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

[Manivannan Sadhasivam]

On Tue, Sep 16, 2025 at 06:28:58PM GMT, Konrad Dybcio wrote:
> 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

Ouch! I didn't know this, thanks for pointing it out.

> Maybe something like of_have_populated_dt()?
> 

Yep, this looks like the correct API.

- Mani

-- 
மணிவண்ணன் சதாசிவம்