Re: [PATCH] usb: dwc3: qcom: Make sure core device is fully initialized before it is used

Matthias Kaehlcke
2020-06-17T21:24:29Z

Hi Stephen,

On Wed, Jun 17, 2020 at 12:49:13PM -0700, Stephen Boyd wrote:
> Quoting Matthias Kaehlcke (2020-06-16 13:37:37)
> > dwc3_qcom_of_register_core() uses of_platform_populate() to add
> > the dwc3 core device. The driver core will try to probe the device,
> > however this might fail (e.g. due to deferred probing) and
> > of_platform_populate() would still return 0 if the device was
> > successully added to the platform bus. Verify that the core device
> > is actually bound to its driver before using it, defer probing of the
> > dwc3_qcom device if the core device isn't ready (yet).
> >
> > Fixes: a4333c3a6ba9 ("usb: dwc3: Add Qualcomm DWC3 glue driver").
> > Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
> > ---
> > depends on:
> > https://lore.kernel.org/patchwork/patch/1251661/ ("driver core:Export
> > the symbol device_is_bound")
> >
> > drivers/usb/dwc3/dwc3-qcom.c | 10 ++++++++++
> > 1 file changed, 10 insertions(+)
> >
> > diff --git a/drivers/usb/dwc3/dwc3-qcom.c b/drivers/usb/dwc3/dwc3-qcom.c
> > index 1dfd024cd06b..5a9036b050c6 100644
> > --- a/drivers/usb/dwc3/dwc3-qcom.c
> > +++ b/drivers/usb/dwc3/dwc3-qcom.c
> > @@ -537,6 +537,16 @@ static int dwc3_qcom_of_register_core(struct platform_device *pdev)
> > return -ENODEV;
> > }
> >
> > + /*
> > + * A successful return from of_platform_populate() only guarantees that
> > + * the core device was added to the platform bus, however it might not
> > + * be bound to its driver (e.g. due to deferred probing). This driver
> > + * requires the core device to be fully initialized, so defer probing
> > + * if it isn't ready (yet).
> > + */
> > + if (!device_is_bound(&qcom->dwc3->dev))
> > + return -EPROBE_DEFER;
>
> Isn't this still broken? i.e. the dwc3 core driver may bind much later
> and then device_is_bound() will return an error here and then we'll
> return to the caller, dwc3_qcom_probe(), and that will depopulate the
> device with of_platform_depopulate(). It seems like we need to run some
> sort of wait for driver to be bound function instead of a one-shot check
> for the driver being bound.

My understanding is that the probing is done synchronously and either done,
failed or deferred when returning from of_platform_populate(). Ideally we
would be able to differentiate between a failure and deferral, and not defer
probing in case of an error, however I'm not aware of a way to do this with
the current driver framework.

The call flow is:

of_platform_populate
of_platform_bus_create
of_platform_device_create_pdata
of_device_add
device_add
bus_probe_device
device_initial_probe
__device_attach
__device_attach_driver
driver_probe_device
really_probe
->probe()

> Also, what about acpi? That has the same problem but it isn't covered by
> the dwc3_qcom_of_register_core() function.

I wouldn't be surprised if it had the same problem. I'm not familiar with ACPI
though and don't have a device for testing, hence I limited the patch to the
platform device.