linux-kernel - Re: [PATCH 1/2] PCI: qcom: Switch to bus notifier for enabling ASPM of PCI devices

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Message-ID: <dbb1d203-9ac3-4c4d-bfd3-2d337a20693c@oss.qualcomm.com>
Date: Tue, 15 Jul 2025 11:46:23 +0200
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Johan Hovold <johan@...nel.org>, Manivannan Sadhasivam <mani@...nel.org>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@....qualcomm.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczyński <kwilczynski@...nel.org>,
        Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-arm-msm@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH 1/2] PCI: qcom: Switch to bus notifier for enabling ASPM
 of PCI devices

On 7/15/25 11:33 AM, Johan Hovold wrote:
> On Tue, Jul 15, 2025 at 02:41:23PM +0530, Manivannan Sadhasivam wrote:
>> On Tue, Jul 15, 2025 at 09:48:30AM GMT, Johan Hovold wrote:
>>> On Mon, Jul 14, 2025 at 11:31:04PM +0530, Manivannan Sadhasivam wrote:
> 
>>>> Obviously, it is the pwrctrl change that caused regression, but it
>>>> ultimately uncovered a flaw in the ASPM enablement logic of the controller
>>>> driver. So to address the actual issue, switch to the bus notifier for
>>>> enabling ASPM of the PCI devices. The notifier will notify the controller
>>>> driver when a PCI device is attached to the bus, thereby allowing it to
>>>> enable ASPM more reliably. It should be noted that the
>>>> 'pci_dev::link_state', which is required for enabling ASPM by the
>>>> pci_enable_link_state_locked() API, is only set by the time of
>>>> BUS_NOTIFY_BIND_DRIVER stage of the notification. So we cannot enable ASPM
>>>> during BUS_NOTIFY_ADD_DEVICE stage.
>>>
>>> A problem with this approach is that ASPM will never be enabled (and
>>> power consumption will be higher) in case an endpoint driver is missing.
>>
>> I'm aware of this limiation. But I don't think we should really worry about that
>> scenario. No one is going to run an OS intentionally with a PCI device and
>> without the relevant driver. If that happens, it might be due to some issue in
>> driver loading or the user is doing it intentionally. Such scenarios are short
>> lived IMO.
> 
> There may not even be a driver (yet). A user could plug in whatever
> device in a free slot. I can also imagine someone wanting to blacklist
> a driver temporarily for whatever reason.
> 
> How would this work on x86? Would the BIOS typically enable ASPM for
> each EP? Then that's what we should do here too, even if the EP driver
> happens to be disabled.

Not sure about all x86, but the Intel VMD controller driver surely doesn't
care what's on the other end:

drivers/pci/controller/vmd.c : vmd_pm_enable_quirk()

Konrad