Re: questions about give back urb in tasklet

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On Wed, Mar 19, 2014 at 11:05 PM, vichy <vichy.kuo@xxxxxxxxx> wrote:
> hi Ming:
>>>> One approach I like to use is trace event.
>>> Would you please let us how to to use trace event for calculate
>>> max/average usb irq time?
>>
>> You can enable irq_handler_entry and irq_handler_exit event, then
>> write a script easily to figure out the time consumed in ehci irq handler,
>> see Documentation/trace/events.txt of linux kernel for detailed steps.
> I find it ^^
>
>>>> But actually the local interrupt is still disabled during completion
>>>> because we need to convert spin_lock in all current usb drivers
>>>> to spin_lock_irqsave() first.
>>> Would you please let me know the function or source you mentioned?
>>
>> The local interrupt is disabled during completion handler in
>> __usb_hcd_giveback_urb().
>>
>>>
>>> BTW, why I cannot see xhci add HCD_BH?
>>
>> There might be some reasons:
>>
>> - in previous following up discussion, Alan thought it should
>> be easier to just split the hcd interrupt handler into two parts
>>
>> - as I said, spin_lock() isn't converted to spin_lock_irqsave()
>> in drivers' completion handler yet
> Do you mean ,
> take uvc/uac for example,
> if we put completion what they register previously in BH,
> the spin_lock() in their completions should be changed to spin_lock_irqsave()?
>
> Is it due to original completion is called in irq context?

Some drivers may assume that the completion handler is
called from hardirq handler, so call spin_lock(), and they
should have called spin_lock_irqsave() if there are shared
resources between its completion handler and other irq
handler.

So the simplest approach for the conversion is to replace
all spin_lock() inside urb completion path into spin_lock_irqsave().

>
>>
>> - maybe no one really care the time in which local interrupt is
>> disabled by USB completion handler.(That might be true
>> because USB devices are commonly used in personal
>> computer, not in big server product)
>>
>>> is there some reason xhci cannot put completion in BH?
>>
>> IMO, the completion handler should be put into tasklet, and
>> can be done. If you like, you can take time to do that.
> Below is so far kernel implementation.
> Isn't it put completion handler into tasklet?

Yes, it is run in tasklet context with local irq disabled.
As you know we need to enable irq during URB completion.


>        .......................
>        ......................
>         spin_lock(&bh->lock);
>         list_add_tail(&urb->urb_list, &bh->head);
>         running = bh->running;
>         spin_unlock(&bh->lock);
>
>         if (running)
>                 ;
>         else if (high_prio_bh)
>                 tasklet_hi_schedule(&bh->bh);
>         else
>                 tasklet_schedule(&bh->bh);
>
>
> Thanks for your help,
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