Re: [PATCH] usb: musb: musb_host: Introduce postponed URB giveback

[Matwey V. Kornilov]

2017-04-28 15:43 GMT+03:00 Bin Liu <b-liu@xxxxxx>:
> On Fri, Apr 28, 2017 at 03:13:55PM +0300, Matwey V. Kornilov wrote:
>>  which i
>>
>> 2017-04-28 14:58 GMT+03:00 Bin Liu <b-liu@xxxxxx>:
>> > On Fri, Apr 28, 2017 at 10:04:30AM +0300, Matwey V. Kornilov wrote:
>> >> 2017-04-27 20:13 GMT+03:00 Bin Liu <b-liu@xxxxxx>:
>> >> > On Thu, Apr 27, 2017 at 07:26:31PM +0300, Matwey V. Kornilov wrote:
>> >> >> 2017-04-27 18:35 GMT+03:00 Bin Liu <b-liu@xxxxxx>:
>> >> >> > Hi Matwey,
>> >> >> >
>> >> >> > On Thu, Apr 27, 2017 at 01:20:33PM +0300, Matwey V. Kornilov wrote:
>> >> >> >> This commit changes the order of actions undertaken in
>> >> >> >> musb_advance_schedule() in order to overcome issue with broken
>> >> >> >> isochronous transfer [1].
>> >> >> >>
>> >> >> >> There is no harm to split musb_giveback into two pieces.  The first
>> >> >> >> unlinks finished urb, the second givebacks it. The issue here that
>> >> >> >> givebacking may be quite time-consuming due to urb->complete() call.
>> >> >> >> As it happens in case of pwc-driven web cameras. It may take about 0.5
>> >> >> >> ms to call __musb_giveback() that calls urb->callback() internally.
>> >> >> >> Under specific circumstances setting MUSB_RXCSR_H_REQPKT in subsequent
>> >> >> >> musb_start_urb() for the next urb will be too late to produce physical
>> >> >> >> IN packet. Since auto req is not used by this module the exchange
>> >> >> >> would be as the following:
>> >> >> >>
>> >> >> >> [        ]   7.220456 d=  0.000997 [182   +  3.667] [  3] IN   : 4.5
>> >> >> >> [    T   ]   7.220459 d=  0.000003 [182   +  7.000] [800] DATA0: [skipped]
>> >> >> >> [        ]   7.222456 d=  0.001997 [184   +  3.667] [  3] IN   : 4.5
>> >> >> >> [        ]   7.222459 d=  0.000003 [184   +  7.000] [  3] DATA0: 00 00
>> >> >> >>
>> >> >> >> It is known that missed IN in isochronous mode makes some
>> >> >> >> perepherial broken. For instance, pwc-driven or uvc-driven
>> >> >> >> web cameras.
>> >> >> >> In order to workaround this issue we postpone calling
>> >> >> >> urb->callback() after setting MUSB_RXCSR_H_REQPKT for the
>> >> >> >> next urb if there is the next urb pending in queue.
>> >> >> >>
>> >> >> >> [1] https://www.spinics.net/lists/linux-usb/msg145747.html
>> >> >> >>
>> >> >> >> Fixes: f551e1352983 ("Revert "usb: musb: musb_host: Enable HCD_BH flag to handle urb return in bottom half"")
>> >> >> >> Signed-off-by: Matwey V. Kornilov <matwey@xxxxxxxxxx>
>> >> >> >
>> >> >> > Thanks for the effort of working on this long standing issue, I know you
>> >> >> > have spent alot of time on it, but what I am thinking is instead of
>> >> >> > workaround the problem we need to understand the root cause - why
>> >> >> > uvc-video takes longer to exec the urb callback, why only am335x
>> >> >> > reported this issue. This is on my backlog, just seems never got time
>> >> >> > for it...
>> >> >>
>> >> >> Have you tried other SoCs with Invetra MUSB?
>> >> >
>> >> > That is the plan, I got an A20 board, but haven't bring it up yet.
>> >> >
>> >> >>
>> >> >> >
>> >> >> > Ideally MUSB driver should be just using HCD_BH flag and let the core to
>> >> >> > handle the urb callback, regardless the usb transfer types.
>> >> >>
>> >> >> I think the only reason why everything worked before with HCD_BH is
>> >> >> that execution of urb->callback() was placed after musb_start(). The
>> >> >> order of operations matters.
>> >> >> However, you said that something was also wrong with HCD_BH and other
>> >> >> peripherals.
>> >> >
>> >> > HCD_BH flag cause some issues which are docummented in the commit log of
>> >> > f551e1352983.
>> >> > But even with HCD_BH flag, it didn't work for uvc webcams, it still misses
>> >> > IN tokens. It might helps pwc webcams though.
>> >>
>> >> pwc webcams work with HCD_BH fine indeed.
>> >
>> > yeah, this is what you told long time ago.
>> >
>> >>
>> >> >
>> >> >> > The MUSB drivers are already messy and complicated enough for
>> >> >> > maintenance, I'd like to understand the root cause of the delay first
>> >> >> > before decide how to solve the issue.
>> >> >> >
>> >> >>
>> >> >> I feel from playing with OpenVizsla that REQPKT should be set well in
>> >> >> advance. So, time window to set the flag is actually smaller than 1
>> >> >> ms.
>> >> >> urb->callback() is never takes longer than 0.4 ms for pwc driver, but
>> >> >> INs are skipped.
>> >> >
>> >> > Setting REQPKT in advance might be the solution, but I'd like to
>> >> > understand why only Isoch transfer shows such issue, and why only am335x
>> >> > reports this issue. The later concerns me more if this would be a
>> >> > system issue not only in usb module.
>> >>
>> >> 0.4 ms is about 100000 CPU cycles given that CPU is running at 275Mhz
>> >> (which is the lowest cpufreq). Long time ago, I run pwc webcam with
>> >> SIS Vortex86 at 200Mhz It worked fine. I would not say that it is
>> >> extraordinary value.
>> >> Do you think that somewhere CPU cycles are wasted globally for some reason?
>> >
>> > Depends on how to interpret 'wasted', my understanding the issue is the
>> > core urb giveback holds a spinlock and in uvc case the giveback takes
>> > longer to finish (I guess the same in pwc case), so the musb driver
>> > doesn't get a chance to re-program the controller on time, which causes
>> > missing issuingIN tokens.
>> >
>> > The questions are, why uvc takes longer to run urb giveback (which holds
>> > a spinlock), and is it am335x specific issue...
>>
>> It is not clear how does it hold a spinlock on a BeagleBone which is
>> single-core system.
>
> Sorry, I have to take it back, urb giveback doesn't hold a spinlock, but
> disables irq. It has been a while (a year?) since the last time I looked
> this issue. Please see the call below flow.
>
> musb_giveback() -->
>   usb_hcd_giveback_urb() -->
>     __usb_hcd_giveback_urb() --> # it gets here regardless HCD_BH flag
>       1765         local_irq_save(flags);
>       1766         urb->complete(urb);
>       1767         local_irq_restore(flags);
>
> so musb driver only gets a chance to re-program the controller after
> line 1766 returns, which is the urb callback in the class driver (uvc in
> this case). If urb->complete() takes too long, the controller will miss
> the IN tokens.
>
> HCD_BH flag could help the situation only if urb->complete() doesn't
> take that long.

Sure, I think that the question is why urb->complete() itself takes so
long only (?) at am335x.

>
>> I mean if it is waiting for blocked lock it will be waiting forever,
>> because it is in interrupt context on single core system.
>
> Hope my message above explains it.
>
>> Here it is waiting for finite time amount. It is also quite unlikely
>> that spinlocks are implemented inefficiently for arm architecture.
>> So, I tested with CONFIG_SMP=y which is default in my distro kernel.
>> Do you think it is worth to try CONFIG_SMP=n or have you already tried
>> UP kernel configuration?
>
> CONFIG_SMP is irrelevant here. BTY, because of my laziness, I use
> CONFIG_SMP in most times, if not all.
>
> Regards,
> -Bin.
>



-- 
With best regards,
Matwey V. Kornilov.
Sternberg Astronomical Institute, Lomonosov Moscow State University, Russia
119234, Moscow, Universitetsky pr-k 13, +7 (495) 9392382