msp: add buffering around the writes.

This bulks up the writes and lets the USB VCP driver send one 20 byte
frame instead of 20 one byte frames.  This speeds up the blackbox
download and makes VCP much more reliable when running under a virtual
machine.

Signed-off-by: Michael Hope <mlhx@google.com>

src/main/io/serial_msp.c

@@ -43,6 +43,7 @@
 #include "drivers/pwm_rx.h"
 #include "drivers/gyro_sync.h"
 
+#include "drivers/buf_writer.h"
 #include "rx/rx.h"
 #include "rx/msp.h"
 
@@ -413,10 +414,11 @@ typedef struct mspPort_s {
 static mspPort_t mspPorts[MAX_MSP_PORT_COUNT];
 
 static mspPort_t *currentPort;
+static bufWriter_t *writer;
 
 static void serialize8(uint8_t a)
 {
-    serialWrite(mspSerialPort, a);
+    bufWriterAppend(writer, a);
     currentPort->checksum ^= a;
 }
 
@@ -1903,7 +1905,7 @@ static bool mspProcessReceivedData(uint8_t c)
     return true;
 }
 
-void setCurrentPort(mspPort_t *port)
+static void setCurrentPort(mspPort_t *port)
 {
     currentPort = port;
     mspSerialPort = currentPort->port;
@@ -1921,6 +1923,10 @@ void mspProcess(void)
         }
 
         setCurrentPort(candidatePort);
+        // Big enough to fit a MSP_STATUS in one write.
+        uint8_t buf[sizeof(bufWriter_t) + 20];
+        writer = bufWriterInit(buf, sizeof(buf),
+                               serialWriteBufShim, currentPort->port);
 
         while (serialRxBytesWaiting(mspSerialPort)) {
 
@@ -1937,6 +1943,8 @@ void mspProcess(void)
             }
         }
 
+        bufWriterFlush(writer);
+
         if (isRebootScheduled) {
             waitForSerialPortToFinishTransmitting(candidatePort->port);
             stopMotors();
@@ -2005,10 +2013,15 @@ void sendMspTelemetry(void)
     }
 
     setCurrentPort(mspTelemetryPort);
+    uint8_t buf[sizeof(bufWriter_t) + 16];
+    writer = bufWriterInit(buf, sizeof(buf),
+                           serialWriteBufShim, currentPort->port);
 
     processOutCommand(mspTelemetryCommandSequence[sequenceIndex]);
     tailSerialReply();
 
+    bufWriterFlush(writer);
+
     sequenceIndex++;
     if (sequenceIndex >= TELEMETRY_MSP_COMMAND_SEQUENCE_ENTRY_COUNT) {
         sequenceIndex = 0;