Made progress on DMA for Speaker output

Considering switching to interrupt updater due to increased flexibility

code/drivers/speaker.c

@@ -1,25 +1,23 @@
 #include "speaker.h"
 
+/** Currently the strategy is to use DMA to transfer data
+ * from a buffer to the PWM driver, updating every cycle.
+ * However, this is impractical for several reasons:
+ * 1, this makes the audio sample rate tied to the PWM top value,
+ * meaning we can't pick an arbitrary sample rate
+ * 2, a 16-bit PWM signal would have an update rate of 488Hz, effectively
+ * generating a 900Hz tone that overpowers our audio. Obviously this is
+ * undesirable. An 11-bit PWM signal could have an update rate of 15kHz,
+ * meaning it generates a 30kHz tone. But to feed this with DMA, we would
+ * have to upsample from 5kHz, taking up RAM with duplicate data.
+ *
+ * The simpler approach is to run a fast interrupt on a different timer,
+ * allowing us to decouple the PWM generation from the audio sample rate.
+ * Even with reduced audio output resolution, high input resolution still has
+ * advantages like increased dynamic range and noise rejection.
+ */
 
-static const DMAConfig spkr_dma = {
-  .stream = STM32_ICU1_CH1_DMA_STREAM,
-  .channel = STM32_ICU1_CH1_DMA_CHANNEL,
-  .dma_priority = STM32_ICU1_CH1_DMA_PRIORITY,
-  .irq_priority = STM32_ICU1_CH1_DMA_IRQ_PRIORITY,
-  .periph_addr = &TIM3->DMAR,
-  .direction = DMA_DIR_P2M,
-  .psize = 2,
-  .msize = 2,
-  .inc_peripheral_addr = false,
-  .inc_memory_addr = true,
-  .circular = true,
-  .error_cb = &error_cb,
-  .end_cb = &end_cb,
-  .mburst = 8,
-  .fifo = 4
-};
-
-//DMA channels is STM32_PWM_TIM3_DMA_STREAM
+uint16_t spkr_dma_buf[spkr_buffer_len];
 
 void speakerUpdate(int16_t data, int len);
 
@@ -33,7 +31,7 @@ void speaker_callback(PWMDriver * pwmp){
 static PWMConfig spkr = {
   .frequency = 4096000,                                    /* 4MHz PWM clock frequency.   */
   .period = 4096,                                      /* Initial PWM period 1ms.       */
-  .callback = speaker_callback,
+  .callback = NULL,
   .channels = {
    {PWM_OUTPUT_ACTIVE_HIGH, NULL},
    {PWM_OUTPUT_DISABLED, NULL},
@@ -44,10 +42,29 @@ static PWMConfig spkr = {
   .dier = STM32_TIM_DIER_CC1DE,   // DMA request on channel 1 compare match
 };
 
+const uint32_t dma_config = 0
+    | STM32_DMA_CR_DIR_M2P      // Memory to peripheral
+    | STM32_DMA_CR_CIRC         // Circular buffer
+    | STM32_DMA_CR_MINC         // Memory Increment Mode
+    | STM32_DMA_CR_PSIZE_WORD   // 16-bit peripheral space
+    | STM32_DMA_CR_MSIZE_WORD   // 16-bit buffer width in RAM
+    | STM32_DMA_CR_PL(3)        // High priority
+    | STM32_DMA_CR_TCIE         // Transfer Complete Interrupt
+    | STM32_DMA_CR_HTIE         // Half-transfer Interrupt
+    ;
+
 void speakerStart(void){
+  static stm32_dma_stream_t * stream =
+      dmaStreamAlloc(STM32_PWM_TIM3_DMA_STREAM, 2, speaker_callback, NULL);
+  dmaStreamSetPeripheral(stream, &TIM3->DMAR);
+  dmaStreamSetMemory0(stream, spkr_dma_buf);
+  dmaStreamSetTransactionSize(stream, spkr_buffer_len);
+  dmaStreamSetMode(stream, dma_config);
+  dmaStreamEnable(stream);
   pwmStart(&PWMD3, &spkr);
 }
 
 void speakerStop(void){
   pwmStop(&PWMD3);
+  dmaStreamDisable(stream);
 }

code/drivers/speaker.h

@@ -1,5 +1,7 @@
 #include "hal.h"
+#include <stdint.h>
 
+const size_t spkr_buffer_len = 64;
 
 void speakerStart(void);
 void speakerStop(void);