I noticed that only 3 out of the 4 input bits were used,
mt.key->flags & FLOW_DIS_IS_FRAGMENT was never checked.

In order to avoid a complicated maze, I converted it to
use a 16 byte mapping table.

As shown in the table below the old heuristics doesn't
always do the right thing, ie. when FLOW_DIS_IS_FRAGMENT=1/1
then it used to only match follow-up fragment packets.

Here are all the combinations, and their resulting new/old
VCAP key/mask filter:

  /- FLOW_DIS_IS_FRAGMENT (key/mask)
  |    /- FLOW_DIS_FIRST_FRAG (key/mask)
  |    |    /-- new VCAP fragment (key/mask)
  v    v    v    v- old VCAP fragment (key/mask)

 0/0  0/0  -/-  -/-     impossible (due to entry cond. on mask)
 0/0  0/1  -/-  0/3 !!  invalid (can't match non-fragment + follow-up frag)
 0/0  1/0  -/-  -/-     impossible (key > mask)
 0/0  1/1  1/3  1/3     first fragment

 0/1  0/0  0/3  3/3 !!  not fragmented
 0/1  0/1  0/3  3/3 !!  not fragmented (+ not first fragment)
 0/1  1/0  -/-  -/-     impossible (key > mask)
 0/1  1/1  -/-  1/3 !!  invalid (non-fragment and first frag)

 1/0  0/0  -/-  -/-     impossible (key > mask)
 1/0  0/1  -/-  -/-     impossible (key > mask)
 1/0  1/0  -/-  -/-     impossible (key > mask)
 1/0  1/1  -/-  -/-     impossible (key > mask)

 1/1  0/0  1/1  3/3 !!  some fragment
 1/1  0/1  3/3  3/3     follow-up fragment
 1/1  1/0  -/-  -/-     impossible (key > mask)
 1/1  1/1  1/3  1/3     first fragment

In the datasheet the VCAP fragment values are documented as:
 0 = no fragment
 1 = initial fragment
 2 = suspicious fragment
 3 = valid follow-up fragment

Result: 3 combinations match the old behavior,
        3 combinations have been corrected,
        2 combinations are now invalid, and fail,
        8 combinations are impossible.

It should now be aligned with how FLOW_DIS_IS_FRAGMENT
and FLOW_DIS_FIRST_FRAG is set in __skb_flow_dissect() in
net/core/flow_dissector.c

Since the VCAP fragment values are not a bitfield, we have
to ignore the suspicious fragment value, eg. when matching
on any kind of fragment with FLOW_DIS_IS_FRAGMENT=1/1.

Only compile tested, and logic tested in userspace, as I
unfortunately don't have access to this switch chip (yet).

Fixes: d6c2964d ("net: microchip: sparx5: Adding more tc flower keys for the IS2 VCAP")
Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Reviewed-by: Steen Hegelund <Steen.Hegelund@microchip.com>
Tested-by: Daniel Machon <daniel.machon@microchip.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20240411111321.114095-1-ast@fiberby.netSigned-off-by: Jakub Kicinski <kuba@kernel.org>