On Fri, 30 Apr 2004, Oliver Neukum wrote: > Very, very tempting. Elegant and simple. But I have a problem with it. > It's entirely legal for a driver to call usb_get_urb() twice. And then we > have a problem. So I don't think that we should use the refcount.
Yes, it is legal for a driver to do that. I suspect that very few of them do, because there's not often a reason for it. In fact, a recursive grep of the kernel source shows that usb_get_urb() is mentioned only within include/linux/usb.h, core/hcd.c, core/urb.c, host/ehci-sched.c, and host/hc_simple.c. No drivers call it. Anyway, we can always document at the start of the routine that it shouldn't be used if the URB's reference count has been monkeyed with. On the other hand, there are some very good reasons for using the refcount in that test. Whatever we test, it needs to be something that is changed in giveback_urb after the completion handler returns. The only field for which that is currently true is the refcount. Remember, at the end of giveback_urb usbcore no longer owns the URB; the driver does. Hence there isn't much that giveback_urb can afford to change. Also, whatever we use, it has to take into account the possibility of resubmission (and even the theoretical possibility of nested invocations of giveback_urb!). The only thing that will handle this properly is a refcount of some sort. Yes, we could introduce another one that would track just the active submissions -- but why go to all that trouble (and introduce yet another field into the already-bloated struct urb) when we have a ready-made reference count sitting right there? Finally, remember this: If a driver plays around with usb_get_urb() or doesn't want to wait the extra couple of milliseconds implied by that polling loop, it can always use asynchronous unlinking and just signal a struct completion from within its completion handler. Alan Stern ------------------------------------------------------- This SF.Net email is sponsored by: Oracle 10g Get certified on the hottest thing ever to hit the market... Oracle 10g. Take an Oracle 10g class now, and we'll give you the exam FREE. http://ads.osdn.com/?ad_id=3149&alloc_id=8166&op=click _______________________________________________ [EMAIL PROTECTED] To unsubscribe, use the last form field at: https://lists.sourceforge.net/lists/listinfo/linux-usb-devel
