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

From: Patel, Mayurkumar
Date: Tue May 02 2017 - 08:03:08 EST

Hi Bjorn

>On Fri, Apr 21, 2017 at 2:46 AM, Patel, Mayurkumar
><mayurkumar.patel@xxxxxxxxx> 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:
>>>> - 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 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

Yes, you are right and per spec also it makes sense that ASPM needs to be disabled.
But, if POLICY_DEFAULT is set then, shouldn't BIOS take care of disabling ASPM?

>> 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?

Yes, it is hot-added device. Also, I understand, for POLICY_DEFAULT, OS would/should not touch ASPM(enable/disable),
but BIOS could still (enable/disable), right?

Currently, what happens in my system is as following, (each 2nd power cycle/hotplug of Endpoint disables ASPM):

First Power cycle (When ASPM L1 is already enabled):
device gets powered off -> there are no Link status events, so no pcie hotplug interrupt and pcie_aspm_exit_link_state() triggered.
When the device gets powered on again -> Link down/Link up events are coming back to back.
First Link down is served. (BIOS checks for the Link status and enables ASPM already, as the device is
actually powered back). OS calls pcie_aspm_exit_link_state() and ASPM gets disabled by OS.

Second Power cycle (When ASPM L1 is disabled after above):
device gets powered off -> there are link status events, pcie hotplug interrupt is triggered and pcie_aspm_exit_link_state() triggered.
OS disables ASPM. BIOS checks Link status and disables ASPM too.
When the device gets powered on -> BIOS enables ASPM and as this is pcie hotplug insertion, OS
does not interfere and we have ASPM enabled.

The above sequence happens each 2nd power cycle of the hotplug device.

So One could still argue if POLICY_DEFAULT is set, then why OS disables ASPM if it is not meant to touch configuration.
This is why I proposed following kind of change, so that OS would not touch ASPM, if POLICY_DEFAULT is set.
Also, With the below change, everything relies on BIOS for ASPM when POLICY_DEFAULT is set and I see above problem
gets resolved. Also, the existing ASPM behavior does not have impact, unless specific BIOS does not disable ASPM on
Root Port when device gets removed.

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