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

[Michael Grzeschik <mgr@pengutronix.de>]

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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?

Michael


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