Re: [PATCH] PM: Allow pm_runtime_suspend() to succeed during system suspend

["Rafael J. Wysocki" <rjw-KKrjLPT3xs0@public.gmane.org>]

On Friday, February 11, 2011, Kevin Hilman wrote:
> "Rafael J. Wysocki" <rjw-KKrjLPT3xs0@public.gmane.org> writes:
> 
> > On Monday, January 31, 2011, Rafael J. Wysocki wrote:
> >> On Monday, January 31, 2011, Alan Stern wrote:
> >> > On Mon, 31 Jan 2011, Kevin Hilman wrote:
> >> > 
> >> > > I understand how this works, but frankly I'm still a bit fuzzy on why.
> >> > > 
> >> > > I guess I'm still missing a good understanding of what "interfering with a
> >> > > system power transition" means, and why a runtime suspend qualifies as
> >> > > interfering but not a runtime resume.
> >> > 
> >> > These are good questions.  Rafael implemented this design originally; 
> >> > my contribution was only to warn him of the potential for problems.  
> >> > Therefore he should explain the rationale for the design.
> >> 
> >> The reason why runtime resume is allowed during system power transitions is
> >> because in some cases during system suspend we simply have to resume devices
> >> that were previously runtime-suspended (for example, the PCI bus type does
> >> that).
> >> 
> >> The reason why runtime suspend is not allowed during system power transitions
> >> if the following race:
> >> 
> >> - A device has been suspended via a system suspend callback.
> >> - The runtime PM framework executes a (scheduled) suspend on that device,
> >>   not knowing that it's already been suspended, which potentially results in
> >>   accessing the device's registers in a low-power state.
> >> 
> >> Now, it can be avoided if every driver does the right thing and checks whether
> >> the device is already suspended in its runtime suspend callback, but that would
> >> kind of defeat the purpose of the runtime PM framework, at least partially.
> >
> > In fact, I've just realized that the above race cannot really occur, because
> > pm_wq is freezable, so I'm proposing the following change.
> >
> > Of course, it still doesn't prevent user space from disabling the runtime PM
> > framework's helpers via /sys/devices/.../power/control.
> >
> > Thanks,
> > Rafael
> >
> >
> > ---
> > From: Rafael J. Wysocki <rjw-KKrjLPT3xs0@public.gmane.org>
> > Subject: PM: Allow pm_runtime_suspend() to succeed during system suspend
> >
> > The dpm_prepare() function increments the runtime PM reference
> > counters of all devices to prevent pm_runtime_suspend() from
> > executing subsystem-level callbacks.  However, this was supposed to
> > guard against a specific race condition that cannot happen, because
> > the power management workqueue is freezable, so pm_runtime_suspend()
> > can only be called synchronously during system suspend and we can
> > rely on subsystems and device drivers to avoid doing that
> > unnecessarily.
> >
> > Make dpm_prepare() drop the runtime PM reference to each device
> > after making sure that runtime resume is not pending for it.
> >
> > Signed-off-by: Rafael J. Wysocki <rjw-KKrjLPT3xs0@public.gmane.org>
> > ---
> 
> Yes!
> 
> Acked-by: Kevin Hilman <khilman-l0cyMroinI0@public.gmane.org>

Well, I hope you realize that it doesn't help you a lot?

Rafael