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

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Message-ID: <CAJZ5v0g_hg9pnnD1DXNPDM9v7kSoiEp-SKqAXcvEntXpxa5Nxw@mail.gmail.com>
Date:   Tue, 14 Nov 2023 14:31:30 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Guan-Yu Lin <guanyulin@...gle.com>
Cc:     linux-pm@...r.kernel.org, "Rafael J. Wysocki" <rafael@...nel.org>,
        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 Tue, Nov 14, 2023 at 10:08 AM Guan-Yu Lin <guanyulin@...gle.com> wrote:
>
> 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?

I will have more time to think about this when I'm back from the
conference I'm participating in now.

In the meantime, I would recommend you to address Alan's comments.