usb: dwc2: gadget: high-bandwidth (mc > 1) status?

[Pavel Hofman <pavel.hofman@ivitera.com>]

Hi Minas at all,

Please does dwc2 (specifically in BCM2835/RPi) support HS ISOC multiple 
transactions mc > 1 reliably? I found this condition 
https://elixir.bootlin.com/linux/v5.16-rc2/source/drivers/usb/dwc2/gadget.c#L4041

	/* High bandwidth ISOC OUT in DDMA not supported */
	if (using_desc_dma(hsotg) && ep_type == USB_ENDPOINT_XFER_ISOC &&
	    !dir_in && mc > 1) {
		dev_err(hsotg->dev,
			"%s: ISOC OUT, DDMA: HB not supported!\n", __func__);
		return -EINVAL;
	}

But I do not know how the Descriptor DMA is critical and whether 
disabling it will affect gadget performance seriously.

I know about the RX FIFO sizing requirement (and TX FIFO too I guess), 
the current default values can be increased for that particular use case 
if needed.

I am trying to learn if it made sense to spend time on adding support 
for high-bandwidth to the UAC2 audio gadget  to allow using larger 
bInterval and mc=2,3 at high samplerates/channel counts (sort of "burst 
mode" similar to UAC3). When doing some CPU-demanding DSP it would help 
to avoid the time-critical handling every 125us microframe. Both OUT and 
IN are important.


Thanks a lot for your expert advice.


Best regards,


Pavel.