Re: [PATCH v2] usb: dwc3: gadget: check drained isoc ep

[Thinh Nguyen <Thinh.Nguyen@synopsys.com>]

Hi Michael,

On Sun, May 05, 2024, Michael Grzeschik wrote:
> On Wed, Apr 24, 2024 at 01:51:01AM +0000, Thinh Nguyen wrote:
> > > 
> > 
> > Right. Unfortunately, dwc3 can only "guess" when UVC function stops
> > pumping more request or whether it's due to some large latency. The
> > logic to workaround this underrun issue will not be foolproof. Perhaps
> > we can improve upon it, but the solution is better implement at the UVC
> > function driver.
> 
> Yes, the best way to solve this is in the uvc driver.
> 
> > I thought we have the mechanism in UVC function now to ensure queuing
> > enough zero-length requests to account for underrun/latency issue?
> > What's the issue now?
> 
> This is actually only partially true. Even with the zero-length packages
> it is possible that we run into underruns. This is why we implemented
> this patch. This has happened because another interrupt thread with the
> same prio on the same CPU as this interrupt thread was keeping the CPU

Do you have the data on the worst latency?

Can this other interrupt thread lower its priority relative to UVC? For
isoc endpoint, data is time critical.

Currently dwc3 can have up to 255 TRBs per endpoint, potentially 255
zero-length requests. That's 255 uframe, or roughly ~30ms. Is your worst
latency more than 30ms? ie. no handling of transfer completion and
ep_queue for the whole 255 intervals or 30ms. If that's the case, we
have problems that cannot just be solved in dwc3.

> busy. As the dwc3 interrupt thread get to its call, the time was already
> over and the hw was already drained, although the started list was not
> yet empty, which was causing the next queued requests to be queued to
> late. (zero length or not)
> 
> Yes, this needed to be solved on the upper level first, by moving the
> long running work of the other interrupt thread to another thread or
> even into the userspace.

Right.

> 
> However I thought it would be great if we could somehow find out in
> the dwc3 core and make the pump mechanism more robust against such
> late enqueues.

The dwc3 core handling of events and ep_queue is relatively quick. I'm
all for any optimization in the dwc3 core for performance. However, I
don't want to just continue looking for workaround in the dwc3 core
without looking to solve the issue where it should be. I don't want to
sacrifice complexity and/or performance to other applications for just
UVC.

> 
> This all started with that series.
> 
> https://lore.kernel.org/all/20240307-dwc3-gadget-complete-irq-v1-0-4fe9ac0ba2b7@pengutronix.de/
> 
> And patch 2 of this series did work well so far. The next move was this
> patch.
> 
> Since the last week debugging we found out that it got other issues.
> It is not allways save to read the HWO bit, from the driver.
> 
> Turns out that after an new TRB was prepared with the HWO bit set
> it is not save to read immideatly back from that value as the hw
> will be doing some operations on that exactly new prepared TRB.
> 
> We ran into this problem when applying this patch. The trb buffer list
> was actually filled but we hit a false positive where the latest HWO bit
> was 0 (probably due to the hw action in the background) and therefor
> went into end transfer.
> 

Thanks for the update.

BR,
Thinh