rp2040: Avoid screeches due to audio underflow during flash writes

By pausing audio during flash writes, the worst screeching of #8121
is avoided. I don't consider this a full fix, but it greatly improves
the by far most common scenario in which the problem occurs.

Tested on rp2040 prop feather with a midi synth playing arpeggios. When
writing to the flash e.g., with
```
dd bs=512 count=32 if=/dev/zero of=/media/jepler/CIRCUITPY/boop
```
the audio goes "tap tap tap tap" during the flash write instead of the
squawking.

This isn't a 100% fix; it will still glitch out, including during USB
enumeration which must be taking a long time without servicing background
tasks. Add a delay if not usb-connected at startup ameliorates this
greatly.

Author: jepler

ports/raspberrypi/audio_dma.c

@@ -388,6 +388,27 @@ bool audio_dma_get_paused(audio_dma_t *dma) {
     return (control & DMA_CH0_CTRL_TRIG_EN_BITS) == 0;
 }
 
+uint32_t audio_dma_pause_all(void) {
+    uint32_t result = 0;
+    for (size_t channel = 0; channel < NUM_DMA_CHANNELS; channel++) {
+        audio_dma_t *dma = MP_STATE_PORT(playing_audio)[channel];
+        if (dma != NULL && !audio_dma_get_paused(dma)) {
+            audio_dma_pause(dma);
+            result |= (1 << channel);
+        }
+    }
+    return result;
+}
+
+void audio_dma_unpause_cookie(uint32_t cookie) {
+    for (size_t channel = 0; channel < NUM_DMA_CHANNELS; channel++) {
+        audio_dma_t *dma = MP_STATE_PORT(playing_audio)[channel];
+        if (dma != NULL && (cookie & (1 << channel))) {
+            audio_dma_resume(dma);
+        }
+    }
+}
+
 void audio_dma_init(audio_dma_t *dma) {
     dma->buffer[0] = NULL;
     dma->buffer[1] = NULL;

ports/raspberrypi/audio_dma.h

@@ -89,4 +89,7 @@ void audio_dma_pause(audio_dma_t *dma);
 void audio_dma_resume(audio_dma_t *dma);
 bool audio_dma_get_paused(audio_dma_t *dma);
 
+uint32_t audio_dma_pause_all(void);
+void audio_dma_unpause_cookie(uint32_t cookie);
+
 #endif  // MICROPY_INCLUDED_RASPBERRYPI_AUDIO_DMA_OUT_H

ports/raspberrypi/supervisor/internal_flash.c

@@ -39,6 +39,7 @@
 #include "lib/oofatfs/ff.h"
 #include "shared-bindings/microcontroller/__init__.h"
 
+#include "audio_dma.h"
 #include "supervisor/flash.h"
 #include "supervisor/usb.h"
 
@@ -97,9 +98,12 @@ void port_internal_flash_flush(void) {
     }
     // Make sure we don't have an interrupt while we do flash operations.
     common_hal_mcu_disable_interrupts();
+    // and audio DMA must be paused as well
+    uint32_t cookie = audio_dma_pause_all();
     flash_range_erase(CIRCUITPY_CIRCUITPY_DRIVE_START_ADDR + _cache_lba, SECTOR_SIZE);
     flash_range_program(CIRCUITPY_CIRCUITPY_DRIVE_START_ADDR + _cache_lba, _cache, SECTOR_SIZE);
     _cache_lba = NO_CACHE;
+    audio_dma_unpause_cookie(cookie);
     common_hal_mcu_enable_interrupts();
 }