linux-kernel - Re: [PATCH v3 1/3] usb: udc: allow adding and removing the same gadget device

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Date:   Wed, 05 Apr 2017 11:34:48 +0300
From:   Felipe Balbi <balbi@...nel.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Roger Quadros <rogerq@...com>, vivek.gautam@...eaurora.org,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/3] usb: udc: allow adding and removing the same gadget device


Hi,

Alan Stern <stern@...land.harvard.edu> writes:
>> >> allow usb_del_gadget_udc() and usb add_gadget_udc() to be called
>> >> repeatedly on the same gadget->dev structure.
>> >> 
>> >> We need to clear the gadget->dev structure so that kobject_init()
>> >> doesn't complain about already initialized object.
>> >> 
>> >> Signed-off-by: Roger Quadros <rogerq@...com>
>> >> ---
>> >>  drivers/usb/gadget/udc/core.c | 1 +
>> >>  1 file changed, 1 insertion(+)
>> >> 
>> >> diff --git a/drivers/usb/gadget/udc/core.c b/drivers/usb/gadget/udc/core.c
>> >> index d685d82..efce68e 100644
>> >> --- a/drivers/usb/gadget/udc/core.c
>> >> +++ b/drivers/usb/gadget/udc/core.c
>> >> @@ -1273,6 +1273,7 @@ void usb_del_gadget_udc(struct usb_gadget *gadget)
>> >>  	flush_work(&gadget->work);
>> >>  	device_unregister(&udc->dev);
>> >>  	device_unregister(&gadget->dev);
>> >> +	memset(&gadget->dev, 0x00, sizeof(gadget->dev));
>> >>  }
>> >>  EXPORT_SYMBOL_GPL(usb_del_gadget_udc);
>> >
>> > Isn't this dangerous?  It's quite possible that the device_unregister() 
>> 
>> not on the gadget API, no.
>> 
>> > call on the previous line invokes the gadget->dev.release callback, 
>> > which might deallocate gadget.  If that happens, your new memset will 
>> > oops.
>> 
>> that won't happen. struct usb_gadget is a member of the UDC's private
>> structure, like this:
>> 
>> struct dwc3 {
>> 	[...]
>> 	struct usb_gadget	gadget;
>> 	struct usb_gadget_driver *gadget_driver;
>> 	[...]
>> };
>
> Yes.  So what?  Can't the UDC driver use the refcount inside struct 
> usb_gadget to control the lifetime of its private structure?

nope, not being used. At least not yet.

> (By the way, can you tell what's going on in net2280.c?  I must be
> missing something; it looks like gadget_release() would quickly run
> into problems because it calls dev_get_drvdata() for &gadget->dev, but
> net2280_probe() never calls dev_set_drvdata() for that device.  
> Furthermore, net2280_remove() continues to reference the net2280 struct
> after calling usb_del_gadget_udc(), and it never does seem to do a
> final put.)

static int net2280_probe(struct pci_dev *pdev, const struct pci_device_id *id)
{
	struct net2280		*dev;
	unsigned long		resource, len;
	void			__iomem *base = NULL;
	int			retval, i;

	/* alloc, and start init */
	dev = kzalloc(sizeof(*dev), GFP_KERNEL);
	if (dev == NULL) {
		retval = -ENOMEM;
		goto done;
	}

	pci_set_drvdata(pdev, dev);
	^^^^^^^^^^^^^^^^^^^^^^^^^^^

>> I'm actually thinking that struct usb_gadget shouldn't have a struct
>> device at all. Just a pointer to a device, that would solve all these
>> issues.
>
> A pointer to which device?  The UDC?  That would change the directory 
> layout in sysfs.

indeed. Would that be a problem?

> Or a pointer to a separate dynamically allocated device (the way struct 
> usb_hcd contains a pointer to the root hub device)?  That would work.  
> If the UDC driver wanted to re-register the gadget, it would have to 
> allocate a new device.

That could be done as well, if maintaining the directory structure is a
must.

-- 
balbi

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