Date:   Sat, 11 Apr 2020 00:03:36 +0200
From:   Michael Grzeschik <mgr@...gutronix.de>
To:     Thinh Nguyen <Thinh.Nguyen@...opsys.com>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        Felipe Balbi <balbi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>
Subject: Re: [PATCH 2/2] usb: dwc3: gadget: restart the transfer if a isoc
 request is queued too late

On Fri, Apr 10, 2020 at 01:09:23AM +0000, Thinh Nguyen wrote:
>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.

Which control bits are those? ISOC-First, Chain and Last bits?

I see the Scatter-Gather preparation is using these pattern.

>>>>> 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.

Right, for now the drivers is doing:

- Strart Transfer (with the right frame number) once.

- Update Transfer On every ep_queue with the corresponding TRB
  setting CHN = 0, IOC = 1, First-ISOC = 1

- End Transfer is somehow not handled right for this case.

See my first comment. I think dwc3_prepare_one_trb_sg does the proper chain
handling already.

>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.

Can the UVC not just enqueue one big sg-request, which will represent one burst
and not one TRB. Also that is  what the SG code already seem to handle right.

>> 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.

Right. This also stops, after the Command Complete Interrupt arrives.

>> 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?

I don't know, probably it was more likely for USB 2.0 controllers to be handled
this way.

As mentioned the current uvc code is also working when we add this changes.

diff --git a/drivers/usb/dwc3/gadget.c b/drivers/usb/dwc3/gadget.c
index ec357f64f319..a5dc44f2e9d8 100644
--- a/drivers/usb/dwc3/gadget.c
+++ b/drivers/usb/dwc3/gadget.c
@@ -2629,6 +2629,9 @@ static void dwc3_gadget_endpoint_transfer_in_progress(struct dwc3_ep *dep,

        dwc3_gadget_ep_cleanup_completed_requests(dep, event, status);

+       if (list_empty(&dep->started_list))
+               stop = true;
+
        if (stop)
                dwc3_stop_active_transfer(dep, true, true);
        else if (dwc3_gadget_ep_should_continue(dep))

diff --git a/drivers/usb/gadget/function/uvc_video.c b/drivers/usb/gadget/function/uvc_video.c
index da6ba8ba4bca..a3dac5d91aae 100644
--- a/drivers/usb/gadget/function/uvc_video.c
+++ b/drivers/usb/gadget/function/uvc_video.c
@@ -183,6 +183,7 @@ uvc_video_complete(struct usb_ep *ep, struct usb_request *req)

        switch (req->status) {
        case 0:
+       case -EXDEV: /* we ignore missed transfers */
                break;

        case -ESHUTDOWN:        /* disconnect from host. */


Regards,
Michael

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