On Wed, 6 Feb 2019 at 13:10, Rafael J. Wysocki <raf...@kernel.org> wrote: > > On Wed, Feb 6, 2019 at 12:24 PM Ulf Hansson <ulf.hans...@linaro.org> wrote: > > > > On Wed, 6 Feb 2019 at 10:56, Rafael J. Wysocki <raf...@kernel.org> wrote: > > > > > > 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. > > > > Well, I would say it sucks in both cases. :-) > > > > > > > > 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. > > > > I get your point, but unfortunate I don't think it's that simple. > > > > For example, someone (like a child) may prevent runtime suspend for > > the consumer. Hence, also the supplier is prevented from being runtime > > suspended. > > Well, in that case the supplier should not be suspended until the > consumer can be suspended too. > > IOW, if you call device_link_del() in that case, it would be a bug if > it allowed the supplier suspend.
Well, maybe the "child" was a bad example. The point is, the driver doesn't control the RPM child count and nor the RPM usage count for its consumer device - solely by itself. Someone, like the driver/PM core can at some point decide to increase the runtime PM usage count for example for the consumer device. For whatever good reason. > > > So, if you want to push this responsibility to the driver, then I > > think we need make __pm_runtime_set_status() to respect device links, > > similar to how it already deals with child/parents. > > > > In that way, the driver could call pm_runtime_set_suspended(), before > > dropping the device link in ->probe(), which would allow the supplier > > to also become runtime suspended. > > I guess you mean that runtime PM would be disabled for the consumer at > that point? Yes. Calling pm_runtime_set_suspended() should be a part of the error path in the driver, which includes disabling runtime PM as well (if it enabled it in the first place of course). > > > I did a quick research of users of device links, unless I am mistaken, > > this seems like an okay approach. > > > > What do you think? > > Well, I think I need to know the exact use case you have in mind. :-) The use case is simply a generic driver that fails to probe by returning -EPROBE_DEFER. So it's hypothetical, but I often tests common sequences by using my RPM testdriver, to make sure it all works as expected. The below sequence is common, so then I have added the use of device links, to see how this plays. And it doesn't. ->probe() ... pm_runtime_get_noresume() pm_runtime_set_active() pm_runtime_enable() ... device_link_add(con, supp, DL_FLAG_STATELESS |DL_FLAG_PM_RUNTIME | DL_FLAG_RPM_ACTIVE); we got some errors... goto err: ... err: pm_runtime_put_noidle() pm_runtime_disable() pm_runtime_set_suspended() device_link_remove() return err; Kind regards Uffe
