Messages in this thread | | | From | Ulf Hansson <> | Date | Wed, 6 Feb 2019 12:23:26 +0100 | Subject | Re: [PATCH v2 0/9] driver core: Fix some device links issues and add "consumer autoprobe" flag |
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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.
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 did a quick research of users of device links, unless I am mistaken, this seems like an okay approach.
What do you think?
> > Because of the above, I'm just going to post a patch to document the > current behavior of the code as a known limitation.
Let's not give up, yet, please. I am sure we can figure something out.
Kind regards Uffe
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