dshot: fix bidirectional dshot stream sharing (#24996)

Freeing the DMA stream in the hrt callback causes other peripherals on that DMA controller to lock up (namely GPS). Moving the free back into thread context, right before allocation, solves the problem

Author: dakejahl

platforms/nuttx/src/px4/stm/stm32_common/dshot/dshot.c

@@ -206,20 +206,24 @@ static void init_timers_dma_up(void)
 		timer_configs[timer_index].dma_handle = stm32_dmachannel(io_timers[timer_index].dshot.dma_map_up);
 
 		if (timer_configs[timer_index].dma_handle == NULL) {
-			PX4_DEBUG("Failed to allocate Timer %u DMA UP", timer_index);
+			PX4_WARN("Failed to allocate Timer %u DMA UP", timer_index);
 			continue;
 		}
 
 		PX4_INFO("Allocated DMA UP Timer Index %u", timer_index);
 		timer_configs[timer_index].initialized = true;
 	}
 
-	// Free the allocated DMA channels
-	for (uint8_t timer_index = 0; timer_index < MAX_IO_TIMERS; timer_index++) {
-		if (timer_configs[timer_index].dma_handle != NULL) {
-			stm32_dmafree(timer_configs[timer_index].dma_handle);
-			timer_configs[timer_index].dma_handle = NULL;
-			PX4_INFO("Freed DMA UP Timer Index %u", timer_index);
+	// Bidirectional DShot will free/allocate DMA stream on every update event. This is required
+	// in order to reconfigure the DMA stream between Timer Burst and CaptureCompare.
+	if (_bidirectional) {
+		// Free the allocated DMA channels
+		for (uint8_t timer_index = 0; timer_index < MAX_IO_TIMERS; timer_index++) {
+			if (timer_configs[timer_index].dma_handle != NULL) {
+				stm32_dmafree(timer_configs[timer_index].dma_handle);
+				timer_configs[timer_index].dma_handle = NULL;
+				PX4_INFO("Freed DMA UP Timer Index %u", timer_index);
+			}
 		}
 	}
 }
@@ -341,8 +345,15 @@ void up_dshot_trigger()
 
 			io_timer_set_dshot_burst_mode(timer_index, _dshot_frequency, channel_count);
 
-			// Allocate DMA
-			if (timer_configs[timer_index].dma_handle == NULL) {
+			if (_bidirectional) {
+				// Deallocate DMA from previous transaction
+				if (timer_configs[timer_index].dma_handle != NULL) {
+					stm32_dmastop(timer_configs[timer_index].dma_handle);
+					stm32_dmafree(timer_configs[timer_index].dma_handle);
+					timer_configs[timer_index].dma_handle = NULL;
+				}
+
+				// Allocate DMA
 				timer_configs[timer_index].dma_handle = stm32_dmachannel(io_timers[timer_index].dshot.dma_map_up);
 
 				if (timer_configs[timer_index].dma_handle == NULL) {
@@ -502,11 +513,8 @@ static void capture_complete_callback(void *arg)
 	// Disable capture DMA
 	io_timer_capture_dma_req(timer_index, capture_channel, false);
 
-	if (timer_configs[timer_index].dma_handle != NULL) {
-		stm32_dmastop(timer_configs[timer_index].dma_handle);
-		stm32_dmafree(timer_configs[timer_index].dma_handle);
-		timer_configs[timer_index].dma_handle = NULL;
-	}
+	// Stop DMA (should already be finished)
+	stm32_dmastop(timer_configs[timer_index].dma_handle);
 
 	// Re-initialize all output channels on this timer
 	for (uint8_t output_channel = 0; output_channel < MAX_TIMER_IO_CHANNELS; output_channel++) {