Re: [PATCH 2/2] driver core: Fix possible supplier PM-usage counter imbalance

[Jon Hunter]

Hi Rafael,

On 12/02/2019 12:08, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> 
> If a stateless device link to a certain supplier with
> DL_FLAG_PM_RUNTIME set in the flags is added and then removed by the
> consumer driver's probe callback, the supplier's PM-runtime usage
> counter will be nonzero after that which effectively causes the
> supplier to remain "always on" going forward.
> 
> Namely, device_link_add() called to add the link invokes
> device_link_rpm_prepare() which notices that the consumer driver is
> probing, so it increments the supplier's PM-runtime usage counter
> with the assumption that the link will stay around until
> pm_runtime_put_suppliers() is called by driver_probe_device(),
> but if the link goes away before that point, the supplier's
> PM-runtime usage counter will remain nonzero.
> 
> To prevent that from happening, first rework pm_runtime_get_suppliers()
> and pm_runtime_put_suppliers() to use the rpm_active refounts of device
> links and make the latter only drop rpm_active and the supplier's
> PM-runtime usage counter for each link by one, unless rpm_active is
> one already for it.  Next, modify device_link_add() to bump up the
> new link's rpm_active refcount and the suppliers PM-runtime usage
> counter by two, to prevent pm_runtime_put_suppliers(), if it is
> called subsequently, from suspending the supplier prematurely (in
> case its PM-runtime usage counter goes down to 0 in there).
> 
> Due to the way rpm_put_suppliers() works, this change does not
> affect runtime suspend of the consumer ends of new device links (or,
> generally, device links for which DL_FLAG_PM_RUNTIME has just been
> set).
> 
> Fixes: e2f3cd831a28 ("driver core: Fix handling of runtime PM flags in device_link_add()")
> Reported-by: Ulf Hansson <ulf.hansson@xxxxxxxxxx> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> ---
> 
> Note that the issue had been there before commit e2f3cd831a28, but it was
> overlooked by that commit and this change is a fix on top of it, so make
> the Fixes: tag point to commit e2f3cd831a28 (instead of an earlier one
> that the patch will not be applicable to).
I noticed that yesterday's and today's -next were no longer booting on
one of our Tegra boards (Tegra210 Jetson TX2) because networking is
failing. The ethernet chip is a USB device and looking at the bootlogs I
can see that the Tegra XHCI driver is failing ...

 tegra-xusb 70090000.usb: xHCI host controller not responding, assume dead
 tegra-xusb 70090000.usb: HC died; cleaning up

The Tegra XHCI driver uses multiple power-domains and uses
device_link_add() to attach them. So now I am wondering if there is
something that we have got wrong in our implementation. However, I don't
see the device being probed deferred on boot or anything like that.

The driver in question is drivers/usb/host/xhci-tegra.c and we add the
links in the function tegra_xusb_powerdomain_init() which is before RPM
is enabled. Let me know if you have any thoughts.

Cheers
Jon

-- 
nvpublic