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

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Message-ID: <aHYHzrl0DE2HV86S@hovoldconsulting.com>
Date: Tue, 15 Jul 2025 09:48:30 +0200
From: Johan Hovold <johan@...nel.org>
To: Manivannan Sadhasivam <manivannan.sadhasivam@....qualcomm.com>
Cc: Manivannan Sadhasivam <mani@...nel.org>,
	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 Mon, Jul 14, 2025 at 11:31:04PM +0530, Manivannan Sadhasivam wrote:
> Commit 9f4f3dfad8cf ("PCI: qcom: Enable ASPM for platforms supporting 1.9.0
> ops") allowed the Qcom controller driver to enable ASPM for all PCI devices
> enumerated at the time of the controller driver probe. It proved to be
> useful for devices already powered on by the bootloader as it allowed
> devices to enter ASPM without user intervention.
> 
> However, it could not enable ASPM for the hotplug capable devices i.e.,
> devices enumerated *after* the controller driver probe. This limitation
> mostly went unnoticed as the Qcom PCI controllers are not hotplug capable
> and also the bootloader has been enabling the PCI devices before Linux
> Kernel boots (mostly on the Qcom compute platforms which users use on a
> daily basis).
> 
> But with the advent of the commit b458ff7e8176 ("PCI/pwrctl: Ensure that
> pwrctl drivers are probed before PCI client drivers"), the pwrctrl driver
> started to block the PCI device enumeration until it had been probed.
> Though, the intention of the commit was to avoid race between the pwrctrl
> driver and PCI client driver, it also meant that the pwrctrl controlled PCI
> devices may get probed after the controller driver and will no longer have
> ASPM enabled. So users started noticing high runtime power consumption with
> WLAN chipsets on Qcom compute platforms like Thinkpad X13s, and Thinkpad
> T14s, etc...
> 
> 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 think that's something we should try to avoid.

> So with this, we can also get rid of the controller driver specific
> 'qcom_pcie_ops::host_post_init' callback.
> 
> Cc: stable@...r.kernel.org # v6.7
> Fixes: 9f4f3dfad8cf ("PCI: qcom: Enable ASPM for platforms supporting 1.9.0 ops")
> Reported-by: Johan Hovold <johan@...nel.org>
> Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@....qualcomm.com>

Note that the patch fails to apply to 6.16-rc6 due to changes in
linux-next. Depending on how fast we can come up with a fix it may be
better to target 6.16.

> -static int qcom_pcie_enable_aspm(struct pci_dev *pdev, void *userdata)
> -{
> -	/*
> -	 * Downstream devices need to be in D0 state before enabling PCI PM
> -	 * substates.
> -	 */
> -	pci_set_power_state_locked(pdev, PCI_D0);
> -	pci_enable_link_state_locked(pdev, PCIE_LINK_STATE_ALL);
> -
> -	return 0;
> -}

I think you should consider leaving this helper in place here to keep
the size of the diff down (e.g. as you intend to backport this).

> +static int qcom_pcie_enable_aspm(struct pci_dev *pdev)
> +{
> +	/*
> +	 * Downstream devices need to be in D0 state before enabling PCI PM
> +	 * substates.
> +	 */
> +	pci_set_power_state_locked(pdev, PCI_D0);
> +	pci_enable_link_state_locked(pdev, PCIE_LINK_STATE_ALL);

You need to use the non-locked helpers here since you no longer hold the
bus semaphore (e.g. as reported by lockdep).

Maybe this makes the previous comment about not moving the helper moot.

> +
> +	return 0;
> +}
> +
> +static int pcie_qcom_notify(struct notifier_block *nb, unsigned long action,
> +			      void *data)
> +{
> +	struct qcom_pcie *pcie = container_of(nb, struct qcom_pcie, nb);

This results in an unused variable warning (presumably until the next
patch in the series is applied).

> +	struct device *dev = data;
> +	struct pci_dev *pdev = to_pci_dev(dev);
> +
> +	switch (action) {
> +	case BUS_NOTIFY_BIND_DRIVER:
> +		qcom_pcie_enable_aspm(pdev);
> +		break;
> +	}
> +
> +	return NOTIFY_DONE;
> +}

Missing newline.

>  static int qcom_pcie_probe(struct platform_device *pdev)
>  {
>  	const struct qcom_pcie_cfg *pcie_cfg;

Johan