linux-kernel - Re: [PATCH] nvme/pci: Use host managed power state for suspend

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Date:   Mon, 13 May 2019 08:55:24 -0600
From:   Keith Busch <kbusch@...nel.org>
To:     Mario.Limonciello@...l.com
Cc:     hch@....de, keith.busch@...el.com, sagi@...mberg.me,
        linux-nvme@...ts.infradead.org, rafael@...nel.org,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        kai.heng.feng@...onical.com
Subject: Re: [PATCH] nvme/pci: Use host managed power state for suspend

On Mon, May 13, 2019 at 02:43:43PM +0000, Mario.Limonciello@...l.com wrote:
> > Well, it sounds like your partners device does not work properly in this
> > case.  There is nothing in the NVMe spec that says queues should be
> > torn down for deep power states, and that whole idea seems rather
> > counter productive to low-latency suspend/resume cycles.
> 
> Well I've got a thought, quoting the NVME spec:
> "After a successful completion of a Set Features command for this feature, the controller shall be in the
> Power State specified. If enabled, autonomous power state transitions continue to occur from the new state."
> 
> If APST is enabled on this disk, what is to stop an autonomous  reverse
> transition from queue activity on the way down?

Regardless of whether APST is enabled or not, the controller may
autonomously transition out of a host requested low power state in
response to host activity. Exiting a low power state should mean some
other task is actively using the device, and if that's the case, why are
you trying to enter a low power state in the first place? Alternatively,
if host activity really is idle, then why is the device autonomously
leaving the requested state?