virtio_buffer: Increase buffer by 1 to meet the minimum requirement

Author: kbumsik

cores/arduino/stm32/OpenAMP/virtio_buffer.h

@@ -31,12 +31,16 @@ extern "C" {
 #endif
 
 /**
- * See virtio_config.h for the size decision.
- * The multiplier should be at least 1.
- * If VIRTIO_BUFFER_SIZE is still too big, RPMSG_VRING_TOTAL_PAYLOAD_SIZE
- * can be reduced by reducing the number of VRING_NUM_BUFFS in virtio_config.h.
+ * @brief Size of virtio ring buffer
+ * @note  See virtio_config.h for the size decision.
+ *        The multiplier should be at least 1. As a result, the minimum
+ *        required size of the initial virtio_buffer_write_available()
+ *        must be RPMSG_VRING_TOTAL_PAYLOAD_SIZE.
+ * @note  If VIRTIO_BUFFER_SIZE is still too big, RPMSG_VRING_TOTAL_PAYLOAD_SIZE
+ *        can be reduced by reducing the number of VRING_NUM_BUFFS in
+ *        virtio_config.h.
  */
-#define VIRTIO_BUFFER_SIZE (RPMSG_VRING_TOTAL_PAYLOAD_SIZE * 2)
+#define VIRTIO_BUFFER_SIZE (RPMSG_VRING_TOTAL_PAYLOAD_SIZE * 2 + 1)
 
 typedef struct {
   uint8_t buffer[VIRTIO_BUFFER_SIZE];