Re: Handling devices that don't have a bus

[Alan Stern]

On Sat, 1 Apr 2006, Russell King wrote:

> On Thu, Mar 30, 2006 at 03:45:50PM -0500, Alan Stern wrote:
> > I recently tried running the dummy_hcd driver for the first time in a 
> > while, and it crashed when the gadget driver was unloaded.  It turns out 
> > this was because the gadget's embedded struct device is registered without 
> > a bus, which triggers an oops when the device's driver is unbound.  The 
> > oops could be fixed by doing this:
> 
> Can you provide the oops itself please?

No, I don't have it any more.  But I can tell you exactly where the oops 
occurred.  In __device_release_driver() (in drivers/base/dd.c), this line 
you added:

		if (dev->bus->remove)

crashed because dev->bus was NULL.  My patch changes the line to:

		if (dev->bus && dev->bus->remove)

Any objection to that?

> > Part of the problem here is that most of the USB controllers are platform
> > devices and so belong on the platform bus.
> 
> You're making a connection where no such connection exists.  Devices
> are only part of the platform bus if they explicitly want to be (in
> much the same way that devices are only part of the PCI bus if they
> explicitly set dev->bus to be the PCI bus.)

I think you have misunderstood my point.  Yes, devices are part of the
platform bus only if they explicitly want to be.  My point was that even
though they _do_ want to be on the platform bus, in this situation they
_can't_ because they are forced to register a struct device, not a struct
platform_device.  The choice is not up to the driver; it is determined by
the USB Gadget framework.  (See the definition of struct usb_gadget in 
include/linux/usb_gadget.h -- there's an embedded struct device, not an 
embedded struct platform_device.)

> > But struct usb_gadget contains an embedded struct device, not an embedded
> > struct platform_device... so the gadget _can't_ be registered on its 
> > parent's bus.
> 
> From what I can see, the embedded device does not belong to any bus at
> all since dev->bus is NULL.  Hence, I don't think it's the embedded
> struct device which is causing the problem here.
> 
> It would be good to see the entire oops to see what's going on.

Yes, the device did not belong to any bus.  Hence the unguarded reference
to dev->bus->remove caused the oops.  If it's legal for devices not to
belong to a bus, then my patch should be applied to guard the reference.  
If it's not legal then the Gadget framework needs to be changed.

Alan Stern


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