Re: [PATCH 1/2] PCI/ASPM: Override the ASPM and Clock PM states set by BIOS for devicetree platforms

[Konrad Dybcio]

On 9/16/25 6:12 PM, Manivannan Sadhasivam via B4 Relay wrote:
> From: Manivannan Sadhasivam <manivannan.sadhasivam@xxxxxxxxxxxxxxxx>
> 
> So far, the PCI subsystem has honored the ASPM and Clock PM states set by
> the BIOS (through LNKCTL) during device initialization. This was done
> conservatively to avoid issues with the buggy devices that advertise
> ASPM capabilities, but behave erratically if the ASPM states are enabled.
> So the PCI subsystem ended up trusting the BIOS to enable only the ASPM
> states that were known to work for the devices.
> 
> But this turned out to be a problem for devicetree platforms, especially
> the ARM based devicetree platforms powering Embedded and *some* Compute
> devices as they tend to run without any standard BIOS. So the ASPM states
> on these platforms were left disabled during boot and the PCI subsystem
> never bothered to enable them, unless the user has forcefully enabled the
> ASPM states through Kconfig, cmdline, and sysfs or the device drivers
> themselves, enabling the ASPM states through pci_enable_link_state() APIs.
> 
> This caused runtime power issues on those platforms. So a couple of
> approaches were tried to mitigate this BIOS dependency without user
> intervention by enabling the ASPM states in the PCI controller drivers
> after device enumeration, and overriding the ASPM/Clock PM states
> by the PCI controller drivers through an API before enumeration.
> 
> But it has been concluded that none of these mitigations should really be
> required and the PCI subsystem should enable the ASPM states advertised by
> the devices without relying on BIOS or the PCI controller drivers. If any
> device is found to be misbehaving after enabling ASPM states that they
> advertised, then those devices should be quirked to disable the problematic
> ASPM/Clock PM states.
> 
> In an effort to do so, start by overriding the ASPM and Clock PM states set
> by the BIOS for devicetree platforms first. Separate helper functions are
> introduced to set the default ASPM and Clock PM states and they will
> override the BIOS set states by enabling all of them if CONFIG_OF is
> enabled. To aid debugging, print the overridden ASPM and Clock PM states.
> 
> In the future, these helpers could be extended to allow other platforms
> like VMD, newer ACPI systems with a cutoff year etc... to follow the path.
> 
> Link: https://lore.kernel.org/linux-pci/20250828204345.GA958461@bhelgaas
> Suggested-by: Bjorn Helgaas <helgaas@xxxxxxxxxx>
> Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@xxxxxxxxxxxxxxxx>
> ---

[...]

> +	/* Override the BIOS disabled Clock PM state for devicetree platforms */
> +	if (IS_ENABLED(CONFIG_OF) && !enabled) {

JFYI CONFIG_OF=y && CONFIG_ACPI=y is valid, at least on arm64
Maybe something like of_have_populated_dt()?

You can then choose which one to use with e.g. acpi=force

Konrad