Date:   Wed, 22 Nov 2017 02:28:58 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Ulf Hansson <ulf.hansson@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Linux Documentation <linux-doc@...r.kernel.org>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Kevin Hilman <khilman@...nel.org>
Subject: Re: [PATCH v4 5/6] PM / core: Direct handling of DPM_FLAG_LEAVE_SUSPENDED

On Wednesday, November 22, 2017 2:10:51 AM CET Rafael J. Wysocki wrote:
> On Monday, November 20, 2017 2:42:26 PM CET Ulf Hansson wrote:
> > On 18 November 2017 at 15:41, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> > > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > >
> > > Make the PM core handle DPM_FLAG_LEAVE_SUSPENDED directly for
> > > devices whose "noirq", "late" and "early" driver callbacks are
> > > invoked directly by it.
> > 
> > This indicates that your target for this particular change isn't
> > ACPI/PCI, but instead this aims to be a more generic solution to be
> > able to optimize the resume path for devices.
> > 
> > Assuming, this is the case, I don't think this is good enough as I
> > pointed out [1] earlier. Simply because it isn't as flexible as is
> > required - to really be able cover generic cases.
> 
> I'll go back to that message, but nothing so far has been flexible enough to
> cover everything you can imagine.
> 
> The case this and the next patch cover really is to allow drivers that can be
> used with or without a PM domain to avoid doing any "are we suspended?" type
> of checks in their callbacks.  Actually, the [6/6] is more important from that
> standpoint, but this one also may play the role because of the dependencies
> between devices involved in the handling of LEAVE_SUSPENDED (eg. say a PCI
> parent has a child platform or I2C or similar devices without a PM domain
> and what happens to the child affects the parent).
> 
> > >
> > > Namely, make it skip all of the system-wide resume callbacks for
> > > such devices with DPM_FLAG_LEAVE_SUSPENDED set if they are in
> > > runtime suspend during the "noirq" phase of system-wide suspend
> > > (or analogous) transitions or the system transition under way is
> > > a proper suspend (rather than anything related to hibernation) and
> > > the device's wakeup settings are compatible with runtime PM (that
> > > is, the device cannot generate wakeup signals at all or it is
> > > allowed to wake up the system from sleep).
> > 
> > As I pointed out by submitting another patch [2], device_may_wakeup()
> > doesn't really tell whether the wakeup is configured as "in-band" or
> > "out-of-band". That knowledge is known by the driver and the subsystem
> > layer - and for that reason I don't think the PM core shall base
> > generic decisions like this on it.
> 
> The "or it is allowed to wake up the system from sleep" case may be overly
> optimistic, but the remaining two (runtime-suspended devices and devices
> that can't generate wakeup signals at all) are quite straightforward to me.

BTW, I'm not sure if the device_may_wakeup() check is really insufficient
in this particular case.

Say the device was not in runtime suspend before, but device_may_wakeup()
returns "true" for it and the system is resuming from suspend.  The device's
state should be suitable to wake up the system in any case, so the "in-band"
vs "out-of-band" difference has had to be taken care of already during system
suspend.

Thanks,
Rafael

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