Date:   Fri, 28 Apr 2017 10:04:30 +0300
From:   "Matwey V. Kornilov" <matwey@....msu.ru>
To:     Bin Liu <b-liu@...com>, "Matwey V. Kornilov" <matwey@....msu.ru>,
        gregkh@...uxfoundation.org, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb: musb: musb_host: Introduce postponed URB giveback

2017-04-27 20:13 GMT+03:00 Bin Liu <b-liu@...com>:
> 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@...com>:
>> > 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@....msu.ru>
>> >
>> > 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.

>
>> > 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?

>
>> At the same time musb_host doesn't utilize AutoReq here. I think many
>> other USB host controllers (OHCI?) just rely on hardware to send IN
>> packets in time.
>
> For Isoch transfer, MUSB has to be programmed for every transaction, but
> urb callback takes too long with spinlock held, which cause MUSB misses
> issuing IN tokens.
>
> 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

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