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

[Michael Grzeschik <mgr@pengutronix.de>]

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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?

Regards,
Michael

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