nRF5340 UAC2: can isochronous IN endpoint use full 1023-byte buffer

[Elliot Metcalf]

I'm trying to advertise an isochronous IN endpoint with wMaxPacketSize = 768 bytes (8 channels × 48 kHz × 16-bit audio, 1 ms polling). USB init fails at usbd_init with Failed to assign endpoint addresses. for wMaxPacketSize > 511 bytes

Looking at zephyr/drivers/usb/udc/udc_nrf.c:

ep_cfg_in[i].caps.mps = NRF_USBD_COMMON_ISOSIZE / 2;   // = 511

with NRF_USBD_COMMON_ISOSIZE = 1023. My understanding based on this is that the iso IN endpoint cap is 511 bytes due to hardware double-buffering, even though USB full-speed spec allows up to 1023.

Questions:

1. Is there a supported way (Kconfig, devicetree, runtime API) to disable double-buffering and use the full 1023 bytes per microframe?
2. If I patch udc_nrf.c to remove the /2, are there known issues? Does the rest of the driver assume double-buffering, or is single-buffer mode supported?

Thank you!

[Elliot Metcalf]

I was able to get a working solution by changing a few lines in the SDK file:

//NRF_USBD->ISOSPLIT = USBD_ISOSPLIT_SPLIT_HalfIN << USBD_ISOSPLIT_SPLIT_Pos;

NRF_USBD->ISOSPLIT = USBD_ISOSPLIT_SPLIT_OneDir << USBD_ISOSPLIT_SPLIT_Pos;

and

ep_cfg_in[i].caps.mps = NRF_USBD_COMMON_ISOSIZE; // / 2;

is there a way to do this without modifying the SDK?

[Amanda Hsieh]

Unfortunately, it could not avoid modifying the current SDK to achieve your requirement, but you could pull a request on https://github.com/zephyrproject-rtos/zephyr to submit the changes. It will be merged into the future release if it gets approval.