Messages in this thread | | | From | "Rafael J. Wysocki" <> | Date | Wed, 6 Feb 2019 13:10:35 +0100 | Subject | Re: [PATCH v2 0/9] driver core: Fix some device links issues and add "consumer autoprobe" flag |
| |
On Wed, Feb 6, 2019 at 12:24 PM Ulf Hansson <ulf.hansson@linaro.org> wrote: > > On Wed, 6 Feb 2019 at 10:56, Rafael J. Wysocki <rafael@kernel.org> wrote: > > > > On Tue, Feb 5, 2019 at 12:27 PM Rafael J. Wysocki <rjw@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 <rafael@kernel.org> wrote: > > > > > > > > > > On Mon, Feb 4, 2019 at 12:40 PM Rafael J. Wysocki <rafael@kernel.org> wrote: > > > > > > > > > > > > On Fri, Feb 1, 2019 at 4:18 PM Ulf Hansson <ulf.hansson@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.
> 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?
> 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. :-)
| |