Re: [PATCH 2/2] usb: dwc3: gadget: restart the transfer if a isoc request is queued too late

[Thinh Nguyen]

Hi,

Michael Grzeschik wrote:
> On Thu, Nov 14, 2019 at 08:11:56PM +0000, Thinh Nguyen wrote:
>> Michael Olbrich wrote:
>>> On Wed, Nov 13, 2019 at 07:14:59PM +0000, Thinh Nguyen wrote:
>>>> Alan Stern wrote:
>>>>> On Wed, 13 Nov 2019, Michael Olbrich wrote:
>>>>>> On Wed, Nov 13, 2019 at 03:55:01AM +0000, Thinh Nguyen wrote:
>>>>>>> Michael Olbrich wrote:
>>>>>>>> Currently, most gadget drivers handle isoc transfers on a best 
>>>>>>>> effort
>>>>>>>> bases: If the request queue runs empty, then there will simply 
>>>>>>>> be gaps in
>>>>>>>> the isoc data stream.
>>>>>>>>
>>>>>>>> The UVC gadget depends on this behaviour. It simply provides 
>>>>>>>> new requests
>>>>>>>> when video frames are available and assumes that they are sent 
>>>>>>>> as soon as
>>>>>>>> possible.
>>>>>>>>
>>>>>>>> The dwc3 gadget currently works differently: It assumes that 
>>>>>>>> there is a
>>>>>>>> contiguous stream of requests without any gaps. If a request is 
>>>>>>>> too late,
>>>>>>>> then it is dropped by the hardware.
>>>>>>>> For the UVC gadget this means that a live stream stops after 
>>>>>>>> the first
>>>>>>>> frame because all following requests are late.
>>>>>>> Can you explain little more how UVC gadget fails?
>>>>>>> dwc3 controller expects a steady stream of data otherwise it 
>>>>>>> will result
>>>>>>> in missed_isoc status, and it should be fine as long as new 
>>>>>>> requests are
>>>>>>> queued. The controller doesn't just drop the request unless 
>>>>>>> there's some
>>>>>>> other failure.
>>>>>> UVC (with a live stream) does not fill the complete bandwidth of an
>>>>>> isochronous endpoint. Let's assume for the example that one video 
>>>>>> frame
>>>>>> fills 3 requests. Because it is a live stream, there will be a 
>>>>>> gap between
>>>>>> video frames. This is unavoidable, especially for compressed 
>>>>>> video. So the
>>>>>> UVC gadget will have requests for the frame numbers 1 2 3 5 6 7 9 
>>>>>> 10 11 13 14
>>>>>> 15 and so on.
>>>>>> The dwc3 hardware tries to send those with frame numbers 1 2 3 4 
>>>>>> 5 6 7 8 9
>>>>>> 10 11 12. So except for the fist few requests, all are late and 
>>>>>> result in a
>>>>>> missed_isoc. I tried to just ignore the missed_isoc but that did 
>>>>>> not work
>>>>>> for me. I only received the first frame at the other end.
>>>>>> Maybe I missing something here, i don't have access to the hardware
>>>>>> documentation, so I can only guess from the existing driver.
>>>> The reason I asked is because your patch doesn't seem to address the
>>>> actual issue.
>>>>
>>>> For the 2 checks you do here
>>>> 1. There are currently no requests queued in the hardware
>>>> 2. The current frame number provided by DSTS does not match the frame
>>>>       number returned by the last transfer.
>>>>
>>>> For #1, it's already done in the dwc3 driver. (check
>>>> dwc3_gadget_endpoint_transfer_in_progress())
>>> But that's only after a isoc_missed occurred. What exactly does that 
>>> mean?
>>> Was the request transferred or not? My tests suggest that it was not
>>> transferred, so I wanted to catch this before it happens.
>>
>> Missed_isoc status means that the controller did not move all the data
>> in an interval.
>
> I read in some Processor documentation that in case the host tries to
> fetch data from the client and no active TRB (HWO=1) is available the
> XferInProgress Interrupt will be produced, with the missed status set.
> This is done because the hardware will produce zero length packets
> on its own, to keep the stream running.

The controller only generates XferInProgress if it had processed a TRB 
(with specific control bits). For IN direction, if the controller is 
starved of TRB, it will send a ZLP if the host requests for data.


>
>>>> For #2, it's unlikely that DSTS current frame number will match 
>>>> with the
>>>> XferNotReady's frame number. So this check doesn't mean much.
>>> The frame number is also updated for each "Transfer In Progress" 
>>> interrupt.
>>> If they match, then there a new request can still be queued 
>>> successfully.
>>> Without this I got unnecessary stop/start transfers in the middle of a
>>> video frame. But maybe something else was wrong here. I'd need to 
>>> recheck.
>>
>> The reason they may not match is 1) the frame_number is only updated
>> after the software handles the XferInProgress interrupt. Depends on
>> system latency, that value may not be updated at the time that we check
>> the frame_number.
>> 2) This check doesn't work if the service interval is greater than 1
>> uframe. That is, it doesn't have to match exactly the time to be
>> consider not late. Though, the second reason can easily be fixed.
>
> In the empty trb case, after the Hardware has send enough zero packets 
> this
> active transfer has to be stopped with endtransfer cmd. Because every 
> next
> update transfer on that active transfer will likely lead to further 
> missed
> transfers, as the newly updated trb will be handled to late anyway.

The controller is expecting the function driver to feed TRBs to the 
controller for every interval. If it's late, then the controller will 
consider that data "missed_isoc".

In your case, the UVC driver seems to queue requests to the controller 
driver as if it is bulk requests, and the UVC expects those data to go 
out at the time it queues. To achieve what UVC needs, then you may want 
to issue END_TRANSFER command before the next burst of data. This way, 
the controller can restart the isoc endpoint and not consider the next 
video frame data late. There are some corner cases that you need to 
watch out for. If you're going for this route, we can look further.

Also, you'd need provide a way for the UVC to communicate to the dwc3 to 
let it know to expect the next burst of data.

>
>
> The odd thing here is, that I don't see the refered XferInProgress
> Interrupts with the missed event, when the started_list is empty.

See my first comment.

>
> But this would be the only case to fall into this condition and handle it
> properly. Like alredy assumed in the following code:
>
> static void dwc3_gadget_endpoint_transfer_in_progress(struct dwc3_ep 
> *dep,
>              const struct dwc3_event_depevt *event)
> {
> ...
>
>        if (event->status & DEPEVT_STATUS_MISSED_ISOC) {
>                status = -EXDEV;
>
>                if (list_empty(&dep->started_list))
>                        stop = true;
>        }
>
> ...
>
>        if (stop)
>                dwc3_stop_active_transfer(dep, true, true);
> ...
> }
>
> In fact I did sometimes see these XferInProgress Interrupts on empty 
> trb queue
> after I stoped the tansfer when the started_list was empty right after
> ep_cleanup_completed_requests has moved all trbs out of the queue.
>
> These Interrupts appeared right after the ENDTRANSFER cmd was send. 
> (But I
> could no verify this every time)

If END_TRANSFER command completes, then you should not see 
XferInProgress event. The next event should ber XferNotReady.

>
> Anyways in that case these Interrupts are not useful anymore, as I 
> already
> implied the same stop, with ENDTRANSFER after we know that there are 
> no other
> trbs in the chain.
>

Just curious, do you know if there's a reason for UVC to behave this 
way? Seems like what it's trying to do is more for bulk. Maybe it wants 
bandwidth priority perhaps?

BR,
Thinh