On Fri, Nov 29, 2019 at 11:08 AM Thierry Reding <thierry.reding@xxxxxxxxx> wrote: > > 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: > > > > > > > > --0F1p//8PRICkK4MW > > > > Content-Type: text/plain; charset=us-ascii > > > > Content-Disposition: inline > > > > Content-Transfer-Encoding: quoted-printable > > > > > > > > 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@xxxxxxxxx>= > > > > wrote: > > > > > > > > > > > > From: Thierry Reding <treding@xxxxxxxxxx> > > > > > > > > > > > > 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. Seeing suggestions like this in messages from seasoned kernel developers is seriously disappointing. :-/ > Currently doing this is likely going to break display support on Tegra, > so this would be a good idea in this case anyway. PM-runtime has always allowed user space to prevent devices from being suspended and it seems that this has not been taken into account by Tegra display support developers at all. > Again, I could avoid all of these issues by avoiding runtime PM in this driver, I don't quite see the connection here. Preventing a device from suspending should never be a functional problem. It may be an energy-efficiency problem, but that's something for user space to consider before writing "on" to a device's control file. > 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. Which doesn't have to be by using PM-runtime suspend for the handling of system-wide suspend, at least in my view.
