[PATCH net-next 7/7] docs: networking: Add documentation for the coalescing support in rmnet

[Subash Abhinov Kasiviswanathan <subash.a.kasiviswanathan@oss.qualcomm.com>]

Add information about the MAPv5 coalescing header covering the layout
and the information from the fields in the header.

Co-developed-by: Sean Tranchetti <sean.tranchetti@oss.qualcomm.com>
Signed-off-by: Sean Tranchetti <sean.tranchetti@oss.qualcomm.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <subash.a.kasiviswanathan@oss.qualcomm.com>
---
 .../cellular/qualcomm/rmnet.rst               | 115 ++++++++++++++++--
 1 file changed, 107 insertions(+), 8 deletions(-)

diff --git a/Documentation/networking/device_drivers/cellular/qualcomm/rmnet.rst b/Documentation/networking/device_drivers/cellular/qualcomm/rmnet.rst
index 5aedbabb7382..ba8e947d54e6 100644
--- a/Documentation/networking/device_drivers/cellular/qualcomm/rmnet.rst
+++ b/Documentation/networking/device_drivers/cellular/qualcomm/rmnet.rst
@@ -125,8 +125,8 @@ 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.
+Next header is used to indicate the presence of another header, currently
+limited to the checksum and coalescing headers.
 
 Padding is the number of bytes to be appended to the payload to
 ensure 4 byte alignment.
@@ -150,11 +150,11 @@ Header Type is to indicate the type of header, this usually is set to CHECKSUM
 
 Header types
 
-= ===============
+= ======================
 0 Reserved
-1 Reserved
+1 coalescing header
 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
@@ -162,8 +162,78 @@ 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)
---------------------------------------
+e. Coalescing header v5
+------------------------
+
+Hardware can coalesce multiple same-flow IP packets of the same length into
+a single MAP frame to reduce per-packet overhead at high data rates.  The
+coalescing header (header type 1) describes the coalesced content.
+
+Packet format::
+
+  Bit         0 - 6         7          8           9-11         12-15
+  Function  Header Type  Next Header  CSUM valid  Num NLOs    (reserved)
+
+  Bit        16-19        20-23
+  Function  Close value  Close type
+
+  Bit        24-27        28-31
+  Function  (reserved)    VEID
+
+  Bit           32 - 47        48 - 55            56 - 63
+  Function   Packet length  CSUM error bitmap  Num packets (NLO 0)
+
+  ... (up to 6 NLO entries total, same 32-bit format per entry)
+
+Header Type is set to 1 (coalescing).
+
+Num NLOs (Number-Length Objects) is the count of active NLO entries
+(1 – 6).  Each NLO describes a group of consecutive coalesced packets
+that all share the same IP packet length.
+
+CSUM valid (bit 8) indicates whether the hardware checksum is valid
+for all packets in the frame.
+
+Close type and close value encode the hardware reason that coalescing
+was terminated for this frame:
+
+Close type values:
+
+= ==============================
+0 non-coalesced (single packet)
+1 IP flow miss
+2 transport flow miss
+3 hardware limit (see value)
+4 coalescing closed (FIN/PSH)
+= ==============================
+
+Close value (used when close type is 3):
+
+= ==================
+0 NL limit reached
+1 packet limit
+2 byte limit
+3 time limit
+4 eviction
+= ==================
+
+VEID is the virtual endpoint ID of the originating flow.
+
+Each NLO entry::
+
+  Bit         0 - 15        16 - 23            24 - 31
+  Function  Pkt length   CSUM error bitmap   Num packets
+
+Pkt length is the full IP packet length, including the IP header,
+transport header, and payload, for every packet in this NLO group.
+
+CSUM error bitmap is a per-packet bitmask. Bit N is set when packet N
+in this NLO has a bad checksum.
+
+Num packets is the count of coalesced packets described by this NLO.
+
+f. MAP packet v1/v5 (command specific)
+---------------------------------------
 
 Packet format::
 
@@ -187,7 +257,7 @@ Command types
 3 is for error during processing of commands
 = ==========================================
 
-f. Aggregation
+g. Aggregation
 --------------
 
 Aggregation is multiple MAP packets (can be data or command) delivered to
@@ -208,3 +278,32 @@ rmnet userspace configuration is done through netlink using iproute2
 https://git.kernel.org/pub/scm/network/iproute2/iproute2.git/
 
 The driver uses rtnl_link_ops for communication.
+
+The data format flags controlling the ingress and egress processing
+pipeline are set via the ``IFLA_RMNET_FLAGS`` attribute
+(``struct ifla_rmnet_flags``).
+
+Relevant ingress flags:
+
+``RMNET_FLAGS_INGRESS_DEAGGREGATION``
+  Enable MAP frame de-aggregation.
+
+``RMNET_FLAGS_INGRESS_MAP_CKSUMV4``
+  Enable MAPv4 downlink checksum offload.
+
+``RMNET_FLAGS_INGRESS_MAP_CKSUMV5``
+  Enable MAPv5 downlink checksum offload (header type 2).
+
+``RMNET_FLAGS_INGRESS_COALESCE``
+  Enable MAPv5 downlink hardware coalescing (header type 1).
+  When set the driver will decode coalescing headers, reconstruct
+  individual IP packets and will deliver batched GSO SKBs to the
+  stack for efficient processing.
+
+Relevant egress flags:
+
+``RMNET_FLAGS_EGRESS_MAP_CKSUMV4``
+  Enable MAPv4 uplink checksum offload.
+
+``RMNET_FLAGS_EGRESS_MAP_CKSUMV5``
+  Enable MAPv5 uplink checksum offload.
-- 
2.34.1