linux-kernel - Re: [PATCH v7 2/4] usb: dwc3: qcom: Add interconnect support in dwc3 driver

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Date:   Thu, 04 Jun 2020 04:16:31 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Doug Anderson <dianders@...omium.org>,
        Felipe Balbi <balbi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mark Rutland <mark.rutland@....com>,
        Matthias Kaehlcke <mka@...omium.org>,
        Rob Herring <robh+dt@...nel.org>,
        Sandeep Maheswaram (Temp) <sanm@...eaurora.org>
Cc:     linux-arm-msm@...r.kernel.org, linux-usb@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        Manu Gautam <mgautam@...eaurora.org>,
        Chandana Kishori Chiluveru <cchiluve@...eaurora.org>
Subject: Re: [PATCH v7 2/4] usb: dwc3: qcom: Add interconnect support in dwc3 driver

Quoting Sandeep Maheswaram (Temp) (2020-06-04 02:43:09)
> 
> On 6/3/2020 11:06 PM, Stephen Boyd wrote:
> > Quoting Sandeep Maheswaram (2020-03-31 22:15:43)
> >> diff --git a/drivers/usb/dwc3/dwc3-qcom.c b/drivers/usb/dwc3/dwc3-qcom.c
> >> index 1dfd024..d33ae86 100644
> >> --- a/drivers/usb/dwc3/dwc3-qcom.c
> >> +++ b/drivers/usb/dwc3/dwc3-qcom.c
> >> @@ -285,6 +307,101 @@ static int dwc3_qcom_resume(struct dwc3_qcom *qcom)
> >>          return 0;
> >>   }
> >>   
> >> +
> >> +/**
> >> + * dwc3_qcom_interconnect_init() - Get interconnect path handles
> >> + * @qcom:                      Pointer to the concerned usb core.
> >> + *
> >> + */
> >> +static int dwc3_qcom_interconnect_init(struct dwc3_qcom *qcom)
> >> +{
> >> +       struct device *dev = qcom->dev;
> >> +       int ret;
> >> +
> >> +       if (!device_is_bound(&qcom->dwc3->dev))
> >> +               return -EPROBE_DEFER;
> > How is this supposed to work? I see that this was added in an earlier
> > revision of this patch series but there isn't any mention of why
> > device_is_bound() is used here. It would be great if there was a comment
> > detailing why this is necessary. It sounds like maximum_speed is
> > important?
> >
> > Furthermore, dwc3_qcom_interconnect_init() is called by
> > dwc3_qcom_probe() which is the function that registers the device for
> > qcom->dwc3->dev. If that device doesn't probe between the time it is
> > registered by dwc3_qcom_probe() and this function is called then we'll
> > fail dwc3_qcom_probe() with -EPROBE_DEFER. And that will remove the
> > qcom->dwc3->dev device from the platform bus because we call
> > of_platform_depopulate() on the error path of dwc3_qcom_probe().
> >
> > So isn't this whole thing racy and can potentially lead us to a driver
> > probe loop where the wrapper (dwc3_qcom) and the core (dwc3) are probing
> > and we're trying to time it just right so that driver for dwc3 binds
> > before we setup interconnects? I don't know if dwc3 can communicate to
> > the wrapper but that would be more of a direct way to do this. Or maybe
> > the wrapper should try to read the DT property for maximum speed and
> > fallback to a worst case high bandwidth value if it can't figure it out
> > itself without help from dwc3 core.
> >
> This was added in V4 to address comments from Matthias in V3
> 
> https://patchwork.kernel.org/patch/11148587/
> 

Yes, that why I said:

"I see that this was added in an earlier
 revision of this patch series but there isn't any mention of why
 device_is_bound() is used here. It would be great if there was a comment
 detailing why this is necessary. It sounds like maximum_speed is
 important?"

Can you please respond to the rest of my email?