[PATCH] nvme/pci: Use host managed power state for suspend

[kbusch@kernel.org (Keith Busch)]

On Mon, May 13, 2019@02:43:43PM +0000, Mario.Limonciello@dell.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?