usb_audio: refill the mic FIFO from tud_audio_tx_done_isr()

ports/espressif/mpconfigport.mk

@@ -417,5 +417,18 @@ endif
 USB_NUM_ENDPOINT_PAIRS = 7
 USB_NUM_IN_ENDPOINTS = 5
 
+# usb_audio is compiled in on the S3 but claims no endpoints until
+# usb_audio.enable() is called from boot.py, so enabling it by default does not
+# affect existing boards. Freeing one IN endpoint (usb_hid.disable()) is then the
+# only thing a user needs, with no custom build.
+ifeq ($(IDF_TARGET),esp32s3)
+# It needs the TinyUSB device stack and audiocore, so leave it off for boards
+# that turn either off. The S3 has no AUDIOIO and no AUDIOPWMIO, so audiocore
+# follows CIRCUITPY_AUDIOBUSIO here.
+ifeq ($(filter 0,$(CIRCUITPY_USB_DEVICE) $(CIRCUITPY_AUDIOCORE) $(CIRCUITPY_AUDIOBUSIO)),)
+CIRCUITPY_USB_AUDIO ?= 1
+endif
+endif
+
 # Usually lots of flash space available
 CIRCUITPY_MESSAGE_COMPRESSION_LEVEL ?= 1

shared-module/usb_audio/__init__.c

@@ -18,6 +18,7 @@
 #include "py/mphal.h"
 #include "py/runtime.h"
 #include "supervisor/shared/tick.h"
+#include "supervisor/usb.h"
 #include "tusb.h"
 
 static bool usb_audio_is_enabled = false;
@@ -465,10 +466,33 @@ bool tud_audio_set_itf_cb(uint8_t rhport, tusb_control_request_t const *p_reques
         usb_audio_usbspeaker_streaming_reset();
     } else if (itf == usb_audio_mic_as_itf) {
         usb_audio_mic_streaming = streaming;
+        if (streaming) {
+            // Prime the FIFO for the first frame; after that each completed
+            // transfer schedules the next refill via tud_audio_tx_done_isr().
+            usb_background_schedule();
+        }
     }
     return true;
 }
 
+// Invoked from the audio class transfer-complete path once the next packet has
+// been loaded into the EP IN buffer. TinyUSB services audio completions in ISR
+// context, so they never queue a USB event. A port that pumps the optional class
+// tasks from a task loop sitting behind tud_task() therefore never refills the
+// microphone FIFO. Scheduling the refill here keeps usb_audio independent of how
+// a port drives TinyUSB.
+bool tud_audio_tx_done_isr(uint8_t rhport, uint16_t n_bytes_sent, uint8_t func_id,
+    uint8_t ep_in, uint8_t cur_alt_setting) {
+    (void)rhport;
+    (void)n_bytes_sent;
+    (void)func_id;
+    (void)ep_in;
+    (void)cur_alt_setting;
+
+    usb_background_schedule();
+    return true;
+}
+
 bool tud_audio_set_itf_close_ep_cb(uint8_t rhport, tusb_control_request_t const *p_request) {
     (void)rhport;
     uint8_t const itf = (uint8_t)tu_u16_low(p_request->wIndex);