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

[Michael Olbrich]

On Fri, Nov 15, 2019 at 04:06:10PM -0500, Alan Stern wrote:
> On Thu, 14 Nov 2019, Thinh Nguyen wrote:
> 
> > Michael Olbrich wrote:
> 
> > >>> How about changing the gadget driver instead?  For frames where the UVC
> > >>> gadget knows no video frame data is available (numbers 4, 8, 12, and so
> > >>> on in the example above), queue a zero-length request.  Then there
> > >>> won't be any gaps in the isochronous packet stream.
> > >> What Alan suggests may work. Have you tried this?
> > > Yes and it works in general. There are however some problems with that
> > > approach that I want to avoid:
> > >
> > > 1. It adds extra overhead to handle the extra zero-length request.
> > > Especially for encoded video the available bandwidth can be quite a bit
> > > larger that what is actually used. I want to avoid that.
> 
> This comment doesn't seem to make sense.  If the available bandwidth is
> much _larger_ than what is actually used, what's the problem?  You
> don't run into difficulties until the available bandwidth is too
> _small_.
> 
> The extra overhead of a zero-length request should be pretty small.  
> After all, the gadget expects to send a packet for every frame anyway,
> more or less.

My current test-case is video frames with 450kB on average at 30fps. This
currently results in ~10 CPU load for the threaded interrupt handler.
At least in my test, filling the actual video data into the frame has very
little impact. So if I reserve 900kB to support occasionally larger video
frames, then I expect that this CPU load will almost double in all cases,
not just when the video frames are larger.

> > > 2. The UVC gadget currently does no know how many zero-length request must
> > > added. So it needs fill all available request until a new video frame
> > > arrives. With the current 4 requests that is not a problem right now. But
> > > that does not scale for USB3 bandwidths. So one thing that I want to do is
> > > to queue many requests but only enable the interrupt for a few of than.
> > >  From what I can tell from the code, the gadget framework and the dwc3
> > > driver should already support this.
> > > This will result in extra latency. There is probably an acceptable
> > > trade-off with an acceptable interrupt load and latency. But I would like
> > > to avoid that if possible.
> 
> There are two different situations to consider:
> 
> 	In the middle of a video stream, latency isn't an issue.
> 	The gadget should expect to send a new packet for each frame,
> 	and it doesn't know what to put in that packet until it
> 	receives the video data or it knows there won't be any data.
> 
> 	At the start of a video stream, latency can be an issue.  But
> 	in this situation the gadget doesn't have to send 0-length
> 	requests until there actually is some data available.
> 
> Either way, it should be okay.
> 
> As far as interrupt load is concerned, I don't see how it relates to
> the issue of sending 0-length requests.

Maybe I don't understand, how 0-length requests work. My current
understanding is, that they are queued like any other request.

If I want to reduce the number of interrupts then I need to queue more
requests and only ask for an interrupt for some of them. This means that
potentially a lot of 0-length requests requests are queued when a new video
frame arrives and this means extra latency for the frame.

I think the worst-case latency is 2x the time between two interrupts.
So less interrupts mean more latency.
The stop/start transfer this patch implements, the video frame can be sent
immediately without any extra latency.

> > I think I understand the problem you're trying to solve now.
> > 
> > The dwc3 driver does not know that there's a gap until after a new 
> > request was queued, which then it will send an END_TRANSFER command and 
> > dequeue all the requests to restart the transfer due to missed_isoc.
> > We do this because the dwc3 driver does not know whether the new request 
> > is actually stale data, and we should not change this behavior.
> > 
> > Now, with UVC, it needs to communicate to the dwc3 driver that there 
> > will be a gap after a certain request (and that the device is expecting 
> > to send 0-length data). This is not a normal operation for isoc 
> > transfer. You may need to introduce a new way for the function driver to 
> > do that, possibly a new field in usb_request structure to indicate that. 
> > However, this seems a little awkward. Maybe others can comment on this.

I'm not sure how this is supposed to work. What exactly can the dwc3 driver
/ hardware do to handle a gap?

> Note that on the host side, there is a difference between receiving 
> a 0-length packet and receiving no packet at all.  As long as both the 
> host and the gadget expect the isochronous stream to be running, there 
> shouldn't be any gaps if you can avoid it.

Huh, so how is this handled on other hardware? From what I can tell the UVC
gadget works with other drivers and I've not found any special handling for
this. Is there no packet sent or are 0-length packet generated implicitly
somewhere?

Regards,
Michael

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