On Fri, Apr 28, 2017 at 04:15:09PM +0300, Matwey V. Kornilov wrote: > 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. This is what I meant. I haven't checked other platforms yet, but at least it seems the issue only reported on am335x as far as I am aware. > > > > >> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
