stmhal: On SysTick IRQ, only process one DMA channel at a time.

This can be generalised if/when more processing is needed by SysTick.

Thanks to @chuckbook for the idea.

stmhal/dma.c

@@ -220,14 +220,16 @@ void dma_invalidate_channel(DMA_Stream_TypeDef *dma_stream, uint32_t dma_channel
     }
 }
 
-// Called from the SysTick handler (once per millisecond)
-void dma_idle_handler() {
+// Called from the SysTick handler
+// We use LSB of tick to select which controller to process
+void dma_idle_handler(int tick) {
     static const uint32_t   controller_mask[] = {
         DMA1_ENABLE_MASK, DMA2_ENABLE_MASK
     };
-    for (int controller = 0; controller < NCONTROLLERS; controller++) {
+    {
+        int controller = tick & 1;
         if (dma_idle.counter[controller] == 0) {
-            continue;
+            return;
         }
         if (++dma_idle.counter[controller] > DMA_IDLE_TICK_MAX) {
             if ((dma_enable_mask & controller_mask[controller]) == 0) {

stmhal/dma.h

@@ -38,7 +38,7 @@ typedef union {
 extern volatile dma_idle_count_t dma_idle;
 #define DMA_IDLE_ENABLED()  (dma_idle.enabled != 0)
 
-#define DMA_SYSTICK_MASK            0x0F
+#define DMA_SYSTICK_MASK            0x0e
 #define DMA_MSECS_PER_SYSTICK       (DMA_SYSTICK_MASK + 1)
 #define DMA_IDLE_TICK_MAX           (8)     // 128 msec
 #define DMA_IDLE_TICK(tick)         (((tick) & DMA_SYSTICK_MASK) == 0)
@@ -48,4 +48,4 @@ extern const DMA_InitTypeDef dma_init_struct_spi_i2c;
 void dma_init(DMA_HandleTypeDef *dma, DMA_Stream_TypeDef *dma_stream, const DMA_InitTypeDef *dma_init, uint32_t dma_channel, uint32_t direction, void *data);
 void dma_deinit(DMA_HandleTypeDef *dma);
 void dma_invalidate_channel(DMA_Stream_TypeDef *dma_stream, uint32_t dma_channel);
-void dma_idle_handler();
+void dma_idle_handler(int controller);

stmhal/stm32_it.c

@@ -269,8 +269,12 @@ void SysTick_Handler(void) {
     // work properly.
     SysTick->CTRL;
 
+    // Right now we just have the DMA controllers to process during this
+    // interrupt and we use a custom dispatch handler.  If this needs to
+    // be generalised in the future then a dispatch table can be used as
+    // follows: ((void(*)(void))(systick_dispatch[uwTick & 0xf]))();
     if (DMA_IDLE_ENABLED() && DMA_IDLE_TICK(uwTick)) {
-        dma_idle_handler(); 
+        dma_idle_handler(uwTick);
     }
 }