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

From: Michael Grzeschik
Date: Thu Apr 09 2020 - 04:00:11 EST

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.

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

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)

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.

Regards,
Michael

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