usbnet: ipheth: break up NCM header size computation

Originally, the total NCM header size was computed as the sum of two
vaguely labelled constants. While accurate, it wasn't particularly clear
where they were coming from.

Use sizes of existing NCM structs where available. Define the total
NDP16 size based on the maximum amount of DPEs that can fit into the
iOS-specific fixed-size header.

This change does not fix any particular issue. Rather, it introduces
intermediate constants that will simplify subsequent commits.
It should also make it clearer for the reader where the constant values
come from.

Cc: [email protected] # 6.5.x
Signed-off-by: Foster Snowhill <[email protected]>
Reviewed-by: Jakub Kicinski <[email protected]>
Signed-off-by: Paolo Abeni <[email protected]>

Author: Forst

drivers/net/usb/ipheth.c

@@ -61,7 +61,18 @@
 #define IPHETH_USBINTF_PROTO    1
 
 #define IPHETH_IP_ALIGN		2	/* padding at front of URB */
-#define IPHETH_NCM_HEADER_SIZE  (12 + 96) /* NCMH + NCM0 */
+/* On iOS devices, NCM headers in RX have a fixed size regardless of DPE count:
+ * - NTH16 (NCMH): 12 bytes, as per CDC NCM 1.0 spec
+ * - NDP16 (NCM0): 96 bytes, of which
+ *    - NDP16 fixed header: 8 bytes
+ *    - maximum of 22 DPEs (21 datagrams + trailer), 4 bytes each
+ */
+#define IPHETH_NDP16_MAX_DPE	22
+#define IPHETH_NDP16_HEADER_SIZE (sizeof(struct usb_cdc_ncm_ndp16) + \
+				  IPHETH_NDP16_MAX_DPE * \
+				  sizeof(struct usb_cdc_ncm_dpe16))
+#define IPHETH_NCM_HEADER_SIZE	(sizeof(struct usb_cdc_ncm_nth16) + \
+				 IPHETH_NDP16_HEADER_SIZE)
 #define IPHETH_TX_BUF_SIZE      ETH_FRAME_LEN
 #define IPHETH_RX_BUF_SIZE_LEGACY (IPHETH_IP_ALIGN + ETH_FRAME_LEN)
 #define IPHETH_RX_BUF_SIZE_NCM	65536