net: dsa: qca8k: fix wrong length value for mgmt eth packet

The assumption that Documentation was right about how this value work was
wrong. It was discovered that the length value of the mgmt header is in
step of word size.

As an example to process 4 byte of data the correct length to set is 2.
To process 8 byte 4, 12 byte 6, 16 byte 8...

Odd values will always return the next size on the ack packet.
(length of 3 (6 byte) will always return 8 bytes of data)

This means that a value of 15 (0xf) actually means reading/writing 32 bytes
of data instead of 16 bytes. This behaviour is totally absent and not
documented in the switch Documentation.

In fact from Documentation the max value that mgmt eth can process is
16 byte of data while in reality it can process 32 bytes at once.

To handle this we always round up the length after deviding it for word
size. We check if the result is odd and we round another time to align
to what the switch will provide in the ack packet.
The workaround for the length limit of 15 is still needed as the length
reg max value is 0xf(15)

Reported-by: Ronald Wahl <[e-mail]>
Tested-by: Ronald Wahl <[e-mail]>
Fixes: 90386223f44e ("net: dsa: qca8k: add support for larger read/write size with mgmt Ethernet")
Signed-off-by: Christian Marangi <[e-mail]>
Cc: [e-mail] # v5.18+
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/dsa/qca/qca8k-8xxx.c

···
 
 	command = get_unaligned_le32(&mgmt_ethhdr->command);
 	cmd = FIELD_GET(QCA_HDR_MGMT_CMD, command);
+
 	len = FIELD_GET(QCA_HDR_MGMT_LENGTH, command);
+	/* Special case for len of 15 as this is the max value for len and needs to
+	 * be increased before converting it from word to dword.
+	 */
+	if (len == 15)
+		len++;
+
+	/* We can ignore odd value, we always round up them in the alloc function. */
+	len *= sizeof(u16);
 
 	/* Make sure the seq match the requested packet */
 	if (get_unaligned_le32(&mgmt_ethhdr->seq) == mgmt_eth_data->seq)
···
 	if (!skb)
 		return NULL;
 
-	/* Max value for len reg is 15 (0xf) but the switch actually return 16 byte
-	 * Actually for some reason the steps are:
-	 * 0: nothing
-	 * 1-4: first 4 byte
-	 * 5-6: first 12 byte
-	 * 7-15: all 16 byte
+	/* Hdr mgmt length value is in step of word size.
+	 * As an example to process 4 byte of data the correct length to set is 2.
+	 * To process 8 byte 4, 12 byte 6, 16 byte 8...
+	 *
+	 * Odd values will always return the next size on the ack packet.
+	 * (length of 3 (6 byte) will always return 8 bytes of data)
+	 *
+	 * This means that a value of 15 (0xf) actually means reading/writing 32 bytes
+	 * of data.
+	 *
+	 * To correctly calculate the length we devide the requested len by word and
+	 * round up.
+	 * On the ack function we can skip the odd check as we already handle the
+	 * case here.
 	 */
-	if (len == 16)
-		real_len = 15;
-	else
-		real_len = len;
+	real_len = DIV_ROUND_UP(len, sizeof(u16));
+
+	/* We check if the result len is odd and we round up another time to
+	 * the next size. (length of 3 will be increased to 4 as switch will always
+	 * return 8 bytes)
+	 */
+	if (real_len % sizeof(u16) != 0)
+		real_len++;
+
+	/* Max reg value is 0xf(15) but switch will always return the next size (32 byte) */
+	if (real_len == 16)
+		real_len--;
 
 	skb_reset_mac_header(skb);
 	skb_set_network_header(skb, skb->len);