On Tue, Feb 5, 2019 at 12:27 PM Rafael J. Wysocki <r...@rjwysocki.net> wrote: > > On Tuesday, February 5, 2019 9:15:49 AM CET Ulf Hansson wrote: > > On Mon, 4 Feb 2019 at 12:45, Rafael J. Wysocki <raf...@kernel.org> wrote: > > > > > > 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:
[cut] > > > > > > 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. > > > > I see what your are saying and I must admit, by looking at the code, > > that it has turned into being rather complicated. Assuming of good > > reasons, of course. > > > > Anyway, I will play a little bit more with my tests to see what I can find > > out. > > > > > > > > > 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. > > > > Let's see. > > So for the record, below is the (untested) patch I'm thinking about. > > Having considered this for some time, I think that it would be better to > try to drop the supplier's PM-runtime usage counter on link removal even if > the link doesn't go away then. That would be more consistent at least IMO. So I can't convince myself that this is the case. Either way, if there are two callers of device_link_add() for one consumer-supplier pair trying to add a stateless link between them and one of these callers passes DL_FLAG_RPM_ACTIVE set in the flags to it, there may be issues regardless of what device_link_del() and device_link_remove() do. However, if they decrement the link's rpm_active refcount (and possibly the supplier's PM-runtime usage counter too), the supplier may be suspended prematurely, whereas in the other case (no decrementation of rpm_active, which how the code works after this series) it may just be prevented from suspending. To me, the former is worse than the latter. Moreover, there is a workaround for the latter issue that seems to be easy enough: it is sufficient to let the consumer runtime suspend after calling device_link_add() to create the link (with DL_FLAG_RPM_ACTIVE set) and before trying to remove it. Because of the above, I'm just going to post a patch to document the current behavior of the code as a known limitation.
