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

[Michael Grzeschik <mgr@pengutronix.de>]

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Hi Thinh

Thanks for the update!

On Wed, Sep 06, 2023 at 11:09:03PM +0000, Thinh Nguyen wrote:
>On Wed, Sep 06, 2023, Thinh Nguyen wrote:
>> On Wed, Sep 06, 2023, Michael Grzeschik wrote:
>> >
>> > > 2) Burst setting
>> > > 	I think this is self-explainatory. Large data request needs
>> > > 	higher burst.
>> >
>> > I will have to find out if the burst setting can be changed on the
>> > rk3568 somehow. This sounds very likely.
>> >
>>
>> The dwc3 driver checks the endpoint descriptor from the UVC function
>> driver to setup the burst. So just setup the max 16 bursts (or 15 in the
>> descriptor). The dwc3 controller supports that. Whether the host would
>> do 16 bursts is another thing. Note that there's no "mult" setting for
>> SuperSpeed.
>
>Clarification: no mult setting for the dwc3 controller when operate in
>SuperSpeed.

I was somehow mistaken by the wording "burst setting" and thought of the
axi-bus burst setting to the controller instead of the usb3 maxburst
setting as you ment actually.

However. This is usefull input anyway. I never thought of maximizing the
burst and packagesize and let the host side make the decision.
But we will probably will eat up a lot of usb bandwidth by doing so.

Before your note I was somehow mistaken that the maxpacket and maxburst
size had to correlate with the actually transfered data we queue into
the hardware per request.

>> I recall that UVC tries to pack a lot of data in a single request. All
>> the while some intervals it would send 0-length data because of idle
>> time. I would spread to more requests with a little less data to give
>> the host a little more breathing room and bandwidth.

The higher load per request is due to the fact that the uvc gadget is
using the multiplier, maxpacket and maxburst parameters for the size
calculation of the request.

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/usb/gadget/function/uvc_video.c#n331

Since it is clear now that those parameters are not necessary coupled
it makes total sense to split the requests into smaller chunks.

Thanks,
Michael

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