Re: [PATCH 0/3] PM: runtime: Fixes related to device links management

[Rafael J. Wysocki]

On Friday, October 23, 2020 5:50:04 AM CEST chenxiang (M) wrote:
> Hi Rafael,
> 
> 在 2020/10/22 3:10, Rafael J. Wysocki 写道:
> > Hi Greg & all,
> >
> > Commit d12544fb2aa9 ("PM: runtime: Remove link state checks in
> > rpm_get/put_supplier()") merged recently introduced a weakness
> > in the handling of device links in the runtime PM framework that
> > may be confusing and even harmful.
> >
> > Namely, the checks removed by that commit prevented PM-runtime from
> > getting or dropping references to the supplier device whose driver
> > was going away via its links to consumers, which specifically allowed
> > the pm_runtime_clean_up_links() called from __device_release_driver()
> > to run without interfering with runtime suspend/resume of consumer
> > devices (which still might happen even though the drivers had been
> > unbound from them by that time).
> >
> > After the above commit, calling pm_runtime_clean_up_links() from
> > __device_release_driver() makes a little sense and it may be interfering
> > destructively with regular PM-runtime suspend/resume control flows, so
> > it needs to be either fixed or dropped altogether.  I prefer the latter,
> > because among other things this removes an arbitrary difference in the
> > handling of managed device links with respect to the stateless ones,
> > so patch [2/3] is doing just that.
> >
> > However, in some rare cases pm_runtime_clean_up_links() may help to clean
> > up leftover PM-runtime references, so if that function goes away, they
> > need to be cleaned up elsewhere.  That's why patch [1/3] modifies
> > __device_link_del() to drop them upon device link removal (which also
> > needs to be done for stateless device links and that's why I'm regarding
> > this patch as a fix).
> >
> > Finally, to avoid pointless overhead related to suspending and resuming
> > the target device for multiple times in a row in __device_release_driver(),
> > it is better to resume it upfront before checking its links to consumers,
> > which is done by patch [3/3].
> 
> 
> I have tested the patchset, and it solves my reported issue, so please 
> feel free to add :
> Tested-by: Xiang Chen <chenxiang66@xxxxxxxxxxxxx>

Thank you!