docs: networking: Add documentation for MAPv5

Adding documentation explaining the new MAPv4/v5 packet formats
and the corresponding checksum offload headers.
Signed-off-by: Sharath Chandra Vurukala <sharathv@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/device_drivers/cellular/qualcomm/rmnet.rst

@@ -27,34 +27,136 @@ these MAP frames and send them to appropriate PDN's.
 2. Packet format
 ================
 
-a. MAP packet (data / control)
+a. MAP packet v1 (data / control)
 
-MAP header has the same endianness of the IP packet.
+MAP header fields are in big endian format.
 
 Packet format::
 
-  Bit             0             1           2-7      8 - 15           16 - 31
+  Bit             0             1           2-7      8-15           16-31
   Function   Command / Data   Reserved     Pad   Multiplexer ID    Payload length
-  Bit            32 - x
-  Function     Raw  Bytes
+
+  Bit            32-x
+  Function      Raw bytes
 
 Command (1)/ Data (0) bit value is to indicate if the packet is a MAP command
-or data packet. Control packet is used for transport level flow control. Data
+or data packet. Command packet is used for transport level flow control. Data
 packets are standard IP packets.
 
-Reserved bits are usually zeroed out and to be ignored by receiver.
+Reserved bits must be zero when sent and ignored when received.
 
-Padding is number of bytes to be added for 4 byte alignment if required by
-hardware.
+Padding is the number of bytes to be appended to the payload to
+ensure 4 byte alignment.
 
 Multiplexer ID is to indicate the PDN on which data has to be sent.
 
 Payload length includes the padding length but does not include MAP header
 length.
 
-b. MAP packet (command specific)::
+b. Map packet v4 (data / control)
+
+MAP header fields are in big endian format.
+
+Packet format::
+
+  Bit             0             1           2-7      8-15           16-31
+  Function   Command / Data   Reserved     Pad   Multiplexer ID    Payload length
+
+  Bit            32-(x-33)      (x-32)-x
+  Function      Raw bytes      Checksum offload header
+
+Command (1)/ Data (0) bit value is to indicate if the packet is a MAP command
+or data packet. Command packet is used for transport level flow control. Data
+packets are standard IP packets.
+
+Reserved bits must be zero when sent and ignored when received.
+
+Padding is the number of bytes to be appended to the payload to
+ensure 4 byte alignment.
+
+Multiplexer ID is to indicate the PDN on which data has to be sent.
+
+Payload length includes the padding length but does not include MAP header
+length.
+
+Checksum offload header, has the information about the checksum processing done
+by the hardware.Checksum offload header fields are in big endian format.
+
+Packet format::
+
+  Bit             0-14        15              16-31
+  Function      Reserved   Valid     Checksum start offset
+
+  Bit                31-47                    48-64
+  Function      Checksum length           Checksum value
+
+Reserved bits must be zero when sent and ignored when received.
+
+Valid bit indicates whether the partial checksum is calculated and is valid.
+Set to 1, if its is valid. Set to 0 otherwise.
+
+Padding is the number of bytes to be appended to the payload to
+ensure 4 byte alignment.
+
+Checksum start offset, Indicates the offset in bytes from the beginning of the
+IP header, from which modem computed checksum.
+
+Checksum length is the Length in bytes starting from CKSUM_START_OFFSET,
+over which checksum is computed.
+
+Checksum value, indicates the checksum computed.
+
+c. MAP packet v5 (data / control)
+
+MAP header fields are in big endian format.
+
+Packet format::
+
+  Bit             0             1         2-7      8-15           16-31
+  Function   Command / Data  Next header  Pad   Multiplexer ID   Payload length
+
+  Bit            32-x
+  Function      Raw bytes
+
+Command (1)/ Data (0) bit value is to indicate if the packet is a MAP command
+or data packet. Command packet is used for transport level flow control. Data
+packets are standard IP packets.
+
+Next header is used to indicate the presence of another header, currently is
+limited to checksum header.
+
+Padding is the number of bytes to be appended to the payload to
+ensure 4 byte alignment.
+
+Multiplexer ID is to indicate the PDN on which data has to be sent.
+
+Payload length includes the padding length but does not include MAP header
+length.
+
+d. Checksum offload header v5
+
+Checksum offload header fields are in big endian format.
+
+  Bit            0 - 6          7               8-15              16-31
+  Function     Header Type    Next Header     Checksum Valid    Reserved
+
+Header Type is to indicate the type of header, this usually is set to CHECKSUM
+
+Header types
+= ==========================================
+0 Reserved
+1 Reserved
+2 checksum header
+
+Checksum Valid is to indicate whether the header checksum is valid. Value of 1
+implies that checksum is calculated on this packet and is valid, value of 0
+indicates that the calculated packet checksum is invalid.
+
+Reserved bits must be zero when sent and ignored when received.
+
+e. MAP packet v1/v5 (command specific)::
 
-    Bit             0             1           2-7      8 - 15           16 - 31
+    Bit             0             1         2-7      8 - 15           16 - 31
     Function   Command         Reserved     Pad   Multiplexer ID    Payload length
     Bit          32 - 39        40 - 45    46 - 47       48 - 63
     Function   Command name    Reserved   Command Type   Reserved
@@ -74,7 +176,7 @@ Command types
 3 is for error during processing of commands
 = ==========================================
 
-c. Aggregation
+f. Aggregation
 
 Aggregation is multiple MAP packets (can be data or command) delivered to
 rmnet in a single linear skb. rmnet will process the individual