linux-kernel - Re: [PATCH] nvme-pci: Use non-operational power state instead of D3 on Suspend-to-Idle

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Date:   Thu, 9 May 2019 18:28:32 +0800
From:   Kai-Heng Feng <kai.heng.feng@...onical.com>
To:     Christoph Hellwig <hch@....de>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Mario Limonciello <Mario.Limonciello@...l.com>,
        Keith Busch <kbusch@...nel.org>,
        Keith Busch <keith.busch@...el.com>, Jens Axboe <axboe@...com>,
        Sagi Grimberg <sagi@...mberg.me>,
        linux-nvme <linux-nvme@...ts.infradead.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] nvme-pci: Use non-operational power state instead of D3
 on Suspend-to-Idle

at 17:56, Christoph Hellwig <hch@....de> wrote:

> On Thu, May 09, 2019 at 05:42:30PM +0800, Kai-Heng Feng wrote:
>>> That would be a set of 6 new suspend and resume callbacks, mind you,
>>> and there's quite a few of them already.  And the majority of drivers
>>> would not need to use them anyway.
>>
>> I think suspend_to_idle() and resume_from_idle() should be enough?
>> What are other 4 callbacks?
>>
>>> Also, please note that, possibly apart from the device power state
>>> setting, the S2I and S2R handling really aren't that different at all.
>>> You basically need to carry out the same preparations during suspend
>>> and reverse them during resume in both cases.
>>
>> But for this case, it’s quite different to the original suspend and
>> resume callbacks.
>
> Let's think of what cases we needed.
>
> The "classic" suspend in the nvme driver basically shuts down the
> device entirely.  This is useful for:
>
>  a) device that have no power management
>  b) System power states that eventually power off the entire PCIe bus.
>     I think that would:
>
>      - suspend to disk (hibernate)
>      - classic suspend to ram
>
> The we have the sequence in your patch.  This seems to be related to
> some of the MS wording, but I'm not sure what for example tearing down
> the queues buys us.  Can you explain a bit more where those bits
> make a difference?

Based on my testing if queues (IRQ) are not disabled, NVMe controller won’t  
be quiesced.
Symptoms can be high power drain or system freeze.

I can check with vendors whether this also necessary under Windows.

>
> Otherwise I think we should use a "no-op" suspend, just leaving the
> power management to the device, or a simple setting the device to the
> deepest power state for everything else, where everything else is
> suspend, or suspend to idle.

I am not sure I get your idea. Does this “no-op” suspend happen in NVMe  
driver or PM core?

>
> And of course than we have windows modern standby actually mandating
> runtime D3 in some case, and vague handwaving mentions of this being
> forced on the platforms, which I'm not entirely sure how they fit
> into the above picture.

I was told that Windows doesn’t use runtime D3, APST is used exclusively.

Kai-Heng