Roger:
Your commit fac323471df6 ("usb: udc: allow adding and removing the same
gadget device") from a few years ago just caught my eye. (I don't
recall whether I noticed it at the time.)
In any case, we need to talk about it. What you're doing --
unregistering and re-registering the struct device embedded in the
gadget structure -- is strictly forbidden by the kernel's device model.
It's even mentioned specifically in the kerneldoc for device_add().
Now, I guess doing this would be okay _if_ you took care not to
re-register the device until all references to the previous incarnation
have been dropped. In particular, setting the structure's memory to 0
should not be done immediately after calling device_unregister() --
which is what the commit does -- but rather in the release routine.
Do you know which UDC drivers actually do re-register their gadgets? In
particular, do they have their own release routines or do they rely on
the default usb_udc_nop_release() provided by the UDC core? Moving the
memset into that routine ought to be okay; leaving it where it is would
be a time bomb waiting to go off. I'm suprised it hasn't caused
problems already.
Furthermore, drivers that do re-register their gadgets should wait until
gadget->dev.release is NULL, indicating that the release routine has
been called and the memory has been wiped. If they re-register before
that, the re-registration will fail for the same reasons you observed
when you wrote the commit.
Of course, a cleaner alternative would be to allocate the gadget
structure dynamically. Then instead of re-registering the old one, the
driver could allocate a new one and register it instead, with no
concerns about reuse.
Alan Stern
