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

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Message-ID: <d3857812-7e03-4b62-f7cb-ac0d139b6123@codeaurora.org>
Date:   Fri, 14 Apr 2017 15:12:35 -0400
From:   Sinan Kaya <okaya@...eaurora.org>
To:     Rajat Jain <rajatja@...gle.com>
Cc:     linux-pci@...r.kernel.org, timur@...eaurora.org,
        "Patel, Mayurkumar" <mayurkumar.patel@...el.com>,
        linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Yinghai Lu <yinghai@...nel.org>,
        David Daney <david.daney@...ium.com>,
        Shawn Lin <shawn.lin@...k-chips.com>,
        Julia Lawall <Julia.Lawall@...6.fr>,
        linux-kernel@...r.kernel.org, Rajat Jain <rajatxjain@...il.com>
Subject: Re: [PATCH V8 4/5] PCI/ASPM: save power on values during bridge init

Bjorn,

On 4/12/2017 3:19 PM, Rajat Jain wrote:
> On Fri, Apr 7, 2017 at 9:55 PM, Sinan Kaya <okaya@...eaurora.org> wrote:
>> Now that we added a hook to be called from device_add, save the
>> default values from the HW registers early in the boot for further
>> reuse during hot device add/remove operations.
>>
>> If the link is down during boot, assume that we want to enable L0s
>> and L1 following hotplug insertion as well as L1SS if supported.
> 
> IIUC, so far POLICY_DEFAULT meant that we'd just use & follow what
> BIOS has done, and play it safe (never try to be more opportunistic).
> With this change however, we'd be slightly overstepping and giving
> ourselves benefit of doubt if the BIOS could not enable ASPM states
> because the link was not up. This may be good, but I think we should
> call it out, and add some more elaborate comment on the POLICY_DEFAULT
> description (what to, and what not to expect in different situations).
> 
> It is important because existing systems today, that used to boot
> without cards and later hotplugged them, didn't have ASPM states
> enabled. They will now suddenly start seeing all ASPM states enabled
> including L1 substates for the first time (if supported).
> 

Rajat has a good point here. Would you like me to update the ASPM document
with this new behavior for hotplug?

Do you have another behavior preference when it comes this?

Sinan

-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.