From 7d7280a5b162800ef72d48a8a3d2afb4f0496b85 Mon Sep 17 00:00:00 2001
From: xengineering <me@xengineering.eu>
Date: Sat, 1 Jun 2024 12:48:44 +0200
Subject: firmware: Minimal frame encoding

This only covers the correct frame size, setting the buffer to 0 and
copy the payload over.

Missing is:

- copy destination address
- copy source address
- set frame type
- calculate and add CRC32 checksum
---
 firmware/src/data_link.c | 12 ++++++++++++
 firmware/src/data_link.h |  3 +++
 firmware/src/main.c      | 10 +++++++++-
 3 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/firmware/src/data_link.c b/firmware/src/data_link.c
index a6d091a..330e973 100644
--- a/firmware/src/data_link.c
+++ b/firmware/src/data_link.c
@@ -1,7 +1,19 @@
+#include <string.h>
 #include <zephyr/drivers/uart.h>
 
 #include "data_link.h"
 
+ssize_t dl_encode_frame(uint8_t *payload, size_t len, uint8_t *dst) {
+	// FIXME check for maximum size
+
+	size_t flen = 8 + 8 + 2 + len + 4;
+
+	memset(dst, 0, flen);
+	memcpy((void *)(dst + 18), payload, len);
+
+	return flen;
+}
+
 void dl_send_frame(const struct device *const uart_dev, uint8_t *buffer,
                    size_t len) {
 	for (size_t i = 0; i < len; i++) {
diff --git a/firmware/src/data_link.h b/firmware/src/data_link.h
index d2a25a1..45f58dc 100644
--- a/firmware/src/data_link.h
+++ b/firmware/src/data_link.h
@@ -9,6 +9,9 @@
 #define DL_SLIP_ESC_END 0xDC
 #define DL_SLIP_ESC_ESC 0xDD
 
+#define DL_MAX_FRAME_SIZE 1500
+
+ssize_t dl_encode_frame(uint8_t *payload, size_t len, uint8_t *dst);
 void dl_send_frame(const struct device *const uart_dev, uint8_t *buffer,
                    size_t len);
 
diff --git a/firmware/src/main.c b/firmware/src/main.c
index e18cc77..8b31505 100644
--- a/firmware/src/main.c
+++ b/firmware/src/main.c
@@ -1,4 +1,5 @@
 #include <stdbool.h>
+#include <sys/types.h>
 
 #include <zephyr/device.h>
 #include <zephyr/drivers/uart.h>
@@ -24,10 +25,17 @@ int main(void)
 	}
 
 	uint8_t payload[] = {0xDE, DL_SLIP_END, DL_SLIP_ESC, 0xAD};
+	uint8_t frame[DL_MAX_FRAME_SIZE];
+
+	ssize_t flen = dl_encode_frame(payload, ARRAY_SIZE(payload), frame);
+	if (flen < 0) {
+		printk("Failed to encode frame");
+		return 0;
+	}
 
 	while (true) {
 		k_sleep(K_MSEC(1000));
-		dl_send_frame(uart_dev, payload, ARRAY_SIZE(payload));
+		dl_send_frame(uart_dev, frame, flen);
 	}
 
 	return 0;
-- 
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