Re: [PATCH 0/2] PCI/ASPM: Enable ASPM and Clock PM by default on devicetree platforms

[Bjorn Helgaas]

[+cc Kai-Heng, Rafael; thread at
https://lore.kernel.org/r/20250916-pci-dt-aspm-v1-0-778fe907c9ad@xxxxxxxxxxxxxxxx]

On Tue, Sep 16, 2025 at 09:42:51PM +0530, Manivannan Sadhasivam via B4 Relay wrote:
> Hi,
> 
> This series is one of the 'let's bite the bullet' kind, where we have decided to
> enable all ASPM and Clock PM states by default on devicetree platforms [1]. The
> reason why devicetree platforms were chosen because, it will be of minimal
> impact compared to the ACPI platforms. So seemed ideal to test the waters.
> 
> Problem Statement
> =================
> 
> Historically, PCI subsystem relied on the BIOS to enable ASPM and Clock PM
> states for PCI devices before the kernel boot. This was done to avoid enabling
> ASPM for the buggy devices that are known to create issues with ASPM (even
> though they advertise the ASPM capability). But BIOS is not at all a thing on
> most of the non-x86 platforms. For instance, the majority of the Embedded and
> Compute ARM based platforms using devicetree have something called bootloader,
> which is not anyway near the standard BIOS used in x86 based platforms. And
> these bootloaders wouldn't touch PCIe at all, unless they boot using PCIe
> storage, even then there would be no guarantee that the ASPM states will get
> enabled. Another example is the Intel's VMD domain that is not at all configured
> by the BIOS. But, this series is not enabling ASPM/Clock PM for VMD domain. I
> hope it will be done similarly in the future patches.
> 
> Solution
> ========
> 
> So to avoid relying on BIOS, it was agreed [2] that the PCI subsystem has to
> enable ASPM and Clock PM states based on the device capability. If any devices
> misbehave, then they should be quirked accordingly.
> 
> First patch of this series introduces two helper functions to enable all ASPM
> and Clock PM states if CONFIG_OF is enabled. Second patch drops the custom ASPM
> enablement code from the pcie-qcom driver as it is no longer needed.
> 
> Testing
> =======
> 
> This series is tested on Lenovo Thinkpad T14s based on Snapdragon X1 SoC. All
> supported ASPM states are getting enabled for both the NVMe and WLAN devices by
> default.
> 
> [1] https://lore.kernel.org/linux-pci/a47sg5ahflhvzyzqnfxvpk3dw4clkhqlhznjxzwqpf4nyjx5dk@bcghz5o6zolk
> [2] https://lore.kernel.org/linux-pci/20250828204345.GA958461@bhelgaas
> 
> Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@xxxxxxxxxxxxxxxx>
> ---
> Manivannan Sadhasivam (2):
>       PCI/ASPM: Override the ASPM and Clock PM states set by BIOS for devicetree platforms
>       PCI: qcom: Remove the custom ASPM enablement code
> 
>  drivers/pci/controller/dwc/pcie-qcom.c | 32 -----------------------
>  drivers/pci/pcie/aspm.c                | 48 ++++++++++++++++++++++++++++++----
>  2 files changed, 43 insertions(+), 37 deletions(-)
> ---
> base-commit: 8f5ae30d69d7543eee0d70083daf4de8fe15d585
> change-id: 20250916-pci-dt-aspm-8b3a7e8d2cf1
> 
> Best regards,
> -- 
> Manivannan Sadhasivam <manivannan.sadhasivam@xxxxxxxxxxxxxxxx>
> 
>