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

[Michael Grzeschik <mgr@pengutronix.de>]

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On Wed, Sep 06, 2023 at 12:44:02AM +0000, Thinh Nguyen wrote:
>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.

Since I did not find proper documentation of the registers I was just hoping.

In the device-tree documentation stanley added he mentioned some device
mode properties. As I look again it seems the RXFIFO parameters are
the only one affecting device mode. So nevermind.

Thanks,
Michael

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