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Subject: Re: [PATCH v7 2/4] usb: dwc3: qcom: Add interconnect support in dwc3 driver
From:   Stephen Boyd <swboyd@chromium.org>
Cc:     linux-arm-msm@vger.kernel.org, linux-usb@vger.kernel.org,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        Manu Gautam <mgautam@codeaurora.org>,
        Chandana Kishori Chiluveru <cchiluve@codeaurora.org>,
        Sandeep Maheswaram <sanm@codeaurora.org>
To:     Andy Gross <agross@kernel.org>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        Doug Anderson <dianders@chromium.org>,
        Felipe Balbi <balbi@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Matthias Kaehlcke <mka@chromium.org>,
        Rob Herring <robh+dt@kernel.org>,
        Sandeep Maheswaram <sanm@codeaurora.org>
Date:   Wed, 03 Jun 2020 10:36:18 -0700
Message-ID: <159120577830.69627.13288547914742515702@swboyd.mtv.corp.google.com>
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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
> @@ -76,8 +85,13 @@ struct dwc3_qcom {
>         enum usb_dr_mode        mode;
>         bool                    is_suspended;
>         bool                    pm_suspended;
> +       struct icc_path         *usb_ddr_icc_path;
> +       struct icc_path         *apps_usb_icc_path;
>  };
> =20
> +static int dwc3_qcom_interconnect_enable(struct dwc3_qcom *qcom);
> +static int dwc3_qcom_interconnect_disable(struct dwc3_qcom *qcom);

Please get rid of these. We shouldn't need forward declarations.

> +
>  static inline void dwc3_qcom_setbits(void __iomem *base, u32 offset, u32=
 val)
>  {
>         u32 reg;
> @@ -285,6 +307,101 @@ static int dwc3_qcom_resume(struct dwc3_qcom *qcom)
>         return 0;
>  }
> =20
> +
> +/**
> + * 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 =3D 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.