Re: [PATCH v3 3/8] PCI/ASPM: Transition the device to D0 (if required) when enabling ASPM link states

[Bjorn Helgaas <helgaas@kernel.org>]

[+cc Rafael because I'm not a power management user]

On Wed, Jul 08, 2026 at 04:30:17PM +0200, Manivannan Sadhasivam wrote:
> From: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
> 
> Per PCIe spec r6.0, sec 5.5.4:
> 
> "If setting either or both of the enable bits for PCI-PM L1 PM Substates,
> both ports must be configured as described in this section while in D0."
> 
> Currently, the callers of pci_enable_link_state_locked() (vmd, pcie-qcom)
> transition the device to D0 themselves before enabling the link state. But
> this is easy to get wrong and has to be duplicated by every caller.
> 
> Move the D0 transition into the shared __pci_enable_link_state() helper so
> that all three APIs pci_enable_link_state(), pci_enable_link_state_locked()
> and pci_force_enable_link_state() perform it, and only when the PCI-PM L1
> PM Substates are getting enabled.
> 
> Now that the helper handles the transition, drop the redundant D0 transition
> from the vmd and pcie-qcom callers.
> 
> Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
> ---
>  drivers/pci/controller/dwc/pcie-qcom.c |  5 -----
>  drivers/pci/controller/vmd.c           |  5 -----
>  drivers/pci/pcie/aspm.c                | 23 +++++++++++++++++------
>  3 files changed, 17 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/pci/controller/dwc/pcie-qcom.c b/drivers/pci/controller/dwc/pcie-qcom.c
> index d8eb52857f69..45f4caeb0814 100644
> --- a/drivers/pci/controller/dwc/pcie-qcom.c
> +++ b/drivers/pci/controller/dwc/pcie-qcom.c
> @@ -1085,11 +1085,6 @@ static int qcom_pcie_post_init_2_7_0(struct qcom_pcie *pcie)
>  
>  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;
> diff --git a/drivers/pci/controller/vmd.c b/drivers/pci/controller/vmd.c
> index d4ae250d4bc6..20597d2cf66e 100644
> --- a/drivers/pci/controller/vmd.c
> +++ b/drivers/pci/controller/vmd.c
> @@ -762,11 +762,6 @@ static int vmd_pm_enable_quirk(struct pci_dev *pdev, void *userdata)
>  	pci_info(pdev, "VMD: Default LTR value set by driver\n");
>  
>  out_state_change:
> -	/*
> -	 * Ensure devices are in D0 before enabling PCI-PM L1 PM Substates, per
> -	 * PCIe r6.0, sec 5.5.4.
> -	 */
> -	pci_set_power_state_locked(pdev, PCI_D0);
>  	pci_enable_link_state_locked(pdev, PCIE_LINK_STATE_ALL);
>  	return 0;
>  }
> diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
> index c04fb71de91c..6d6862fd2ebb 100644
> --- a/drivers/pci/pcie/aspm.c
> +++ b/drivers/pci/pcie/aspm.c
> @@ -1524,6 +1524,17 @@ static int __pci_enable_link_state(struct pci_dev *pdev, int state, bool locked,
>  		return -EPERM;
>  	}
>  
> +	/*
> +	 * Ensure the device is in D0 before enabling PCI-PM L1 PM Substates, per
> +	 * PCIe r6.0, sec 5.5.4.
> +	 */
> +	if (state & PCIE_LINK_STATE_L1_SS_PCIPM) {
> +		if (locked)
> +			pci_set_power_state_locked(pdev, PCI_D0);
> +		else
> +			pci_set_power_state(pdev, PCI_D0);
> +	}

I think it's a great thing to get the power state management out of
the callers of pci_enable_link_state().

But shouldn't we really have done this with pm_runtime_get_sync() and
a corresponding pci_runtime_put()?  Setting the power state with
pci_set_power_state() feels like it's too low-level and possibly racy
for use like this.

>  	if (!locked)
>  		down_read(&pci_bus_sem);
>  	mutex_lock(&aspm_lock);
> @@ -1550,8 +1561,8 @@ static int __pci_enable_link_state(struct pci_dev *pdev, int state, bool locked,
>   * touch the LNKCTL register. Also note that this does not enable states
>   * disabled by pci_disable_link_state(). Return 0 or a negative errno.
>   *
> - * Note: Ensure devices are in D0 before enabling PCI-PM L1 PM Substates, per
> - * PCIe r6.0, sec 5.5.4.
> + * Note: The device will be transitioned to D0 state if the PCI-PM L1 Substates
> + * are getting enabled.
>   *
>   * @pdev: PCI device
>   * @state: Mask of ASPM link states to enable
> @@ -1569,8 +1580,8 @@ EXPORT_SYMBOL(pci_enable_link_state);
>   * can't touch the LNKCTL register. Also note that this does not enable states
>   * disabled by pci_disable_link_state(). Return 0 or a negative errno.
>   *
> - * Note: Ensure devices are in D0 before enabling PCI-PM L1 PM Substates, per
> - * PCIe r6.0, sec 5.5.4.
> + * Note: The device will be transitioned to D0 state if the PCI-PM L1 Substates
> + * are getting enabled.
>   *
>   * @pdev: PCI device
>   * @state: Mask of ASPM link states to enable
> @@ -1600,8 +1611,8 @@ EXPORT_SYMBOL(pci_enable_link_state_locked);
>   * Note that if the BIOS didn't grant ASPM control to the OS, this does nothing
>   * because we can't touch the LNKCTL register.
>   *
> - * Note: Ensure devices are in D0 before enabling PCI-PM L1 PM Substates, per
> - * PCIe r6.0, sec 5.5.4.
> + * Note: The device will be transitioned to D0 state if the PCI-PM L1 Substates
> + * are getting enabled.
>   *
>   * Return: 0 on success, a negative errno otherwise.
>   */
> 
> -- 
> 2.43.0
> 
>