Re: [PATCH v20 2/5] usb: dwc3: core: Host wake up support from system suspend

[Pavan Kondeti <quic_pkondeti@quicinc.com>]

On Mon, Jun 27, 2022 at 01:02:49PM -0700, Stephen Boyd wrote:
> Quoting Pavan Kondeti (2022-06-20 01:54:15)
> > +Felipe, Bjorn
> >
> > On Thu, Jun 16, 2022 at 10:15:49AM -0700, Matthias Kaehlcke wrote:
> > > On Thu, Jun 16, 2022 at 02:41:10PM +0530, Pavan Kondeti wrote:
> > >
> > > Good point! It doesn't really ensure that the child is probed (actually it
> > > won't be probed and DL_FLAG_AUTOPROBE_CONSUMER doesn't make sense here), it
> > > could happen that dwc3_qcom_probe() is deferred multiple times, but eventually
> > > the PHYs should be ready and dwc3_probe() be invoked through
> > > of_platform_populate().
> >
> > This is a generic problem i.e if a parent can only proceed after the child
> > devices are bounded (i.e probed successfully), how to ensure this behavior
> > from the parent's probe? Since we can't block the parent probe (async probe is
> > not the default behavior), we have to identify the condition that the children
> > are deferring probe, so that parent also can do that.
> >
> > Can we add a API in drivers core to tell if a device probe is deferred or
> > not? This can be done by testing list_empty(&dev->p->deferred_probe) under
> > deferred_probe_mutex mutex. The parent can return EPROBE_DEFER based on this
> > API return value.
> >
> > Another alternative would be explicitly checking if the child device suppliers
> > are ready or not before adding child device. That would require decoupling
> > of_platform_populate() to creating devices and adding devices.
> >
> > Note that this problem is not just limited to suppliers not ready. if the
> > dwc3-qcom is made asynchronous probe, then its child also probed
> > asynchronously and there is no guarantee that child would be probed by the
> > time of_platform_populate() is returned.  The bus notifier might come handy
> > in this case. The parent can register for this notifier and waiting for
> > the children device's BUS_NOTIFY_BOUND_DRIVER/BUS_NOTIFY_DRIVER_NOT_BOUND
> > notifications. This would also work in our case, if we move to
> > of_platform_populate() outside the probe().
> >
> > Would like to hear other people thoughts on this.
> >
> 
> I'm not following very closely but it sounds like a problem that may be
> solved by using the component driver code (see
> include/linux/component.h). That would let you move anything that needs
> to be done once the child devices probe to the aggregate driver 'bind'
> function (see struct component_master_ops::bind).

Thanks Stephen for letting us know about the component device framework.

IIUC, 

- dwc3-qcom (parent of the dwc3 core) registers as a component master by
calling component_master_add_with_match() before calling
of_platform_populate(). The match callback could be as simple as comparing
the device against our child device.

- The dwc3 core (child) at the end of its probe can add as a component by calling
component_add(). 

- The above triggers the component_master_ops::bind callback implemented in
  dwc3-qcom driver which signals that we are good to go.

- The dwc-qcom can call component_bind_all() to finish the formality i.e
  telling the dwc3 core that we are good to go.

Is my understanding correct? This is what we are looking for i.e a way for
the child device(s) to signal the parent when the former is bounded.

Also what happens when the child device probe fails for any reason. i.e
component_add() would never be called so the master driver i.e dwc3-qcom would
wait indefinitely. May be it needs to implement a timeout or runtime suspend
etc should take care of keeping the resoures in suspend state.

Thanks,
Pavan