Re: [PATCH] USB: musb: fix external abort on suspend

[Johan Hovold]

On Mon, Jul 24, 2017 at 10:38:41AM -0400, Alan Stern wrote:
> On Mon, 24 Jul 2017, Johan Hovold wrote:
> 
> > Make sure that the controller is runtime resumed when system suspending
> > to avoid an external abort when accessing the interrupt registers:
> > 
> >   Unhandled fault: external abort on non-linefetch (0x1008) at 0xd025840a
> >   ...
> >   [<c05481a4>] (musb_default_readb) from [<c0545abc>] (musb_disable_interrupts+0x84/0xa8)
> >   [<c0545abc>] (musb_disable_interrupts) from [<c0546b08>] (musb_suspend+0x38/0xb8)
> >   [<c0546b08>] (musb_suspend) from [<c04a57f8>] (platform_pm_suspend+0x3c/0x64)
> > 
> > This is easily reproduced on a BBB by enabling the peripheral port only
> > (as the host port may enable the shared clock) and keeping it
> > disconnected so that the controller is runtime suspended. (Well, you
> > would also need to the not-yet-merged am33xx-suspend patches by Dave
> > Gerlach to be able to suspend the BBB.)
> > 
> > This is a regression that was introduced by commit 1c4d0b4e1806 ("usb:
> > musb: Remove pm_runtime_set_irq_safe") which allowed the parent glue
> > device to runtime suspend and thereby exposed a couple of older issues:
> > 
> > Register accesses without explicitly making sure the controller is
> > runtime resumed during suspend was first introduced by commit
> > c338412b5ded ("usb: musb: unconditionally save and restore the context
> > on suspend") in 3.14.
> > 
> > Commit a1fc1920aaaa ("usb: musb: core: make sure musb is in RPM_ACTIVE on
> > resume") later started setting the RPM status to active during resume
> > without first making sure that the parent was runtime resumed. This was
> > also implicitly relying on the parent always being active. Since commit
> > 71723f95463d ("PM / runtime: print error when activating a child to
> > unactive parent") this now also results in following warning:
> > 
> >   musb-hdrc musb-hdrc.0: runtime PM trying to activate child device
> >     musb-hdrc.0 but parent (47401400.usb) is not active
> 
> I don't understand this.  Why wouldn't the parent be in RPM_ACTIVE at
> this time?  After all, how could the system be expected to resume a
> child device if its parent wasn't fully active?

The parent for a musb controller is a "glue" device (e.g. musb_dsps)
which previously was always kept active, but that's no longer the case
as mentioned above.

In a system with two controllers (e.g. a Beagle Bone Black), the host
port may be active and keep the shared clock enabled (managed by the
grandparent device). Thereby the external-abort crash can be avoided
when suspending a disconnected (and runtime suspended) peripheral port.

When the system is later resumed, you would hit that broken activation
code of the runtime suspended device, with a likewise runtime suspended
parent, and the warning would be printed.

> In general, during a system resume callback we should bring a device
> back to full power, tell the PM core that this has been done, and leave
> it at full power until the whole system resume is finished.  For
> efficiency we can avoid doing this in cases where the device was in
> runtime suspend before the system suspend began, but you have to be
> very careful about it -- see the documentation for the ->prepare
> callback in Documentation/driver-api/pm/devices.rst.

Right, this is how things should have been implemented if it is at all
possible too keep the device runtime suspended across system suspend.

Thanks,
Johan