On Mon, Feb 4, 2019 at 12:40 PM Rafael J. Wysocki <raf...@kernel.org> wrote: > > On Fri, Feb 1, 2019 at 4:18 PM Ulf Hansson <ulf.hans...@linaro.org> wrote: > > > > On Fri, 1 Feb 2019 at 02:04, Rafael J. Wysocki <r...@rjwysocki.net> wrote: > > > > > > Hi Greg at al, > > > > > > This is a combination of the two device links series I have posted > > > recently (https://lore.kernel.org/lkml/2493187.oiopcwj...@aspire.rjw.lan/ > > > and https://lore.kernel.org/lkml/2405639.4es7prl...@aspire.rjw.lan/) > > > rebased > > > on top of your driver-core-next branch. > > > > > > Recently I have been looking at the device links code because of the > > > recent discussion on possibly using them in the DRM subsystem (see for > > > example https://marc.info/?l=linux-pm&m=154832771905309&w=2) and I have > > > found a few issues in that code which should be addressed by this patch > > > series. Please refer to the patch changelogs for details. > > > > > > None of the problems addressed here should be manifesting themselves in > > > mainline kernel today, but if there are more device links users in the > > > future, they most likely will be encountered sooner or later. Also they > > > need to be fixed for the DRM use case to be supported IMO. > > > > > > On top of this the series makes device links support the "composite > > > device" > > > use case in the DRM subsystem mentioned above (essentially, the last patch > > > in the series is for that purpose). > > > > > > > Rafael, Greg, I have reviewed patch 1 -> 7, they all look good to me. > > > > If not too late, feel free to add for the first 7 patches: > > > > Reviewed-by: Ulf Hansson <ulf.hans...@linaro.org> > > Thanks! > > > Although, I want to point out one problem that I have found when using > > device links. I believe it's already there, even before this series, > > but just wanted to described it for your consideration. > > > > This is what happens: > > I have a platform driver is being probed. During ->probe() the driver > > adds a device link like this: > > > > link = device_link_add(consumer-dev, supplier-dev, DL_FLAG_STATELESS | > > DL_FLAG_PM_RUNTIME | DL_FLAG_RPM_ACTIVE); > > > > At some point later in ->probe(), the driver realizes that it must > > remove the device link, either because it encountered an error or > > simply because it doesn't need the device link to be there anymore. > > Thus it calls: > > > > device_link_del(link); > > > > When probe finished of the driver, the runtime PM usage count for the > > supplier-dev remains increased to 1 and thus it never becomes runtime > > suspended. > > OK, so this is a tricky one. > > With this series applied, if the link actually goes away after the > cleanup device_link_del(), device_link_free() should take care of > dropping the PM-runtime count of the supplier. If it doesn't do that, > there is a mistake in the code that needs to be fixed. > > However, if the link doesn't go away after the cleanup > device_link_del(), the supplier's PM-runtime count will not be > dropped, because the core doesn't know whether or not the > device_link_del() has been called by the same entity that caused the > supplier's PM-runtime count to be incremented. For example, if the > consumer device is suspended after the device_link_add() that > incremented the supplier's PM-runtime count and then suspended again,
I was distracted while writing this, sorry for the confusion. So let me rephrase: For example, if the consumer device is suspended after the device_link_add() that incremented the supplier's PM-runtime count and then resumed again, the rpm_active refcount will be greater than one because of the last resume and not because of the initial link creation. In that case, dropping the supplier's PM-runtime count on link deletion may not work as expected. > Arguably, device_link_del() could be made automatically drop the > supplier's PM-runtime count by one if the link's rpm_active refcount > is not one, but there will be failing scenarios in that case too > AFAICS.
