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

From: "Sandeep Maheswaram (Temp)" <sanm@codeaurora.org>
To: Matthias Kaehlcke <mka@chromium.org>, Stephen Boyd <swboyd@chromium.org>
Cc: 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>,
	Rob Herring <robh+dt@kernel.org>,
	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>
Subject: Re: [PATCH v7 2/4] usb: dwc3: qcom: Add interconnect support in dwc3 driver
Date: Tue, 16 Jun 2020 10:22:47 +0530	[thread overview]
Message-ID: <3f8fcb0e-387d-e902-9f6b-1fde9d6ae404@codeaurora.org> (raw)
In-Reply-To: <20200615194239.GW4525@google.com>

On 6/16/2020 1:12 AM, Matthias Kaehlcke wrote:
> On Thu, Jun 04, 2020 at 04:16:31AM -0700, Stephen Boyd wrote:
>> 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?
> I agree with Stephen that using device_is_bound() isn't a good option
> in this case, when I suggested it I wasn't looking at the big picture
> of how probing the core driver is triggered, sorry about that.
>
> Reading the speed from the DT with usb_get_maximum_speed() as Stephen
> suggests would be an option, the inconvenient is that we then
> essentially require the property to be defined, while the core driver
> gets a suitable value from hardware registers. Not sure if the wrapper
> driver could read from the same registers.
>
> One option could be to poll device_is_bound() for 100 ms (or so), with
> sleeps between polls. It's not elegant but would probably work if we
> don't find a better solution.
if (np)
         ret = dwc3_qcom_of_register_core(pdev);
     else
         ret = dwc3_qcom_acpi_register_core(pdev);

     if (ret) {
         dev_err(dev, "failed to register DWC3 Core, err=%d\n", ret);
         goto depopulate;
     }

     ret = dwc3_qcom_interconnect_init(qcom);
     if (ret)
         goto depopulate;

     qcom->mode = usb_get_dr_mode(&qcom->dwc3->dev);

Before calling dwc3_qcom_interconnect_init we are checking

     if (ret) {
         dev_err(dev, "failed to register DWC3 Core, err=%d\n", ret);
         goto depopulate;
     }

Doesn't  this condition confirm the core driver is probed?

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