can: gw: support modification of Classical CAN DLCs

Add support for data length code modifications for Classical CAN.

The netlink configuration interface always allowed to pass any value
that fits into a byte, therefore only the modification process had to be
extended to handle the raw DLC represenation of Classical CAN frames.

When a DLC value from 0 .. F is provided for Classical CAN frame
modifications the 'len' value is modified as-is with the exception that
potentially existing 9 .. F DLC values in the len8_dlc element are moved
to the 'len' element for the modification operation by mod_retrieve_ccdlc().

After the modification the Classical CAN frame DLC information is brought
back into the correct format by mod_store_ccdlc() which is filling 'len'
and 'len8_dlc' accordingly.

Signed-off-by: Oliver Hartkopp <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Marc Kleine-Budde <[e-mail]>

include/uapi/linux/can/gw.h

···
 
 /* CAN frame elements that are affected by curr. 3 CAN frame modifications */
 #define CGW_MOD_ID	0x01
-#define CGW_MOD_DLC	0x02		/* contains the data length in bytes */
-#define CGW_MOD_LEN	CGW_MOD_DLC	/* CAN FD length representation */
+#define CGW_MOD_DLC	0x02		/* Classical CAN data length code */
+#define CGW_MOD_LEN	CGW_MOD_DLC	/* CAN FD (plain) data length */
 #define CGW_MOD_DATA	0x04
 #define CGW_MOD_FLAGS	0x08		/* CAN FD flags */
 

net/can/gw.c

···
 	memcpy(cf->data, mod->modframe.set.data, CANFD_MAX_DLEN);
 }
 
+/* retrieve valid CC DLC value and store it into 'len' */
+static void mod_retrieve_ccdlc(struct canfd_frame *cf)
+{
+	struct can_frame *ccf = (struct can_frame *)cf;
+
+	/* len8_dlc is only valid if len == CAN_MAX_DLEN */
+	if (ccf->len != CAN_MAX_DLEN)
+		return;
+
+	/* do we have a valid len8_dlc value from 9 .. 15 ? */
+	if (ccf->len8_dlc > CAN_MAX_DLEN && ccf->len8_dlc <= CAN_MAX_RAW_DLC)
+		ccf->len = ccf->len8_dlc;
+}
+
+/* convert valid CC DLC value in 'len' into struct can_frame elements */
+static void mod_store_ccdlc(struct canfd_frame *cf)
+{
+	struct can_frame *ccf = (struct can_frame *)cf;
+
+	/* clear potential leftovers */
+	ccf->len8_dlc = 0;
+
+	/* plain data length 0 .. 8 - that was easy */
+	if (ccf->len <= CAN_MAX_DLEN)
+		return;
+
+	/* potentially broken values are catched in can_can_gw_rcv() */
+	if (ccf->len > CAN_MAX_RAW_DLC)
+		return;
+
+	/* we have a valid dlc value from 9 .. 15 in ccf->len */
+	ccf->len8_dlc = ccf->len;
+	ccf->len = CAN_MAX_DLEN;
+}
+
+static void mod_and_ccdlc(struct canfd_frame *cf, struct cf_mod *mod)
+{
+	mod_retrieve_ccdlc(cf);
+	mod_and_len(cf, mod);
+	mod_store_ccdlc(cf);
+}
+
+static void mod_or_ccdlc(struct canfd_frame *cf, struct cf_mod *mod)
+{
+	mod_retrieve_ccdlc(cf);
+	mod_or_len(cf, mod);
+	mod_store_ccdlc(cf);
+}
+
+static void mod_xor_ccdlc(struct canfd_frame *cf, struct cf_mod *mod)
+{
+	mod_retrieve_ccdlc(cf);
+	mod_xor_len(cf, mod);
+	mod_store_ccdlc(cf);
+}
+
+static void mod_set_ccdlc(struct canfd_frame *cf, struct cf_mod *mod)
+{
+	mod_set_len(cf, mod);
+	mod_store_ccdlc(cf);
+}
+
 static void canframecpy(struct canfd_frame *dst, struct can_frame *src)
 {
 	/* Copy the struct members separately to ensure that no uninitialized
···
 			if (mb.modtype & CGW_MOD_ID)
 				mod->modfunc[modidx++] = mod_and_id;
 
-			if (mb.modtype & CGW_MOD_LEN)
-				mod->modfunc[modidx++] = mod_and_len;
+			if (mb.modtype & CGW_MOD_DLC)
+				mod->modfunc[modidx++] = mod_and_ccdlc;
 
 			if (mb.modtype & CGW_MOD_DATA)
 				mod->modfunc[modidx++] = mod_and_data;
···
 			if (mb.modtype & CGW_MOD_ID)
 				mod->modfunc[modidx++] = mod_or_id;
 
-			if (mb.modtype & CGW_MOD_LEN)
-				mod->modfunc[modidx++] = mod_or_len;
+			if (mb.modtype & CGW_MOD_DLC)
+				mod->modfunc[modidx++] = mod_or_ccdlc;
 
 			if (mb.modtype & CGW_MOD_DATA)
 				mod->modfunc[modidx++] = mod_or_data;
···
 			if (mb.modtype & CGW_MOD_ID)
 				mod->modfunc[modidx++] = mod_xor_id;
 
-			if (mb.modtype & CGW_MOD_LEN)
-				mod->modfunc[modidx++] = mod_xor_len;
+			if (mb.modtype & CGW_MOD_DLC)
+				mod->modfunc[modidx++] = mod_xor_ccdlc;
 
 			if (mb.modtype & CGW_MOD_DATA)
 				mod->modfunc[modidx++] = mod_xor_data;
···
 			if (mb.modtype & CGW_MOD_ID)
 				mod->modfunc[modidx++] = mod_set_id;
 
-			if (mb.modtype & CGW_MOD_LEN)
-				mod->modfunc[modidx++] = mod_set_len;
+			if (mb.modtype & CGW_MOD_DLC)
+				mod->modfunc[modidx++] = mod_set_ccdlc;
 
 			if (mb.modtype & CGW_MOD_DATA)
 				mod->modfunc[modidx++] = mod_set_data;