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

[Matthias Kaehlcke <mka@chromium.org>]

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