Date:   Tue, 7 Jul 2020 10:41:24 +0530
From:   "Sandeep Maheswaram (Temp)" <sanm@...eaurora.org>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Stephen Boyd <swboyd@...omium.org>, 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>,
        Rob Herring <robh+dt@...nel.org>,
        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


On 7/1/2020 4:12 AM, Matthias Kaehlcke wrote:
> On Tue, Jun 16, 2020 at 01:38:49PM -0700, Matthias Kaehlcke wrote:
>> On Tue, Jun 16, 2020 at 10:22:47AM +0530, Sandeep Maheswaram (Temp) wrote:
>>> 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?
>> Not really:
>>
>> // called under the hood by of_platform_populate()
>> static int really_probe(struct device *dev, struct device_driver *drv)
>> {
>> 	...
>>
>> 	if (dev->bus->probe) {
>> 		ret = dev->bus->probe(dev);
>> 		if (ret)
>> 			goto probe_failed;
>> 	} else if (drv->probe) {
>> 		ret = drv->probe(dev);
>> 	        if (ret)
>> 	       		goto probe_failed;
>>          }
>>
>> 	...
>>
>> probe_failed:
>> 	...
>>
>> 	/*
>>           * Ignore errors returned by ->probe so that the next driver can try
>>           * its luck.
>>           */
>>          ret = 0;
>>
>> 	...
>>
>> 	return ret;
>> }
>>
>> As a result of_platform_populate() in dwc3_qcom_of_register_core()
>> returns 0 even when probing the device failed:
>>
>> [    0.244339] dwc3-qcom a6f8800.usb: DBG: populate
>> [    0.244772] dwc3 a600000.dwc3: DBG: dwc3_probe
>> [    0.245237] dwc3 a600000.dwc3: DBG: dwc3_probe err: -517
>> [    0.245264] dwc3-qcom a6f8800.usb: DBG: populate (done)
>> [    0.245317] dwc3-qcom a6f8800.usb: DBG: dwc3_qcom_interconnect_init() failed: -517
>>
>> Probe fails because the interconnect stuff isn't ready yet, otherwise
>> it could access invalid data.
>>
>> A later _populate() is successful and the probing of the core is done
>> synchronously, i.e. after _populate() the core driver is fully
>> initialized:
>>
>> [    3.898106] dwc3-qcom a6f8800.usb: DBG: populate
>> [    3.908356] dwc3 a600000.dwc3: DBG: dwc3_probe
>> [    4.205104] dwc3 a600000.dwc3: DBG: dwc3_probe (done)
>> [    4.210305] dwc3-qcom a6f8800.usb: DBG: populate (done)
>>
>> The synchronous probing in _populate() suggests that using device_is_bound()
>> would actually be a valid option, either the core device was successfully
>> probed or not, there should be no race.
>>
>> I sent a patch that adds this check to dwc3_qcom_of_register_core(), which
>> is less confusing and makes clear that the core device is valid unless
>> this function returns an error:
>>
>>    https://lore.kernel.org/patchwork/patch/1257279/
>>
>> It might make sense to add your "driver core:Export the symbol
>> device_is_bound" patch, mine and this one to a single series.
>  From the discussion on "driver core:Export the symbol device_is_bound"
> (https://patchwork.kernel.org/patch/11584225/) it is clear that
> this won't fly. The split dwc3 driver is considered a broken
> design.
>
> This is what Rob Herring said:
>
>    We never should have had this split either in the DT binding nor
>    driver(s) as if the SoC wrapper crap and licensed IP block are
>    independent things. The thing to do here is either make the DWC3 code
>    a library which drivers call (e.g. SDHCI) or add hooks into the DWC3
>    driver for platform specifics (e.g. Designware PCI). Neither is a
>    simple solution though.
>
> That seems to be the desirable solution in the longer term, but it
> doesn't seem reasonable to me to expect you to fix this design issue
> to add interconnect support.
>
> Some possible options to move forward:
>
> - try to determine the max speed without involving the core device
> - select a reasonable default when 'maximum-speed' is not specified
> - use the core device to determine the max speed and pray

Can we do as below to get speed

qcom->max_speed = usb_get_maximum_speed(&qcom->dwc3->dev);

as they were doing similarly in below code to get mode

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

-- 
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