Messages in this thread | | | From | "Rafael J. Wysocki" <> | Date | Mon, 4 Feb 2019 12:40:25 +0100 | Subject | Re: [PATCH v2 0/9] driver core: Fix some device links issues and add "consumer autoprobe" flag |
| |
On Fri, Feb 1, 2019 at 4:18 PM Ulf Hansson <ulf.hansson@linaro.org> wrote: > > On Fri, 1 Feb 2019 at 02:04, Rafael J. Wysocki <rjw@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.oiOpCWJBV7@aspire.rjw.lan/ > > and https://lore.kernel.org/lkml/2405639.4es7pRLqn0@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.hansson@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, the link's rpm_active refcount is one already and so the supplier's PM-runtime count should not be dropped.
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.
| |