modem: modem_cmux: optimize modem_cmux_transmit_frame to save ROM

This optimization aggregates frame headers before adding to the ring buffer
and computes the FCS of the frame header in one operation.
This approach improves execution efficiency and reduces memory footprint.
This code adjustment aligns with the changes proposed in zephyrproject-rtos/zephyr#67062.

Signed-off-by: Pisit Sawangvonganan <[email protected]>

subsys/modem/modem_cmux.c

@@ -205,57 +205,55 @@ static void modem_cmux_bus_callback(struct modem_pipe *pipe, enum modem_pipe_eve
 static uint16_t modem_cmux_transmit_frame(struct modem_cmux *cmux,
 					  const struct modem_cmux_frame *frame)
 {
-	uint8_t byte;
+	uint8_t buf[MODEM_CMUX_FRAME_SIZE_MAX];
 	uint8_t fcs;
 	uint16_t space;
 	uint16_t data_len;
+	uint16_t buf_idx;
 
 	space = ring_buf_space_get(&cmux->transmit_rb) - MODEM_CMUX_FRAME_SIZE_MAX;
 	data_len = (space < frame->data_len) ? space : frame->data_len;
 
 	/* SOF */
-	byte = 0xF9;
-	ring_buf_put(&cmux->transmit_rb, &byte, 1);
+	buf[0] = 0xF9;
 
 	/* DLCI Address (Max 63) */
-	byte = 0x01 | (frame->cr << 1) | (frame->dlci_address << 2);
-	fcs = crc8(&byte, 1, MODEM_CMUX_FCS_POLYNOMIAL, MODEM_CMUX_FCS_INIT_VALUE, true);
-	ring_buf_put(&cmux->transmit_rb, &byte, 1);
+	buf[1] = 0x01 | (frame->cr << 1) | (frame->dlci_address << 2);
 
 	/* Frame type and poll/final */
-	byte = frame->type | (frame->pf << 4);
-	fcs = crc8(&byte, 1, MODEM_CMUX_FCS_POLYNOMIAL, fcs, true);
-	ring_buf_put(&cmux->transmit_rb, &byte, 1);
+	buf[2] = frame->type | (frame->pf << 4);
 
 	/* Data length */
 	if (data_len > 127) {
-		byte = data_len << 1;
-		fcs = crc8(&byte, 1, MODEM_CMUX_FCS_POLYNOMIAL, fcs, true);
-		ring_buf_put(&cmux->transmit_rb, &byte, 1);
-		byte = data_len >> 7;
-		ring_buf_put(&cmux->transmit_rb, &byte, 1);
+		buf[3] = data_len << 1;
+		buf[4] = data_len >> 7;
+		buf_idx = 5;
 	} else {
-		byte = 0x01 | (data_len << 1);
-		ring_buf_put(&cmux->transmit_rb, &byte, 1);
+		buf[3] = 0x01 | (data_len << 1);
+		buf_idx = 4;
 	}
 
+	/* Compute FCS for the header (exclude SOF) */
+	fcs = crc8(&buf[1], (buf_idx - 1), MODEM_CMUX_FCS_POLYNOMIAL, MODEM_CMUX_FCS_INIT_VALUE,
+		   true);
+
 	/* FCS final */
 	if (frame->type == MODEM_CMUX_FRAME_TYPE_UIH) {
-		fcs = 0xFF - crc8(&byte, 1, MODEM_CMUX_FCS_POLYNOMIAL, fcs, true);
+		fcs = 0xFF - fcs;
 	} else {
-		fcs = crc8(&byte, 1, MODEM_CMUX_FCS_POLYNOMIAL, fcs, true);
 		fcs = 0xFF - crc8(frame->data, data_len, MODEM_CMUX_FCS_POLYNOMIAL, fcs, true);
 	}
 
+	/* Frame header */
+	ring_buf_put(&cmux->transmit_rb, buf, buf_idx);
+
 	/* Data */
 	ring_buf_put(&cmux->transmit_rb, frame->data, data_len);
 
-	/* FCS */
-	ring_buf_put(&cmux->transmit_rb, &fcs, 1);
-
-	/* EOF */
-	byte = 0xF9;
-	ring_buf_put(&cmux->transmit_rb, &byte, 1);
+	/* FCS and EOF will be put on the same call */
+	buf[0] = fcs;
+	buf[1] = 0xF9;
+	ring_buf_put(&cmux->transmit_rb, buf, 2);
 	k_work_schedule(&cmux->transmit_work, K_NO_WAIT);
 	return data_len;
 }