usb: dwc3: of-simple: fix use-after-free on remove

Hi,

Adding Rafael and linux-pm to Cc as well.

* Felipe Balbi [off-list ref] [180619 01:23]:

This is a direct consequence of not paying attention to the order of
things. If driver were to assume that pm_domain->activate() would do the
right thing for the device -- meaning that probe would run with an
active device --, then we wouldn't need that pm_runtime_get() call on
probe at all. Rather we would follow the sequence:

	pm_runtime_forbid()
	pm_runtime_set_active()
	pm_runtime_enable()

	/* do your probe routine */

	pm_runtime_put_noidle()

Then you remove you would need to call pm_runtime_get_noresume() to
balance out the pm_runtime_put_noidle() there.

How about let's create some prettier interface for the above runtime PM
trickery?

How about something like pm_runtime_init_enabled() for the above
sequence?

It might be then able to do the trick even if activate is not
implemented..

Right now it has the feeling of "oh well we can't get runtime PM to
work so let's bypass it with activate call and then trick runtime PM
to start in enabled mode" :)

Anyway, with an assumption like this, after all platform_devices are
converted over, the assumption can be moved into the bus code and, low
and behold, to enable runtime pm for your driver, all you have to is
implement your callbacks and add pm_runtime_put_noidle() to probe and
pm_runtime_get_noresume() to remove (apart from, of course, making sure
the device isn't allowed to runtime_suspend when it's actually busy).

Do you see the end goal?

It certainly would be nice to make runtime PM generic for drivers :)

(If you need to know why the pm_runtime_put_noidle(), remember that
pm_runtime_set_active() increments the usage counter, so
pm_runtime_put_noidle is basically allowing pm_runtime to happen as soon
as userspace writes "auto" to /sys/..../power/control)

I wonder if we could also remove the need for drivers to call
pm_runtime_putnoidle() at the end of the probe? If we had
pm_runtime_init_enabled() implemented.

Regards,

Tony
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