RE: [PATCH v7 01/10] usb: gadget: udc: Add timer support for usb requests

From: Anurag Kumar Vulisha
Date: Fri Jan 04 2019 - 09:18:07 EST

Hi Felipe,


Since I am waiting on your suggestion, thought of giving remainder.

Anurag Kumar Vulisha

>-----Original Message-----
>From: Anurag Kumar Vulisha
>Sent: Wednesday, December 12, 2018 8:41 PM
>To: 'Alan Stern' <stern@xxxxxxxxxxxxxxxxxxx>; Felipe Balbi <balbi@xxxxxxxxxx>
>Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>; Shuah Khan
><shuah@xxxxxxxxxx>; Johan Hovold <johan@xxxxxxxxxx>; Jaejoong Kim
><>; Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>;
>Roger Quadros <rogerq@xxxxxx>; Manu Gautam <mgautam@xxxxxxxxxxxxxx>;
>martin.petersen@xxxxxxxxxx; Bart Van Assche <bvanassche@xxxxxxx>; Mike
>Christie <mchristi@xxxxxxxxxx>; Matthew Wilcox <willy@xxxxxxxxxxxxx>; Colin Ian
>King <colin.king@xxxxxxxxxxxxx>; linux-usb@xxxxxxxxxxxxxxx; linux-
>kernel@xxxxxxxxxxxxxxx; v.anuragkumar@xxxxxxxxx; Thinh Nguyen
><thinhn@xxxxxxxxxxxx>; Tejas Joglekar <tejas.joglekar@xxxxxxxxxxxx>; Ajay
>Yugalkishore Pandey <APANDEY@xxxxxxxxxx>
>Subject: RE: [PATCH v7 01/10] usb: gadget: udc: Add timer support for usb requests
>Hi Felipe,
>>-----Original Message-----
>>From: Alan Stern [mailto:stern@xxxxxxxxxxxxxxxxxxx]
>>Sent: Friday, December 07, 2018 10:40 PM
>>To: Felipe Balbi <balbi@xxxxxxxxxx>
>>Cc: Anurag Kumar Vulisha <anuragku@xxxxxxxxxx>; Greg Kroah-Hartman
>><gregkh@xxxxxxxxxxxxxxxxxxx>; Shuah Khan <shuah@xxxxxxxxxx>; Johan Hovold
>><johan@xxxxxxxxxx>; Jaejoong Kim <>; Benjamin
>>Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>; Roger Quadros <rogerq@xxxxxx>;
>>Gautam <mgautam@xxxxxxxxxxxxxx>; martin.petersen@xxxxxxxxxx; Bart Van
>>Assche <bvanassche@xxxxxxx>; Mike Christie <mchristi@xxxxxxxxxx>; Matthew
>>Wilcox <willy@xxxxxxxxxxxxx>; Colin Ian King <colin.king@xxxxxxxxxxxxx>; linux-
>>usb@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; v.anuragkumar@xxxxxxxxx;
>>Thinh Nguyen <thinhn@xxxxxxxxxxxx>; Tejas Joglekar
>><tejas.joglekar@xxxxxxxxxxxx>; Ajay Yugalkishore Pandey <APANDEY@xxxxxxxxxx>
>>Subject: RE: [PATCH v7 01/10] usb: gadget: udc: Add timer support for usb requests
>>On Fri, 7 Dec 2018, Felipe Balbi wrote:
>>> hi,
>>> Anurag Kumar Vulisha <anuragku@xxxxxxxxxx> writes:
>>> >>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
>>> >>> >controller driver implement its own fix. At least, that's how it appears to me.
>>> Why, if the UDC implementation will, anyway, be a timer?
>>It's mostly a question of what happens when the timer expires. (After
>>all, starting a timer is just as easy to do in a UDC driver as it is in
>>the core.) When the timer expires, what can the core do?
>>Don't say it can cancel the offending request and resubmit it. That
>>leads to the sort of trouble we discussed earlier in this thread. In
>>particular, we don't want the class driver's completion routine to be
>>called when the cancel occurs. Furthermore, this leads to a race:
>>Suppose the class driver decides to cancel the request at the same time
>>as the core does a cancel and resubmit. Then the class driver's cancel
>>could get lost and the request would remain on the UDC's queue.
>>What you really want to do is issue the appropriate stop and restart
>>commands to the hardware, while leaving the request logically active
>>the entire time. The UDC core can't do this, but a UDC driver can.
>I agree with Alan's comment as it looks like there may be some corner cases
>where the issue may occur with dequeue approach. Are you okay if the timeout
>handler gets moved to the dwc3 driver (the timers still added into udc layer)?
>Please let us know your suggestion on this
>Anurag Kumar Vulisha
>>> >>(Especially if dwc3 is the only driver affected.)
>>> >
>>> > As discussed above, the issue may happen with other gadgets too. As I got divide
>>> > on this implementation and both the implementations looks fine to me, I am
>>> > on which should be implemented.
>>> >
>>> > @Felipe: Do you agree with Alan's implementation? Please let us know your
>>> > on this.
>>> I still think a generic timer is a better solution since it has other uses.
>>Putting a struct timer into struct usb_request is okay with me, but I
>>wouldn't go any farther than that.
>>> >>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.
>>> I wouldn't call this a HW bug though. This is just how the UDC
>>> behaves. There are N streams and host can move data in any stream at any
>>> time. This means that host & gadget _can_ disagree on what stream to
>>> start next.
>>But the USB 3 spec says what should happen when the host and gadget
>>disagree in this way, doesn't it? And it doesn't say that they should
>>deadlock. :-) Or have I misread the spec?
>>> One way to avoid this would be to never pre-start any streams and always
>>> rely on XferNotReady, but that would mean greatly reduced throughput for
>>> streams.
>>It would be good if there was some way to actively detect the problem
>>instead of passively waiting for a timer to expire.
>>Alan Stern