i40e: document drivers use of ntuple filters

Add documentation describing the drivers use of ethtool ntuple filters,
including the limitations that it has due to hardware, as well as how it
reads and parses the user-def data block.
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

Documentation/networking/i40e.txt

@@ -63,6 +63,78 @@ Additional Configurations
   The latest release of ethtool can be found from
   https://www.kernel.org/pub/software/network/ethtool
 
+
+  Flow Director n-ntuple traffic filters (FDir)
+  ---------------------------------------------
+  The driver utilizes the ethtool interface for configuring ntuple filters,
+  via "ethtool -N <device> <filter>".
+
+  The sctp4, ip4, udp4, and tcp4 flow types are supported with the standard
+  fields including src-ip, dst-ip, src-port and dst-port. The driver only
+  supports fully enabling or fully masking the fields, so use of the mask
+  fields for partial matches is not supported.
+
+  Additionally, the driver supports using the action to specify filters for a
+  Virtual Function. You can specify the action as a 64bit value, where the
+  lower 32 bits represents the queue number, while the next 8 bits represent
+  which VF. Note that 0 is the PF, so the VF identifier is offset by 1. For
+  example:
+
+    ... action 0x800000002 ...
+
+  Would indicate to direct traffic for Virtual Function 7 (8 minus 1) on queue
+  2 of that VF.
+
+  The driver also supports using the user-defined field to specify 2 bytes of
+  arbitrary data to match within the packet payload in addition to the regular
+  fields. The data is specified in the lower 32bits of the user-def field in
+  the following way:
+
+  +----------------------------+---------------------------+
+  | 31    28    24    20    16 | 15    12     8     4     0|
+  +----------------------------+---------------------------+
+  | offset into packet payload |  2 bytes of flexible data |
+  +----------------------------+---------------------------+
+
+  As an example,
+
+    ... user-def 0x4FFFF ....
+
+  means to match the value 0xFFFF 4 bytes into the packet payload. Note that
+  the offset is based on the beginning of the payload, and not the beginning
+  of the packet. Thus
+
+    flow-type tcp4 ... user-def 0x8BEAF ....
+
+  would match TCP/IPv4 packets which have the value 0xBEAF 8bytes into the
+  TCP/IPv4 payload.
+
+  For ICMP, the hardware parses the ICMP header as 4 bytes of header and 4
+  bytes of payload, so if you want to match an ICMP frames payload you may need
+  to add 4 to the offset in order to match the data.
+
+  Furthermore, the offset can only be up to a value of 64, as the hardware
+  will only read up to 64 bytes of data from the payload. It must also be even
+  as the flexible data is 2 bytes long and must be aligned to byte 0 of the
+  packet payload.
+
+  When programming filters, the hardware is limited to using a single input
+  set for each flow type. This means that it is an error to program two
+  different filters with the same type that don't match on the same fields.
+  Thus the second of the following two commands will fail:
+
+    ethtool -N <device> flow-type tcp4 src-ip 192.168.0.7 action 5
+    ethtool -N <device> flow-type tcp4 dst-ip 192.168.15.18 action 1
+
+  This is because the first filter will be accepted and reprogram the input
+  set for TCPv4 filters, but the second filter will be unable to reprogram the
+  input set until all the conflicting TCPv4 filters are first removed.
+
+  Note that the user-defined flexible offset is also considered part of the
+  input set and cannot be programmed separately for multiple filters of the
+  same type. However, the flexible data is not part of the input set and
+  multiple filters may use the same offset but match against different data.
+
   Data Center Bridging (DCB)
   --------------------------
   DCB configuration is not currently supported.