UAC2 Panic "ep 81 was already available" when polling Audio TX (Loopback)

[ashafq]

I am developing a minimal UAC2 audio loopback example on the RP2350 (Raspberry Pi Pico 2). My goal is a simple 48kHz, 24-bit (32-bit slot) stereo configuration.

I am using a polling approach in main (calling usb_audio_task) rather than using interrupt callbacks. I am encountering a persistent panic: WARN: starting new transfer on already active ep 81, followed by *** PANIC *** ep 81 was already available.

This occurs shortly after the host enumerates the device and begins streaming.

See below for serial debug logs:

Build date: Dec  8 2025 22:40:11
TinyUSB version: TINYUSB_VERSION
Colahall, LLC (colahall.io) Audio Interface
Get sample rate RANGE (48000 Hz)
Get sample rate RANGE (48000 Hz)
Clock get is valid 1
Get sample rate CUR (48000 Hz)  
Clock get is valid 1
Get sample rate RANGE (48000 Hz)
Get sample rate RANGE (48000 Hz)
Linein streaming stopped
Clock get is valid 1
Get sample rate CUR (48000 Hz)
Clock get is valid 1
Linein streaming started
Linein streaming stopped
Get mute CUR (0)
Get mute CUR (0)
Get mute CUR (0)
Get volume CUR (0)
Get volume CUR (0)
Get volume CUR (0)
Get volume CUR (0)
Get volume CUR (0)
Get volume CUR (0)
Get volume CUR (0)
Get volume CUR (0)
Get volume CUR (0)
Clock get is valid 1
Get sample rate CUR (48000 Hz)
Clock get is valid 1
WARN: starting new transfer on already active ep 81

*** PANIC ***

ep 81 was already available

Here is the full example of the code: https://gist.github.com/ashafq/c45e48b874e1e76fe11597556a8b9120

[HiFiPhile]

Likely #3142

[ashafq]

I tried to manually update TinyUSB to version 0.20.0 with the pico-sdk at 2.2.1. You can see my docker file here, but I now get a lot of build errors. I tried cleaning the build cache.

I also had to change a bunch of code like this:

1. On top of usb_callback.c

   #if TUSB_VERSION_MINOR >= 20
   #define audio_control_request_t audio20_control_request_t
   #define audio_control_cur_4_t audio20_control_cur_4_t
   #define audio_control_range_4_n_t(n) audio20_control_range_4_n_t(n)
   
   #define AUDIO_CS_REQ_CUR AUDIO20_CS_REQ_CUR
   #define AUDIO20_CS_REQ_RANGE AUDIO20_CS_REQ_RANGE
   #define AUDIO20_CS_CTRL_CLK_VALID AUDIO20_CS_CTRL_CLK_VALID
   #define AUDIO20_FU_CTRL_MUTE AUDIO20_FU_CTRL_MUTE
   #define AUDIO20_FU_CTRL_VOLUME AUDIO20_FU_CTRL_VOLUME
   #define AUDIO_CS_REQ_RANGE AUDIO20_CS_REQ_RANGE
   #define AUDIO_CS_CTRL_SAM_FREQ AUDIO20_CS_CTRL_SAM_FREQ
   #define AUDIO_FU_CTRL_VOLUME AUDIO20_FU_CTRL_VOLUME
   #define AUDIO_FU_CTRL_MUTE AUDIO20_FU_CTRL_MUTE
   #endif

2. On top of usb_descriptor.h

   #if TUSB_VERSION_MINOR >= 20
   
   #define TUD_AUDIO_DESC_IAD_LEN TUD_AUDIO20_DESC_IAD_LEN
   
   #endif 

Here is the build error log https://gist.github.com/ashafq/143dcd509cff9bb31280e0380dbb44c8

[ashafq]

Looks like there were many API breaking changes in version 0.20.0. I have updated the code, and now it's building and functioning.

Here is the updated code https://gist.github.com/ashafq/c45e48b874e1e76fe11597556a8b9120