[PATCH 1/2] PM / runtime: Allow drivers to override runtime PM behaviour on sleep

[Thierry Reding]

On Thu, Nov 28, 2019 at 11:20:01PM +0100, Rafael J. Wysocki wrote:
> On Thursday, November 28, 2019 11:03:57 PM CET Rafael J. Wysocki wrote:
> > On Thursday, November 28, 2019 5:50:26 PM CET Thierry Reding wrote:
> > > 
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> > > On Thu, Nov 28, 2019 at 05:14:51PM +0100, Rafael J. Wysocki wrote:
> > > > On Thu, Nov 28, 2019 at 5:03 PM Thierry Reding <thierry.reding at gmail.com>=
> > >  wrote:
> > > > >
> > > > > From: Thierry Reding <treding at nvidia.com>
> > > > >
> > > > > Currently the driver PM core will automatically acquire a runtime PM
> > > > > reference for devices before system sleep is entered. This is needed
> > > > > to avoid potential issues related to devices' parents getting put to
> > > > > runtime suspend at the wrong time and causing problems with their
> > > > > children.
> > > >=20
> > > > Not only for that.
> > > >=20
> > > > > In some cases drivers are carefully written to avoid such issues and
> > > > > the default behaviour can be changed to allow runtime PM to operate
> > > > > regularly during system sleep.
> > > >=20
> > > > But this change breaks quite a few assumptions in the core too, so no,
> > > > it can't be made.
> > > 
> > > Anything in particular that I can look at? I'm not seeing any issues
> > > when I test this, which could of course mean that I'm just getting
> > > lucky.
> > 
> > There are races and such that you may never hit during casual testing.
> > 
> > > One thing that irritated me is that I think this used to work. I do
> > > recall testing suspend/resume a few years ago and devices would get
> > > properly runtime suspended/resumed.
> > 
> > Not true at all.
> > 
> > The PM core has always taken PM-runtime references on all devices pretty much
> > since when PM-runtime was introduced.
> > 
> > > I did some digging but couldn't
> > > find anything that would have had an impact on this.
> > > 
> > > Given that this is completely opt-in feature, why are you categorically
> > > NAK'ing this?
> > 
> > The general problem is that if any device has been touched by system-wide
> > suspend code, it should not be subject to PM-runtime any more until the
> > subsequent system-wide resume is able to undo whatever the suspend did.
> > 
> > Moreover, if a device is runtime-suspended, the system-wide suspend code
> > may mishandle it, in general.  That's why PM-runtime suspend is not allowed
> > during system-wide transitions at all.  And it has always been like that.
> > 
> > For a specific platform you may be able to overcome these limitations if
> > you are careful enough, but certainly they are there in general and surely
> > you cannot prevent people from using your opt-in just because they think
> > that they know what they are doing.
> 
> BTW, what if user space prevents PM-runtime from suspending devices by writing
> "on" to their "control" files?
> 
> System-wide suspend is (of course) still expected to work in that case, so how
> exactly would you overcome that?

I suppose one way to overcome that would be to make it an error to write
"on" to the "control" files for these devices.

Currently doing this is likely going to break display support on Tegra,
so this would be a good idea in this case anyway.

Again, I could avoid all of these issues by avoiding runtime PM in this
driver, but I would end up reimplementing some of the same concepts. I'd
rather use something that's supported by the PM core and that might be
useful to other drivers than reinvent the wheel.

Thierry
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