[PATCH V8 4/5] PCI/ASPM: save power on values during bridge init

On Fri, Apr 14, 2017 at 5:17 PM, Sinan Kaya [off-list ref] wrote:

On 4/14/2017 5:44 PM, Bjorn Helgaas wrote:

quoted

I think there's an argument to be made that if we care about ASPM
configuration, we should be using a non-POLICY_DEFAULT setting.  And I
think there's value in having POLICY_DEFAULT be the absolute lowest-
risk setting, which I think means option 1.

What do you think?

I see your point. The counter argument is that most of the users do not
know what an ASPM kernel command line is unless they understand PCI
language.

I don't think the answer is using the "pcie_aspm.policy=" boot
argument.  I certainly don't want users to have to deal with that.  I
wish we didn't even have that parameter.

I think we need runtime knobs instead (and I guess we already have
/sys/module/pcie_aspm/parameters/policy and /sys/.../link_state), and
distro userspace should use them.  I'm envisioning something in
"System Settings / Power" or similar.  Basically I think the policy
doesn't *have* to be dictated by a kernel boot-time parameter, so it
should not be.

I have been using the powersave policy option until now. I recently realized
that nobody except me is using this option. Therefore, we are wasting
power by default following a hotplug insertion.

This is the case where I'm trying to avoid. With the introduction of NVMe
u.2 drives, hotplug is becoming more and more mainstream. I decided to
take the matters into my hand with this series for this very reason.

Like you said, BIOS is out of the picture with pciehp. There is nobody
to configure ASPM following a hotplug insertion.

I can also claim that If user wants performance, they should boot with
the performance policy or pcie_aspm=off parameters.

I saw this recommendation in multiple DPDK tuning documents.

Like you said, what do we do by default is the question. Should we opt
for safe like we are doing, or try to save some power.

I think safety is paramount.  Every user should be able to boot safely
without any kernel parameters.  We don't want users to have a problem
booting and then have to search for a workaround like booting with
"pcie_aspm=off".  Most users will never do that.

Here's a long-term strawman proposal, see what you think:

  - Deprecate CONFIG_PCIEASPM_DEFAULT, CONFIG_PCIEASPM_POWERSAVE, etc.
  - Default aspm_policy is POLICY_DEFAULT always.
  - POLICY_DEFAULT means Linux doesn't touch anything: if BIOS enabled
ASPM, we leave it that way; we leave ASPM disabled on hot-added
devices.
  - Deprecate kernel boot parameters (possibly keep pcie_aspm=off for
debugging use).
  - Use /sys/module/pcie_aspm/parameters/policy for run-time
system-wide  control, including for future hot-added devices.
  - Remove CONFIG_PCIEASPM_DEBUG and enable that code always, so we
have fine-grained run-time control.

Maybe, we are missing a HPP option from the PCI spec.

That's an interesting idea.  _HPX does have provision for manipulating
Link Control bits (see ACPI r5.0, sec 6.2.8.3), but I don't think very
many systems implement it.  And there's currently no connection
between program_hpp_type2() and aspm.c, so I'm a little worried that
we might have issues if a system did implement an _HPX that sets any
of the ASPM bits.

Bjorn