Re: DWC3-Gadget: Flickering with ISOC Streaming (UVC) while multiplier set on Superspeed

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

Hi Michael,

On Fri, Sep 01, 2023, Michael Grzeschik wrote:
> Hi Thinh!
> 
> I just stumbled over a similar issue we already solved for the High
> Speed Case when streaming ISOC packages and using a multiplier higher
> then one. Last time we saw some bad frame artefacts when using the
> higher multiplier value. The Frames were distorted due to truncated
> transfers.
> 
> Since the last case we have patch
> 
> 8affe37c525d ("usb: dwc3: gadget: fix high speed multiplier setting")
> 
> that fixes the calculation of the mult PCM1 parameter when using high
> speed transfers. After that no truncations were reported again.
> 
> However I came across a similar issue which is just a little less easy
> to trigger and only occurs with Superspeed. Now, while the memory
> bandwidth of the machine runs on higher load, the UVC frames are
> similarly distorted when we use a multiplier higher then one.
> 
> I looked over the implications the multiplier has on the Superspeed case
> in the dwc3 gadget driver, but could only find some TXFIFO adjustments
> and no other extra bits e.g. in the transfer descriptors. Do you have
> some pointers how the multiplier parameter of the endpoint is used in
> hardware?
> 

As you already know, PCM1 is only for highspeed not Superspeed. What
failure did the dwc3 driver reported? Missed isoc? What's the request
transfer size?

Perhaps you can capture some tracepoints of the problem?

Thanks,
Thinh