linux-kernel - Re: [PATCH v2] PCI: Make d3cold_allowed sysfs attribute read only

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Message-ID: <87bca301-f50b-4164-9b3c-c597dc26288d@amd.com>
Date:   Tue, 10 Oct 2023 11:34:39 -0500
From:   Mario Limonciello <mario.limonciello@....com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     bhelgaas@...gle.com, lukas@...ner.de, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, mika.westerberg@...ux.intel.com
Subject: Re: [PATCH v2] PCI: Make d3cold_allowed sysfs attribute read only

On 10/10/2023 11:33, Bjorn Helgaas wrote:
> On Thu, Oct 05, 2023 at 01:56:27PM -0500, Mario Limonciello wrote:
>> On 10/5/2023 13:53, Bjorn Helgaas wrote:
>>> On Wed, Oct 04, 2023 at 09:47:31AM -0500, Mario Limonciello wrote:
>>>> Before d3cold was stable userspace was allowed to influence the kernel's
>>>> decision of whether to enable d3cold for a device by a sysfs file
>>>> `d3cold_allowed`.  This potentially allows userspace to break the suspend
>>>> for the system.
>>>
>>> Is "Before d3cold was stable" referring to a "d3cold" read-only
>>> variable, or to Linux functionality of using D3cold, or ...?
>>
>> I was referring to the previous thread's comments when I asked about the
>> history on it.
>>
>>> In what sense does the `d3cold_allowed` sysfs file break suspend?
>>
>> SoCs might not be able to get into their deepest sleep state if userspace
>> messes with it.
>>
>>>> For debugging purposes `pci_port_pm=` can be used to control whether
>>>> a PCI port will go into D3cold and runtime PM can be turned off by
>>>> sysfs on PCI end points.
>>>
>>> I guess this should be "pcie_port_pm=", which affects *all* PCIe
>>> ports?
>>
>> Yes.
>>
>>> Which sysfs file turns off runtime PM for endpoints?
>>
>> /sys/bus/pci/devices/*/power/control
> 
> To close the loop on this, I think these are questions that should be
> answered in the commit log (actually, that's usually the case when I
> have questions, because future readers of the git history may have the
> same questions, and it's not practical to dig the answers out of the
> lore archive).
> 
> Bjorn

OK thanks, sometimes it's unclear if you just want to know more or want 
it in the commit message.

I'll respin a v2 with the commit message adjusted.