Re: Query on ASPM driver design

[Bjorn Helgaas <helgaas@kernel.org>]

On Tue, Jul 27, 2021 at 07:51:37AM +0530, Om Prakash Singh wrote:
> Hi Bjorn,
> I think it makes sense to have the scope of keeping default ASPM
> policy disable and API pci_enable_link_state() to selectively enable
> by EP Driver.
>
> sysfs interface for ASPM also does not allow enabling ASPM for a
> device if the default policy (policy_to_aspm_state()) does not allow
> it.

The ASPM policy implementation may require changes.  I think the
current setup where a policy is compiled into the kernel via Kconfig
options is seriously flawed.

We need a fail-safe kernel parameter, i.e., "pcie_aspm=off", for cases
where devices don't work at all with ASPM.  We need quirks to work
around devices known to be broken, e.g., those that advertise ASPM
support that doesn't actually work, or those that advertise incorrect
exit latencies.  I think most other configuration should be done via
sysfs.

> Consider a situation, for a platform one wants to utilize ASPM
> capability of an onboard PCIe device because it is well evaluated,
> at the same time they want to keep ASPM disabled for other PCIe
> devices that can be connected on open PCIe slot to avoid possible
> performance issue.
> 
> I see ASPM is broken on many devices, though the device shows ASPM
> capabilities but has performance issues when it is enabled.

I'll wait to see your proposal and use case before commenting on this.

Bjorn