Date:   Wed, 5 Dec 2018 15:43:41 +0000
From:   Anurag Kumar Vulisha <anuragku@...inx.com>
To:     Alan Stern <stern@...land.harvard.edu>
CC:     Felipe Balbi <balbi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Shuah Khan <shuah@...nel.org>, Johan Hovold <johan@...nel.org>,
        Jaejoong Kim <climbbb.kim@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Roger Quadros <rogerq@...com>,
        Manu Gautam <mgautam@...eaurora.org>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        Bart Van Assche <bvanassche@....org>,
        Mike Christie <mchristi@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Colin Ian King <colin.king@...onical.com>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "v.anuragkumar@...il.com" <v.anuragkumar@...il.com>,
        Thinh Nguyen <thinhn@...opsys.com>,
        Tejas Joglekar <tejas.joglekar@...opsys.com>,
        Ajay Yugalkishore Pandey <APANDEY@...inx.com>
Subject: RE: [PATCH v7 01/10] usb: gadget: udc: Add timer support for usb
 requests

Hi Alan,

>-----Original Message-----
>From: Alan Stern [mailto:stern@...land.harvard.edu]
>Sent: Wednesday, December 05, 2018 12:59 AM
>To: Anurag Kumar Vulisha <anuragku@...inx.com>
>Cc: Felipe Balbi <balbi@...nel.org>; Greg Kroah-Hartman
><gregkh@...uxfoundation.org>; Shuah Khan <shuah@...nel.org>; Johan Hovold
><johan@...nel.org>; Jaejoong Kim <climbbb.kim@...il.com>; Benjamin
>Herrenschmidt <benh@...nel.crashing.org>; Roger Quadros <rogerq@...com>; Manu
>Gautam <mgautam@...eaurora.org>; martin.petersen@...cle.com; Bart Van
>Assche <bvanassche@....org>; Mike Christie <mchristi@...hat.com>; Matthew
>Wilcox <willy@...radead.org>; Colin Ian King <colin.king@...onical.com>; linux-
>usb@...r.kernel.org; linux-kernel@...r.kernel.org; v.anuragkumar@...il.com;
>Thinh Nguyen <thinhn@...opsys.com>; Tejas Joglekar
><tejas.joglekar@...opsys.com>; Ajay Yugalkishore Pandey <APANDEY@...inx.com>
>Subject: RE: [PATCH v7 01/10] usb: gadget: udc: Add timer support for usb requests
>
>On Tue, 4 Dec 2018, Anurag Kumar Vulisha wrote:
>
>> >Is there any way for the device controller driver to detect the problem without
>relying
>> >on a timeout?
>> >
>> >In fact, is this problem a known bug in the existing device controller hardware?
>Have
>> >the controller manufacturers published a suggested scheme for working around it?
>> >
>>
>> Yes, this problem is mentioned in dwc3 controller data book and the workaround
>> mentioned  is the same that we are doing , to implement timeout and if no valid
>> stream event is found , after timeout issue end transfer followed by start transfer.
>
>Okay.  But this implies that the problem is confined to DWC3 and
>doesn't affect other types of controllers, which would mean modifying
>the UDC core would be inappropriate.
>

Both host & gadget are equally responsible for this issue and it may potentially occur
with other controllers also(which are incapable of handling this situation) . Because of
this reason I would not call this issue as a dwc3 alone bug. One thing is that, with dwc3 the
issue is easily reproduced. Because of these reasons I feel that it would be better to have
a generic udc timers rather than having driver specific workaround. We had this same
discussion earlier in this mailing list https://lkml.org/lkml/2018/10/9/638

>Does the data book suggest a value for the timeout?
>

No, the databook doesn't mention about the timeout value

>> >At this point, it seems that the generic approach will be messier than having every
>> >controller driver implement its own fix.  At least, that's how it appears to me.
>
>(Especially if dwc3 is the only driver affected.)
>

As discussed above, the issue may happen with other gadgets too. As I got divide opinions
on this implementation and both the implementations looks fine to me, I am little confused
on which should be implemented.

@Felipe: Do you agree with Alan's implementation? Please let us know your suggestion
on this.

>> With the dequeue approach there is an advantage(not related to this issue) that the
>> class driver can have control of the timed out transfer whether to requeue it or
>consider
>> it as an error (based on implementation). This approach gives more flexibility to the
>class
>> drivers. This may be out of context of this issue but wanted to point out that we
>may lose
>> this advantage on switching to older implementation.
>
>Class drivers can cancel and requeue requests at any time.  There's no
>extra flexibility.
>

I agree with you, but the class drivers have to implement their own logic instead of
having a generic logic to implement the timeouts. 

>> >Ideally it would not be necessary to rely on a timeout at all.
>> >
>> >Also, maintainers dislike module parameters.  It would be better not to add one.
>>
>> Okay. I would be happy if any alternative for this issue is present but unfortunately
>> I am not able to figure out any alternative other than timers. If not
>module_params()
>> we can add an configfs entry in stream gadget to update the timeout. Please
>provide
>> your opinion on this approach.
>
>Since the purpose of the timeout is to detect a deadlock caused by a
>hardware bug, I suggest a fixed and relatively short timeout value such
>as one second.  Cancelling and requeuing a few requests at 1-second
>intervals shouldn't add very much overhead.
>

Thanks for suggesting. 1 sec timeout should be fair enough. I will change this
in next series 

Best Regards,
Anurag Kumar Vulisha

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