modem: cmux: replace crc8 with crc8_rohc for FCS calculation

To improve performance in FCS calculation, this commit replaces
the usage of the generic `crc8` function with the specific `crc8_rohc`
function in `modem_cmux.c`.
The `crc8_rohc` function utilizes a small table approach, enhancing
the efficiency of CRC-8/ROHC variant calculations while optimizing
memory usage.

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

Author: ndrs-pst

subsys/modem/modem_cmux.c

@@ -295,14 +295,13 @@ static uint16_t modem_cmux_transmit_frame(struct modem_cmux *cmux,
 	}
 
 	/* 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 = crc8_rohc(MODEM_CMUX_FCS_INIT_VALUE, &buf[1], (buf_idx - 1));
 
 	/* FCS final */
 	if (frame->type == MODEM_CMUX_FRAME_TYPE_UIH) {
 		fcs = 0xFF - fcs;
 	} else {
-		fcs = 0xFF - crc8(frame->data, data_len, MODEM_CMUX_FCS_POLYNOMIAL, fcs, true);
+		fcs = 0xFF - crc8_rohc(fcs, frame->data, data_len);
 	}
 
 	/* Frame header */
@@ -845,16 +844,12 @@ static void modem_cmux_process_received_byte(struct modem_cmux *cmux, uint8_t by
 		}
 
 		/* Compute FCS */
+		fcs = crc8_rohc(MODEM_CMUX_FCS_INIT_VALUE, cmux->frame_header,
+				cmux->frame_header_len);
 		if (cmux->frame.type == MODEM_CMUX_FRAME_TYPE_UIH) {
-			fcs = 0xFF - crc8(cmux->frame_header, cmux->frame_header_len,
-					  MODEM_CMUX_FCS_POLYNOMIAL, MODEM_CMUX_FCS_INIT_VALUE,
-					  true);
+			fcs = 0xFF - fcs;
 		} else {
-			fcs = crc8(cmux->frame_header, cmux->frame_header_len,
-				   MODEM_CMUX_FCS_POLYNOMIAL, MODEM_CMUX_FCS_INIT_VALUE, true);
-
-			fcs = 0xFF - crc8(cmux->frame.data, cmux->frame.data_len,
-					  MODEM_CMUX_FCS_POLYNOMIAL, fcs, true);
+			fcs = 0xFF - crc8_rohc(fcs, cmux->frame.data, cmux->frame.data_len);
 		}
 
 		/* Validate FCS */