Improve header definitions

- Define MIN_OVERHEAD
- Rename `MAX_PAYLOAD` to `MIN_MAX_PAYLOAD` to namespace the def
- Define `MIN_MAX_PACKET_SIZE` to explicitly provide the maximum packet size that MIN will send. This is useful for declaring buffers when the UART tx implementation uses a buffer (DMA).

Signed-off-by: Nick Brook <[email protected]>

Author: nrbrook

target/min.c

@@ -9,7 +9,7 @@
 
 // Number of bytes needed for a frame with a given payload length, excluding stuff bytes
 // 3 header bytes, ID/control byte, length byte, seq byte, 4 byte CRC, EOF byte
-#define ON_WIRE_SIZE(p) ((p) + 11U)
+#define ON_WIRE_SIZE(p) ((p) + MIN_OVERHEAD)
 
 // Special protocol bytes
 enum {
@@ -504,11 +504,11 @@ static void rx_byte(struct min_context *self, uint8_t byte)
         crc32_step(&self->rx_checksum, byte);
         if (self->rx_frame_length > 0) {
             // Can reduce the RAM size by compiling limits to frame sizes
-            if (self->rx_frame_length <= MAX_PAYLOAD) {
+            if (self->rx_frame_length <= MIN_MAX_PAYLOAD) {
                 self->rx_frame_state = RECEIVING_PAYLOAD;
             } else {
                 // Frame dropped because it's longer than any frame we can buffer
-                min_debug_print("Dropping frame because length %d > MAX_PAYLOAD %d", self->rx_frame_length, MAX_PAYLOAD);
+                min_debug_print("Dropping frame because length %d > MIN_MAX_PAYLOAD %d", self->rx_frame_length, MIN_MAX_PAYLOAD);
                 self->rx_frame_state = SEARCHING_FOR_SOF;
             }
         } else {

target/min.h

@@ -70,8 +70,29 @@
 #define TRANSPORT_PROTOCOL
 #endif
 
-#ifndef MAX_PAYLOAD
-#define MAX_PAYLOAD (255U)
+// Number of bytes needed for a frame with a given payload length, excluding stuff bytes
+// 3 header bytes, ID/control byte, length byte, seq byte, 4 byte CRC, EOF byte
+#define MIN_OVERHEAD 11
+
+#if defined(MAX_PAYLOAD)
+// for backwards compatibility
+#define MIN_MAX_PAYLOAD MAX_PAYLOAD
+#endif
+
+#ifdef MIN_MAX_PAYLOAD
+#define MIN_MAX_PACKET_SIZE (MIN_MAX_PAYLOAD + MIN_OVERHEAD)
+#elif defined(MIN_MAX_PACKET_SIZE)
+#if (MIN_MAX_PACKET_SIZE < MIN_OVERHEAD)
+#error "MIN_MAX_PACKET_SIZE must be greater than MIN_OVERHEAD"
+#endif
+#define MIN_MAX_PAYLOAD (MIN_MAX_PACKET_SIZE - MIN_OVERHEAD)
+#else
+#define MIN_MAX_PAYLOAD 255
+#define MIN_MAX_PACKET_SIZE (MIN_MAX_PAYLOAD + MIN_OVERHEAD)
+#endif
+
+#if (MIN_MAX_PAYLOAD > 255)
+#error "MIN frame payloads can be no bigger than 255 bytes"
 #endif
 
 // Powers of two for FIFO management. Default is 16 frames in the FIFO, total of 1024 bytes for frame data
@@ -85,10 +106,6 @@
 #define TRANSPORT_FIFO_MAX_FRAMES (1U << TRANSPORT_FIFO_SIZE_FRAMES_BITS)
 #define TRANSPORT_FIFO_MAX_FRAME_DATA (1U << TRANSPORT_FIFO_SIZE_FRAME_DATA_BITS)
 
-#if (MAX_PAYLOAD > 255)
-#error "MIN frame payloads can be no bigger than 255 bytes"
-#endif
-
 // Indices into the frames FIFO are uint8_t and so can't have more than 256 frames in a FIFO
 #if (TRANSPORT_FIFO_MAX_FRAMES > 256)
 #error "Transport FIFO frames cannot exceed 256"
@@ -143,7 +160,7 @@ struct min_context {
 #ifdef TRANSPORT_PROTOCOL
     struct transport_fifo transport_fifo; // T-MIN queue of outgoing frames
 #endif
-    uint8_t rx_frame_payload_buf[MAX_PAYLOAD]; // Payload received so far
+    uint8_t rx_frame_payload_buf[MIN_MAX_PAYLOAD]; // Payload received so far
     uint32_t rx_frame_checksum;                // Checksum received over the wire
     struct crc32_context rx_checksum;          // Calculated checksum for receiving frame
     struct crc32_context tx_checksum;          // Calculated checksum for sending frame