OP-980 Extended the memory barrier macros and fixed barrier kind in pios_usb_hid

+review OPReview-501

flight/pios/inc/pios_helpers.h

@@ -36,6 +36,34 @@
  */
 #define NELEMENTS(x)       (sizeof(x) / sizeof((x)[0]))
 
-#define WRITE_MEMORY_BARRIER() asm volatile("":::"memory")
+
+/**
+ * @brief Compiler barrier: Disables compiler load/store reordering across the barrier
+ *
+ */
+#define COMPILER_BARRIER() asm volatile ("" ::: "memory")
+
+// Memory barriers:
+// Note that on single core Cortex M3 & M4, the is generally no need to use a processor memory barrier instruction such as DMB.
+// See http://infocenter.arm.com/help/topic/com.arm.doc.dai0321a/DAI0321A_programming_guide_memory_barriers_for_m_profile.pdf
+// However, it makes sense to use these if we want to reduce issues if we ever port to a multicore processor in the future.
+// An important exception for STM32 is when setting up the DMA engine - see the above reference for details.
+
+/**
+ * @brief  Read Acquire memory barrier
+ */
+#define READ_MEMORY_BARRIER()  COMPILER_BARRIER()
+/**
+ * @brief Write Release memory barrier
+ */
+#define WRITE_MEMORY_BARRIER() COMPILER_BARRIER()
+
+/**
+ * @brief Full fence memory barrier
+ */
+#define MEMORY_BARRIER()       COMPILER_BARRIER()
+
+// For future multicore ARM v7 or later:
+// The above three macros would be replaced with: asm volatile("dmb":::"memory")
 
 #endif // PIOS_HELPERS_H

flight/pios/stm32f4xx/pios_usb.c

@@ -181,7 +181,9 @@ bool PIOS_USB_CheckAvailable(__attribute__((unused)) uint32_t id)
     bool status = usb_found != 0 && transfer_possible ? 1 : 0;
 
 #ifdef PIOS_INCLUDE_FREERTOS
-    while(xSemaphoreTakeFromISR(usb_dev->statusCheckSemaphore, NULL) != pdTRUE);
+    while (xSemaphoreTakeFromISR(usb_dev->statusCheckSemaphore, NULL) != pdTRUE) {
+        ;
+    }
 #endif
     bool reconnect = (lastStatus && !status);
     lastStatus = status;

flight/pios/stm32f4xx/pios_usb_hid.c

@@ -187,7 +187,7 @@ static bool PIOS_USB_HID_SendReport(struct pios_usb_hid_dev *usb_hid_dev)
     if (!usb_hid_dev->tx_out_cb) {
         return false;
     }
-    WRITE_MEMORY_BARRIER();
+    READ_MEMORY_BARRIER();
     bool need_yield = false;
 #ifdef PIOS_USB_BOARD_BL_HID_HAS_NO_LENGTH_BYTE
     bytes_to_tx = (usb_hid_dev->tx_out_cb)(usb_hid_dev->tx_out_context,
@@ -510,7 +510,7 @@ static bool PIOS_USB_HID_EP_OUT_Callback(uint32_t usb_hid_id, __attribute__((unu
         usb_hid_dev->rx_active = false;
         return false;
     }
-    WRITE_MEMORY_BARRIER();
+    READ_MEMORY_BARRIER();
     /* The first byte is report ID (not checked), the second byte is the valid data length */
     uint16_t headroom;
     bool need_yield = false;