Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1035878AbdD1Mnp (ORCPT ); Fri, 28 Apr 2017 08:43:45 -0400 Received: from fllnx210.ext.ti.com ([198.47.19.17]:13816 "EHLO fllnx210.ext.ti.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1035855AbdD1Mnh (ORCPT ); Fri, 28 Apr 2017 08:43:37 -0400 Date: Fri, 28 Apr 2017 07:43:28 -0500 From: Bin Liu To: "Matwey V. Kornilov" CC: , , Subject: Re: [PATCH] usb: musb: musb_host: Introduce postponed URB giveback Message-ID: <20170428124328.GD25403@uda0271908> Mail-Followup-To: Bin Liu , "Matwey V. Kornilov" , gregkh@linuxfoundation.org, linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org References: <20170427102033.5500-1-matwey@sai.msu.ru> <20170427153506.GA25403@uda0271908> <20170427171345.GB25403@uda0271908> <20170428115824.GC25403@uda0271908> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6873 Lines: 144 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 : > > On Fri, Apr 28, 2017 at 10:04:30AM +0300, Matwey V. Kornilov wrote: > >> 2017-04-27 20:13 GMT+03:00 Bin Liu : > >> > On Thu, Apr 27, 2017 at 07:26:31PM +0300, Matwey V. Kornilov wrote: > >> >> 2017-04-27 18:35 GMT+03:00 Bin Liu : > >> >> > 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 > >> >> > > >> >> > 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. > 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.