drivers: add a buffering writer.

This wraps around the serial API and buffers a configurable number of
characters before flushing.

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

Makefile

@@ -288,6 +288,7 @@ COMMON_SRC = build_config.c \
 		   drivers/sound_beeper.c \
 		   drivers/system.c \
 		   drivers/gyro_sync.c \
+		   drivers/buf_writer.c \
 		   io/beeper.c \
 		   io/rc_controls.c \
 		   io/rc_curves.c \

src/main/drivers/buf_writer.c

@@ -0,0 +1,47 @@
+/*
+ * This file is part of Cleanflight.
+ *
+ * Cleanflight is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Cleanflight is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Cleanflight.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <stdint.h>
+
+#include "buf_writer.h"
+
+bufWriter_t *bufWriterInit(uint8_t *b, int total_size, bufWrite_t writer, void *arg)
+{
+    bufWriter_t *buf = (bufWriter_t *)b;
+    buf->writer = writer;
+    buf->arg = arg;
+    buf->at = 0;
+    buf->capacity = total_size - sizeof(*buf);
+
+    return buf;
+}
+
+void bufWriterAppend(bufWriter_t *b, uint8_t ch)
+{
+    b->data[b->at++] = ch;
+    if (b->at >= b->capacity) {
+        bufWriterFlush(b);
+    }
+}
+
+void bufWriterFlush(bufWriter_t *b)
+{
+    if (b->at != 0) {
+        b->writer(b->arg, b->data, b->at);
+        b->at = 0;
+    }
+}

src/main/drivers/buf_writer.h

@@ -0,0 +1,38 @@
+/*
+ * This file is part of Cleanflight.
+ *
+ * Cleanflight is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Cleanflight is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Cleanflight.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+// Called to flush the buffer.
+typedef void (*bufWrite_t)(void *arg, void *data, int count);
+
+typedef struct bufWriter_s {
+    bufWrite_t writer;
+    void *arg;
+    uint8_t capacity;
+    uint8_t at;
+    uint8_t data[];
+} bufWriter_t;
+
+// Initialise a block of memory as a buffered writer.
+//
+// b should be sizeof(bufWriter_t) + the number of bytes to buffer.
+// total_size should be the total size of b.
+//
+bufWriter_t *bufWriterInit(uint8_t *b, int total_size, bufWrite_t writer, void *p);
+void bufWriterAppend(bufWriter_t *b, uint8_t ch);
+void bufWriterFlush(bufWriter_t *b);