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

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

On Mon, Sep 04, 2023, Michael Grzeschik wrote:
> Cc'ing: Stanley Chang <stanley_chang@realtek.com>
> 
> On Mon, Sep 04, 2023 at 12:41:33AM +0200, Michael Grzeschik wrote:
> > Hi Thinh
> > 
> > On Fri, Sep 01, 2023 at 01:35:16AM +0000, Thinh Nguyen wrote:
> > > On Fri, Sep 01, 2023, Michael Grzeschik wrote:
> > > > 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?
> > 
> > Yes, I see missed isoc errors. But this is just a symptom in this case.
> > 
> > I can increase the maxburst or maxpacket parameters stepwise and on
> > one point see the flickering appear. But when I increase the TXFIFOSIZE
> > for the endpoint the flickering is gone again. Until I increase one of
> > the parameters maxpacket or maxburst to much again.
> > 
> > So due to the memory bandwidth is under pressure, it seems like the
> > hardware is not fast enough with sending the expected data per transfer,
> > due to the txfifo is not long enough and needs to be refilled more
> > often.
> > 
> > This sounds like a fifo underrun issue in the hardware.
> 
> This whole issue sound like a case of stanleys patches:
> 
> https://lore.kernel.org/all/20230828055212.5600-1-stanley_chang@realtek.com/
> 
> For now I was unluky while adjusting the paramaters. Are those registers
> anywhere documented?
> 

How does his change impact isoc endpoint? His change should only affect
non-periodic endpoint. Also periodic settings is only for host.

Thanks,
Thinh