cangen: disable generation of unsupported CAN frame types in mixed mode

The mixed mode is able to automatically detect the potential supported
CAN frame types CAN CC/FD/XL by checking the CAN device MTU at startup.

Usually the MTU shows which CAN frame types can be sent but in the case of
CAN XL in CANXL-only mode CC and FD frames can not be sent on the CAN_RAW
socket.

Since this patch [1] the CAN_RAW socket rejects unsupported CAN frames and
returns -EINVAL as error code. With this change in cangen the CC and FD
frame generation can be disabled in mixed mode at runtime.

[1] https://lore.kernel.org/linux-can/20251125123859.3924-17-socketcan@hartkopp.net/T/#u

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>

cangen.c

@@ -180,7 +180,7 @@ static void print_usage(char *prg)
 	fprintf(stderr, "         -X            (generate CAN XL CAN frames)\n");
 	fprintf(stderr, "         -R            (generate RTR frames)\n");
 	fprintf(stderr, "         -8            (allow DLC values greater then 8 for Classic CAN frames)\n");
-	fprintf(stderr, "         -m            (mix -e -f -b -E -R -X frames)\n");
+	fprintf(stderr, "         -m            (mix CC [-e -R] FD [-f -b -E] XL [-X] on capable devices)\n");
 	fprintf(stderr, "         -I <mode>     (CAN ID generation mode - see below)\n");
 	fprintf(stderr, "         -L <mode>     (CAN data length code (dlc) generation mode - see below)\n");
 	fprintf(stderr, "         -D <mode>     (CAN data (payload) generation mode - see below)\n");
@@ -339,10 +339,9 @@ resend:
 	nbytes = sendmsg(fd, &msg, 0);
 	if (nbytes < 0) {
 		ret = -errno;
-		if (ret != -ENOBUFS) {
-			perror("write");
+		if (ret != -ENOBUFS)
 			return ret;
-		}
+
 		if (!ignore_enobufs && !timeout) {
 			perror("write");
 			return ret;
@@ -455,6 +454,9 @@ int main(int argc, char **argv)
 	unsigned char extended = 0;
 	unsigned char canfd = 0;
 	unsigned char canxl = 0;
+	unsigned char mixcc = 1; /* mix default */
+	unsigned char mixfd = 1; /* mix default */
+	unsigned char mixxl = 1; /* mix default */
 	unsigned char brs = 0;
 	unsigned char esi = 0;
 	unsigned char mix = 0;
@@ -574,7 +576,6 @@ int main(int argc, char **argv)
 
 		case 'm':
 			mix = 1;
-			canfd = 1; /* to switch the socket into CAN FD mode */
 			view |= CANLIB_VIEW_INDENT_SFF;
 			break;
 
@@ -777,34 +778,32 @@ int main(int argc, char **argv)
 			   &loopback, sizeof(loopback));
 	}
 
-	if (canfd || canxl) {
-
-		/* check if the frame fits into the CAN netdevice */
+	/* get CAN netdevice MTU for frame type capabilities */
 	if (ioctl(s, SIOCGIFMTU, &ifr) < 0) {
 		perror("SIOCGIFMTU");
 		return 1;
 	}
 
-		if (canfd) {
-			/* ensure discrete CAN FD length values 0..8, 12, 16, 20, 24, 32, 64 */
-			cu.fd.len = can_fd_dlc2len(can_fd_len2dlc(cu.fd.len));
-		} else {
-			/* limit fixed CAN XL data length to 64 */
-			if (cu.fd.len > CANFD_MAX_DLEN)
-				cu.fd.len = CANFD_MAX_DLEN;
-		}
-
-		if (canxl && (ifr.ifr_mtu < (int)CANXL_MIN_MTU)) {
+	/* check CAN XL support */
+	if (ifr.ifr_mtu < (int)CANXL_MIN_MTU) {
+		mixxl = 0;
+		if (canxl) {
 			printf("CAN interface not CAN XL capable - sorry.\n");
 			return 1;
 		}
+	}
 
-		if (canfd && (ifr.ifr_mtu < (int)CANFD_MTU)) {
+	/* check CAN FD support */
+	if (ifr.ifr_mtu < (int)CANFD_MTU) {
+		mixfd = 0;
+		if (canfd) {
 			printf("CAN interface not CAN FD capable - sorry.\n");
 			return 1;
 		}
+	}
 
-		if (ifr.ifr_mtu == (int)CANFD_MTU) {
+	/* enable CAN FD on the socket */
+	if (mixfd) {
 		/* interface is ok - try to switch the socket into CAN FD mode */
 		if (setsockopt(s, SOL_CAN_RAW, CAN_RAW_FD_FRAMES,
 			       &enable_canfx, sizeof(enable_canfx))){
@@ -813,7 +812,8 @@ int main(int argc, char **argv)
 		}
 	}
 
-		if (ifr.ifr_mtu >= (int)CANXL_MIN_MTU) {
+	/* enable CAN XL on the socket */
+	if (mixxl) {
 		/* interface is ok - try to switch the socket into CAN XL mode */
 		if (setsockopt(s, SOL_CAN_RAW, CAN_RAW_XL_FRAMES,
 			       &enable_canfx, sizeof(enable_canfx))){
@@ -828,6 +828,16 @@ int main(int argc, char **argv)
 		}
 	}
 
+	/* sanitize given values */
+	if (canfd || canxl) {
+		if (canfd) {
+			/* ensure discrete CAN FD length values 0..8, 12, 16, 20, 24, 32, 64 */
+			cu.fd.len = can_fd_dlc2len(can_fd_len2dlc(cu.fd.len));
+		} else {
+			/* limit fixed CAN XL data length to 64 */
+			if (cu.fd.len > CANFD_MAX_DLEN)
+				cu.fd.len = CANFD_MAX_DLEN;
+		}
 	} else {
 		/* sanitize Classical CAN 2.0 frame length */
 		if (len8_dlc) {
@@ -1040,8 +1050,31 @@ int main(int argc, char **argv)
 		}
 
 		ret = do_send_one(s, &cu, mtu, polltimeout);
-		if (ret)
+		if ((ret == -EINVAL) && mix) {
+			/* mix mode: disable unsupported CAN frame type */
+			switch (mtu) {
+			case CAN_MTU:
+				mixcc = 0;
+				break;
+			case CANFD_MTU:
+				mixfd = 0;
+				break;
+			case CANXL_MTU:
+				mixxl = 0;
+				break;
+			default:
+				printf ("mix mode: unknown MTU");
 				return 1;
+			}
+			if (!mixcc && !mixfd && !mixxl) {
+				printf ("mix mode: no valid CAN frame types\n");
+				return 1;
+			}
+		} else if (ret) {
+			/* other error than -ENOBUFS and -EINVAL */
+			perror("write");
+			return 1;
+		}
 
 		if (burst_sent_count >= burst_count)
 			burst_sent_count = 0;
@@ -1082,18 +1115,41 @@ int main(int argc, char **argv)
 		}
 
 		if (mix) {
+			canfd = 0;
+			canxl = 0;
 			i = random();
 			extended = i & 1;
+			if (mixfd) {
 				canfd = i & 2;
 				if (canfd) {
 					brs = i & 4;
 					esi = i & 8;
 				}
-			/* generate CAN XL traffic if the interface is capable */
-			if (ifr.ifr_mtu >= (int)CANXL_MIN_MTU)
-				canxl = ((i & 96) == 96);
-
-			rtr_frame = ((i & 24) == 24); /* reduce RTR frames to 1/4 */
+			}
+			if (mixxl) {
+				if (mixfd)
+					canxl = ((i & 96) == 96); /* 1/4 */
+				else
+					canxl = ((i & 32) == 32); /* 1/2 */
+			}
+			if (mixcc) {
+				rtr_frame = ((i & 24) == 24); /* reduce RTR to 1/4 */
+			} else {
+				/* no CC frames allowed - CAN XL-only mode? */
+				if (!canxl && !canfd) {
+					/* force XL or FD frames */
+					if (mixxl)
+						canxl = 1;
+					else if (mixfd) {
+						canfd = 1;
+						brs = i & 4;
+						esi = i & 8;
+					} else {
+						printf ("mix mode: no valid CAN frame types\n");
+						return 1;
+					}
+				}
+			}
 		}
 	}