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

[Stephen Boyd <swboyd@chromium.org>]

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).