Re: [PATCH v2 0/4] PM / Domains: Fix race conditions during boot

[Kevin Hilman <khilman@kernel.org>]

Ulf, Rafael,

Ulf Hansson <ulf.hansson@linaro.org> writes:

> When there are more than one device in a PM domain these will obviously
> be probed at different times. Depending on timing and the implemented
> support for runtime PM in a driver/subsystem, genpd may be advised to
> power off a PM domain after a successful probe sequence.
>
> Ideally we should have relied on the driver/subsystem, through runtime
> PM, to bring their device's PM domain into powered state prior doing
> probing if such requirement exist.

I think I've stumbled on a related problem, or maybe the same one.

Even if platform-specific init code has initialized a device with
pm_runtime_set_active(), it seems that the genpd domain can still
power off before before all of its devices are probed.

This is because pm_genpd_poweroff() requires there to be a driver
when it's checking if a device is pm_runtime_suspended() which will not
be the case if the driver has not been probed yet.

Consider this case: There are several devices in the domain that haven't
been probed yet (dev->driver == NULL), but have been marked with
pm_runtime_set_active() + _get_noresume(), so pm_runtime_suspended() == false.  

Then, one of devices is in the domain is probed, and during the probe it
does a _get_sync(), sets some stuff up, and then does _put_sync().
After the probe, because of the _put_sync(), the genpd
->runtime_suspend() will be triggered, causing it to attempt a
_genpd_poweroff().  Since the rest of the devices in the domain haven't
(yet) been probed, their dev->driver pointers are all still NULL, so the
pm_runtime_suspended() check will not be attempted for them.

The result is that the genpd will poweroff after the first device is
probed, but before the others have had a chance to probe, which is not
exactly desired behavior for a genpd that has been initialized as
powered on.

With the hack below[1], I'm able to avoid that problem, but am not
completely sure yet if this is safe in general.

Rafael, do you remember why that check for dev->driver is needed?
Without digging deeper (which I'll do tomorrow), seems to me that
checking pm_runtime_suspended() on devices without drivers is a
reasonable thing to do since they can be initailzed by platform code
before they are probed.   If you think this is OK, I'll cook up a real
patch with a changelog.

Ulf, I'm not sure if this is the same problem you're having, but do you
think this would solve your problem if the drivers are properly
initialized?

Kevin


[1]
diff --git a/drivers/base/power/domain.c b/drivers/base/power/domain.c
index 568bf3172bef..17b0d9466d98 100644
--- a/drivers/base/power/domain.c
+++ b/drivers/base/power/domain.c
@@ -471,7 +471,7 @@ static int pm_genpd_poweroff(struct
generic_pm_domain *genpd)
                if (stat > PM_QOS_FLAGS_NONE)
                        return -EBUSY;

-               if (pdd->dev->driver && (!pm_runtime_suspended(pdd->dev)
+               if ((!pm_runtime_suspended(pdd->dev)
                    || pdd->dev->power.irq_safe))
                        not_suspended++;
        }