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Date:   Thu, 23 Jun 2022 11:38:06 -0700
From:   Matthias Kaehlcke <mka@chromium.org>
To:     Pavan Kondeti <quic_pkondeti@quicinc.com>
Cc:     Bjorn Andersson <bjorn.andersson@linaro.org>,
        Felipe Balbi <balbi@kernel.org>,
        Krishna Kurapati <quic_kriskura@quicinc.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Rob Herring <robh+dt@kernel.org>,
        Andy Gross <agross@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Stephen Boyd <swboyd@chromium.org>,
        Doug Anderson <dianders@chromium.org>,
        Mathias Nyman <mathias.nyman@intel.com>,
        devicetree@vger.kernel.org, linux-arm-msm@vger.kernel.org,
        linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-pm@vger.kernel.org, quic_ppratap@quicinc.com,
        quic_vpulyala@quicinc.com
Subject: Re: [PATCH v20 2/5] usb: dwc3: core: Host wake up support from
 system suspend
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On Mon, Jun 20, 2022 at 02:24:15PM +0530, Pavan Kondeti wrote:
> +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:
> > > Hi Matthias/Krishna,
> > > 
> > > On Tue, Jun 14, 2022 at 10:53:35AM -0700, Matthias Kaehlcke wrote:
> > > > On Mon, Jun 13, 2022 at 11:08:32AM -0700, Matthias Kaehlcke wrote:
> > > > > On Mon, Jun 06, 2022 at 01:45:51PM -0700, Matthias Kaehlcke wrote:
> > > > > > On Thu, Jun 02, 2022 at 12:35:42PM -0700, Matthias Kaehlcke wrote:
> > > > > > > Hi Krishna,
> > > > > > > 
> > > > > > > with this version I see xHCI errors on my SC7180 based system, like
> > > > > > > these:
> > > > > > > 
> > > > > > > [   65.352605] xhci-hcd xhci-hcd.13.auto: xHC error in resume, USBSTS 0x401, Reinit
> > > > > > > 
> > > > > > > [  101.307155] xhci-hcd xhci-hcd.13.auto: WARN: xHC CMD_RUN timeout
> > > > > > > 
> > > > > > > After resume a downstream hub isn't enumerated again.
> > > > > > > 
> > > > > > > So far I didn't see those with v13, but I aso saw the first error with
> > > > > > > v16.
> > > > > > 
> > > > > > It also happens with v13, but only when a wakeup capable vUSB <= 2
> > > > > > device is plugged in. Initially I used a wakeup capable USB3 to
> > > > > > Ethernet adapter to trigger the wakeup case, however older versions
> > > > > > of this series that use usb_wakeup_enabled_descendants() to check
> > > > > > for wakeup capable devices didn't actually check for vUSB > 2
> > > > > > devices.
> > > > > > 
> > > > > > So the case were the controller/PHYs is powered down works, but
> > > > > > the controller is unhappy when the runtime PM path is used during
> > > > > > system suspend.
> > > > > 
> > > > > The issue isn't seen on all systems using dwc3-qcom and the problem starts
> > > > > during probe(). The expected probe sequence is something like this:
> > > > > 
> > > > > dwc3_qcom_probe
> > > > >   dwc3_qcom_of_register_core
> > > > >     dwc3_probe
> > > > > 
> > > > >   if (device_can_wakeup(&qcom->dwc3->dev))
> > > > >     ...
> > > > > 
> > > > > The important part is that device_can_wakeup() is called after dwc3_probe()
> > > > > has completed. That's what I see on a QC SC7280 system, where wakeup is
> > > > > generally working with these patches.
> > > > > 
> > > > > However on a QC SC7180 system dwc3_probe() is deferred and only executed after
> > > > > dwc3_qcom_probe(). As a result the device_can_wakeup() call returns false.
> > > > > With that the controller/driver ends up in an unhappy state after system
> > > > > suspend.
> > > > > 
> > > > > Probing is deferred on SC7180 because device_links_check_suppliers() finds
> > > > > that '88e3000.phy' isn't ready yet.
> > > > 
> > > > It seems device links could be used to make sure the dwc3 core is present:
> > > > 
> > > >   Another example for an inconsistent state would be a device link that
> > > >   represents a driver presence dependency, yet is added from the consumer’s
> > > >   ->probe callback while the supplier hasn’t probed yet: Had the driver core
> > > >   known about the device link earlier, it wouldn’t have probed the consumer
> > > >   in the first place. The onus is thus on the consumer to check presence of
> > > >   the supplier after adding the link, and defer probing on non-presence.
> > > > 
> > > >   https://www.kernel.org/doc/html/v5.18/driver-api/device_link.html#usage
> > > > 
> > > > 
> > > > You could add something like this to dwc3_qcom_of_register_core():
> > > > 
> > > > 
> > > >   device_link_add(dev, &qcom->dwc3->dev,
> > > >   		  DL_FLAG_AUTOREMOVE_CONSUMER | DL_FLAG_AUTOPROBE_CONSUMER);
> > > > 
> > > >   if (qcom->dwc3->dev.links.status != DL_DEV_DRIVER_BOUND)
> > > >       ret = -EPROBE_DEFER;
> > > > 
> > > > 
> > > I am not very sure how the device_link_add() API works. we are the parent and
> > > creating a depdency on child probe. That does not sound correct to me.
> > 
> > The functional dependency is effectively there, the driver already assumes that
> > the dwc3 core was probed when of_platform_populate() returns.
> > 
> > The device link itself doesn't create the dependency on the probe(), the check
> > of the link status below does.
> > 
> > Another option would be to add a link to the PHYs to the dwc3-qcom node in
> > the device tree, but I don't think that would be a better solution (and I
> > expect Rob would oppose this).
> > 
> > I'm open to other solutions, so far the device link is the cleanest that came
> > to my mind.
> > 
> > I think the root issue is the driver architecture, with two interdependent
> > drivers for the same IP block, instead of a single framework driver with a
> > common part (dwc3 core) and vendor specific hooks/data.
> > 
> > > Any ways, I have another question.
> > > 
> > > When dwc3_qcom_of_register_core() returns error back to dwc3_qcom_probe(), we
> > > goto depopulate label which calls of_platform_depopulate() which destroy the
> > > child devices that are populated. how does that ensure that child probe is
> > > completed by the time, our probe is called again. The child device it self is
> > > gone. Is this working because when our probe is called next time, the child
> > > probe depenencies are resolved?
> > 
> > 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.

That could be an option.

> 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.

It might require a new API since there are plenty of users of
of_platform_populate() that rely on the current behavior.

> 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().

If I understand correctly the outcome would be a probe() in two stages. The
first does as much as it can do without the dwc3 core and leaves the device
in a state where it isn't really functional, and the second stage does the
rest when BUS_NOTIFY_BOUND_DRIVER is received for the dwc3 core device.

A concern could be the need for additional conditions in some code paths to
deal with the half-initialized device.

Why would of_platform_populate() be moved outside of probe()?

To avoid the half-initialized device probe() could block until
BUS_NOTIFY_BOUND_DRIVER is received. Probably that should be done with a
timeout to avoid blocking forever in case of a problem with probing the
dwc3 core.