aug 01, 2023 at 10:17:11PM -0500, Mario Limonciello wrote:
>>>>> Consequently, platform_pci_bridge_d3() will return false and the only
>>>>> thing that may allow the port to go into D0 is the dmi_get_bios_year()
>>>>> check at the end of pci_bridge_d3_possible().
>>>>>
>>>>> However, that was added, because there are Intel platforms on which
>>>>> Root Ports need to be programmed into D3hot on suspend (which allows
>>>>> the whole platform to reduce power significantly) and there are no
>>>>> ACPI device power management objects associated with them (Mika should
>>>>> know the gory details related to this).  It looks like under Windows
>>>>> the additional power reduction would not be possible on those systems,
>>>>> but that would be a problem, wouldn't it?
>>>>>
>>>>
>>>> I've been thinking on this today, and I at least have a hypothesis about
>>>> this behavior.  Perhaps Windows is actually utilizing enabled PEP
>>>> constraints to enforce what state device should be put into over Modern
>>>> Standby cycles in the absence of ACPI objects.
>>>>
>>>> In the case of one of my problematic system the PEP constraints for the root
>>>> port are:
>>>>
>>>> Package (0x04)
>>>> {
>>>> 	0x00,
>>>> 	"\\_SB.PCI0.GP17",
>>>> 	0x00,
>>>> 	0x00
>>>> },
>>>>
>>>> That first 0x00 means the constraint isn't actually enabled for the root
>>>> port.
>>>>
>>>> Mika,
>>>>
>>>> Could you get an acpidump from one of these problematic Intel systems so we
>>>> can check the PEP constraints to see if this theory works? Or maybe you have
>>>> some other ideas why this is different?
>>>
>>> The patch adding this was merged in 2016 and unfortunately I don't have
>>> any of the ACPI dumps from them available anymore (and do not recall the
>>> details either). I think these were Apollo Lake-P based systems with the
>>> initial runtime D3cold and S0ix support at the time.
>>
>>
>> I scoured the web looking for acpidumps a bit an Apollo Lake system and came
>> across this random bug report:
>>
>> https://bugzilla.redhat.com/show_bug.cgi?id=1591307
>>
>> "Intel(R) Celeron(R) CPU N3450 @ 1.10GHz (family: 0x6, model: 0x5c,
>> stepping: 0x9)"
>>
>> I looked at the acpidump, and I notice:
>>
>> Low Power S0 Idle (V5) : 0
>>
>> That means that Windows wouldn't actually be putting it into Modern Standby
>> at suspend but would rather use S3.
> 
> Same goes for Linux AFAICT. The ones needed this actually used S0ix so
> the bit should definitely be set.

OK.

> 
>> Considering that result, could we perhaps adjust the check to:
>>
>> if ((c->x86_vendor == X86_VENDOR_INTEL) && !(acpi_gbl_FADT.flags &
>> ACPI_FADT_LOW_POWER_S0))
>>
>> Or could we quirk the PCI root ports from Apollo Lake to opt into D3?
> 
> It is not just Apollo Lake, but all "modern" systems as well (sorry if
> this was unclear). Apollo Lake just was the first one that needed this.
> We also have the Low Power S0 Idle bit set in recent systems too.

Ah got it; I misunderstood it as Apollo Lake was the only one that 
needed it.

So modern systems that set the bit in the FADT, do they also lack _S0W 
and _S0D on the root ports?

Does my PEP constraints theory hold steam at all?