linux-kernel - Re: [PATCH] rpm: pm: enable PM_RPM

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Date:   Tue, 14 Nov 2023 17:08:29 +0800
From:   Guan-Yu Lin <guanyulin@...gle.com>
To:     linux-pm@...r.kernel.org, "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     gregkh@...uxfoundation.org, len.brown@...el.com, pavel@....cz,
        heikki.krogerus@...ux.intel.com, mkl@...gutronix.de,
        hadess@...ess.net, mailhol.vincent@...adoo.fr,
        ivan.orlov0322@...il.com, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, pumahsu@...gle.com,
        raychi@...gle.com, albertccwang@...gle.com,
        Alan Stern <stern@...land.harvard.edu>
Subject: Re: [PATCH] rpm: pm: enable PM_RPM_EXCEPTION config flag

On Wed, Nov 8, 2023 at 11:56 PM Alan Stern <stern@...land.harvard.edu> wrote:
>
> On Wed, Nov 08, 2023 at 04:45:43PM +0800, Guan-Yu Lin wrote:
> > Thanks for the questions. Let me first introduce my motivation for
> > proposing this feature. We can discuss the implementation details later.
> >
> > Motivation:
> > Currently, system PM operations always override runtime PM operations.
> > As runtime PM reflects the power status of devices, there is a
> > possibility that runtime PM states that a device is in use, but system
> > PM decides to suspend it. Up to now, we have assumed that a device can't
> > function without resources from the system, so the device should acquire
> > a wakelock to prevent this from happening. However, what if the device
>
> [From the fact that you mention wakelocks, I assume that you're trying
> to implement something for Android systems rather than Linux systems
> in general.]
>

Apologies, I should use "wakeup source" here.

> > does not need the system's support to function? Or only needs limited
> > resources (e.g., only limited power source or clock) to function? In this
> > situation, we would like to keep the device on but allow the system to
> > suspend. This is an example where we would like devices to follow runtime
> > PM rather than system PM.
>
> To put it more simply, you want a way to leave some devices in an active
> state while the rest of the system is suspended.  It's not clear why you
> have dragged runtime PM into the discussion (apart from the obvious fact
> that you won't want to keep a device active if it isn't active already).
>
> This sounds like a major change, not something to be done with a simple
> override.  You should discuss it with Rafael Wysocki and the linux-pm
> mailing list before trying to implement anything.
>
> > Feature Supported:
> > 1. Devices could control the priority of system PM and runtime PM during
> >    runtime.
>
> This seems like a totally unnecessary side issue.  Forget about runtime
> PM for the time being and concentrate instead on which devices you want
> to keep active.
>
> > 2. The control should be at the device level, meaning that different
> >    devices should control their own priorities.
> >
> > Goal of This Patch:
> > 1. Design a framework to support features above.
> > 2. Apply it into usb for demonstration.
>
> You may find that it is easier (and less work in the long run) to design
> the general framework and get it working than to concentrate on one
> particular subsystem.
>
> Alan Stern

Hi Rafael,
We'd like to implement a feature to allow system suspend with several
devices still active. Do you have any consideration on this?

Thanks,
Guan-Yu