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

[Bjorn Helgaas]

On Fri, Apr 21, 2017 at 2:46 AM, Patel, Mayurkumar
<mayurkumar.patel at intel.com> wrote:
> Hi Bjorn/Kaya,
>
>
>>
>>On 4/17/2017 12:38 PM, Bjorn Helgaas wrote:
>>>> 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.
>>>
>>
>>OK, no problem with leaving the behavior as it is.
>>
>>My initial approach was #2. We knew this way that user had full control
>>over the ASPM policy by changing the BIOS option. Then, Mayurkumar
>>complained that ASPM is not enabled following a hotplug insertion to an
>>empty slot. That's when I switched to #3 as it sounded like a good thing
>>to have for us.
>>
>>> 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.
>>
> I am also ok with leaving the same behavior as now.
> But still following is something open I feel besides, Which may be there in your comments redundantly.
> The current problem is, pcie_aspm_exit_link_state() disables the ASPM configuration even
> if POLICY_DEFAULT was set.

We call pcie_aspm_exit_link_state() when removing an endpoint.  When
we remove an endpoint, I think disabling ASPM is the right thing to
do.  The spec (PCIe r3.1, sec 5.4.1.3) says "Software must not enable
L0s in either direction on a given Link unless components on both
sides of the Link each support L0s; otherwise, the result is
undefined."

> I am seeing already following problem(or may be influence) with it. The Endpoint I have does not have
> does not have "Presence detect change" mechanism. Hot plug is working with Link status events.
> When link is in L1 or L1SS and if EP is powered off, no Link status change event are triggered (It might be
> the expected behavior in L1 or L1SS).  When next time EP is powered on there are link down and
> link up events coming one after other. BIOS enables ASPM on Root port and Endpoint, but while
> processing link status down, pcie_aspm_exit_link_state() clears the ASPM already which were enabled by BIOS.
> If we want to follow above approach then shall we consider having something similar as following?

The proposal was to leave ASPM disabled on hot-added devices.  If the
endpoint was powered off and powered back on again, I think that
device looks like a hot-added device, doesn't it?

Bjorn