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

[Anurag Kumar Vulisha]

Hi Alan,

>-----Original Message-----
>From: Alan Stern [mailto:stern@xxxxxxxxxxxxxxxxxxx]
>Sent: Tuesday, December 04, 2018 4:38 AM
>To: Anurag Kumar Vulisha <anuragku@xxxxxxxxxx>
>Cc: Felipe Balbi <balbi@xxxxxxxxxx>; Greg Kroah-Hartman
><gregkh@xxxxxxxxxxxxxxxxxxx>; Shuah Khan <shuah@xxxxxxxxxx>; Johan Hovold
><johan@xxxxxxxxxx>; Jaejoong Kim <climbbb.kim@xxxxxxxxx>; 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
>
>On Mon, 3 Dec 2018, Anurag Kumar Vulisha wrote:
>
>> >On Mon, 3 Dec 2018, Anurag Kumar Vulisha wrote:
>> >
>> >> >First of all, if some sort of deadlock causes a transfer to fail to
>> >> >complete, the host is expected to cancel and restart it.  Not the
>> >> >gadget.
>> >> >
>> >>
>> >> Thanks for spending your time in reviewing this patch.  The deadlock
>> >> is  a very rare case scenario and is happening because both the gadget
>> >> controller & host controllers get out of sync and are stuck waiting for the
>> >> relevant event. For example this issue is observed in stream protocol where
>> >> the gadget controller is waiting on Host controller to issue PRIME transaction
>> >> and  Host controller is waiting on gadget to issue ERDY transaction. Since
>> >> the stream protocol is gadget driven, the host may not proceed further until it
>> >> receives a valid Start Stream (ERDY) transaction from gadget.
>> >
>> >That's not entirely true.  Can't the host cancel the transfer and then
>> >restart it?
>> >
>>
>> Yes the host can cancel the transfer. This issue originated from the endpoints using
>bulk
>> streaming protocol and may not occur with normal endpoints. AFAIK bulk streaming
>is
>> gadget driven, where the gadget is allowed to select which stream id transfer the
>host
>> should work on . Since the host doesn't have control on when the transfer would be
>> selected by gadget, it may wait for longer timeouts before cancelling the transfer.
>
>You're missing the point.  Although the device selects which stream ID
>gets transferred, the _host_ decides whether a stream transfer should
>occur in the first place.  No matter how many ERDY packets the device
>controller tries to send, no transfer will occur until the host wants
>to do it.
>
>In this sense, stream transfers (like all other USB interactions except
>wakeup requests) are host-driven.
>

I agree with you that without host willing to transfer, the transfer wouldn't have
started and also agree the host always has the capability of detecting the hang. But
we are not sure on what host platform and operating system the gadget would be
tested and also not sure whether the host software is capable of handling the deadlock.
Even if the host has a timer , it would be very long and waiting for the timer to get
timedout  would reduce the performance. In this patch we are giving the user the
flexibility to opt the timeout value, so that the deadlock can be avoided without
effecting the performance. So I think adding the timer in gadget is more easier solution
than handling in host. I have seen this workaround working for both linux & windows.

>> >> Since the gadget
>> >> controller driver is aware that the controller is stuck , makes it responsible
>> >> to recover the controller from hang condition by restarting the transfer (which
>> >> triggers the controller FSM to issue ERDY to host).
>> >
>> >Isn't there a cleaner way to recover than by cancelling the request and
>> >resubmitting it?
>> >
>>
>> dequeuing the request issues the stop transfer command to the controller, which
>> cleans all the hardware resource allocated for that endpoint. This also resets the
>> hardware FSMs for that endpoint . So, when re-queuing of the transfer happens
>> the controller starts allocating hardware resources again, thus avoiding the
>probability
>> of entering into the issue. I am not sure of other controllers, but for dwc3, issuing
>> the stop transfer is the only way to handle this issue.
>
>Again you're missing the point.  Can't the controller driver issue the
>Stop Transfer command but still consider the request to be in progress
>(i.e., don't dequeue the request) so that the gadget driver's
>completion callback isn't invoked and the request does not need to be
>explicitly resubmitted?
>

Yes , what you are saying is correct. My initial patches were following the
same approach that you have suggested. We switched to the dequeue
approach because there can be different gadgets which may enter into
this issue and it would be better to follow a generic approach rather than
having every controller driver implementing their own workaround.
Please find the conservation in this link  https://patchwork.kernel.org/patch/10640145/
   
>> @Felipe:  Can you please provide your suggestion on this.
>
>> >How can the gadget driver know what timeout to use?  The host is
>> >allowed to be as slow as it wants; the gadget driver doesn't have any
>> >way to tell when the host wants to start the transfer.
>>
>> Yes , I agree with you that the timeout may vary from usage to usage. This timeout
>> should be decided by the class driver which queues the request. As discussed above
>> this issue was observed in streaming protocol , which  is very much faster than
>normal
>> BOT modes and it works on super speed .
>
>Although USB mass storage is currently the only user of the stream
>protocol, that may not be true in the future.  You should think in more
>general terms.  A timeout which is appropriate for mass storage may not
>be appropriate for some other application.
>

You are correct , this timeout may not be ideal. Since  I was not able to reproduce this issue
on non-stream capable transfers , at  present the only user of usb_ep_queue_timeout() API
is the streaming gadget.  As we are not aware of the optimal timeout value,  instead of
hardcoding the timeout value we can add module_param() for it.  So that the user can configure
timeout based on their use case. Please let me know your suggestion on this.

Thanks,
Anurag Kumar Vulisha 

>>  More over the gadget controller decides
>> the selection of the stream id on which the host should work , so taking all these
>into
>> consideration I kept 50ms timeout for stream transfers, so that the performance
>may
>> not get decreased.
>>
>> Thanks,
>> Anurag Kumar Vulisha