taprio: support dumping and setting per-tc max SDU

taprio: support dumping and setting per-tc max SDU

The 802.1Q queueMaxSDU table is technically implemented in Linux as
the TCA_TAPRIO_TC_ENTRY_MAX_SDU attribute of the TCA_TAPRIO_ATTR_TC_ENTRY
nest. Multiple TCA_TAPRIO_ATTR_TC_ENTRY nests may appear in the netlink
message, one per traffic class. Other configuration items that are per
traffic class are also supposed to go there.

This is done for future extensibility of the netlink interface (I have
the feeling that the struct tc_mqprio_qopt passed through
TCA_TAPRIO_ATTR_PRIOMAP is not exactly extensible, which kind of defeats
the purpose of using netlink). But otherwise, the max-sdu is parsed from
the user, and printed, just like any other fixed-size 16 element array.

I've modified the example for a fully offloaded configuration (flags 2)
to also show a max-sdu use case. The gate intervals were 0x80 (for TC 7),
0xa0 (for TCs 7 and 5) and 0xdf (for TCs 7, 6, 4, 3, 2, 1, 0).
I modified the last gate to exclude TC 7 (df -> 5f), so that TC 7 now
only interferes with TC 5.

Output after running the full offload command from the man page example
(the new attribute is "max-sdu"):

$ tc qdisc show dev swp0 root
qdisc taprio 8002: root tc 8 map 0 1 2 3 4 5 6 7 0 0 0 0 0 0 0 0
queues offset 0 count 1 offset 1 count 1 offset 2 count 1 offset 3 count 1 offset 4 count 1 offset 5 count 1 offset 6 count 1 offset 7 count 1
 flags 0x2      base-time 200 cycle-time 100000 cycle-time-extension 0
        index 0 cmd S gatemask 0x80 interval 20000
        index 1 cmd S gatemask 0xa0 interval 20000
        index 2 cmd S gatemask 0x5f interval 60000
max-sdu 0 0 0 0 0 200 0 0 0 0 0 0 0 0 0 0

$ tc -j -p qdisc show dev eno0 root
[ {
        "kind": "taprio",
        "handle": "8002:",
        "root": true,
        "options": {
            "tc": 8,
            "map": [ 0,1,2,3,4,5,6,7,0,0,0,0,0,0,0,0 ],
            "queues": [ {
                    "offset": 0,
                    "count": 1
                },{
                    "offset": 1,
                    "count": 1
                },{
                    "offset": 2,
                    "count": 1
                },{
                    "offset": 3,
                    "count": 1
                },{
                    "offset": 4,
                    "count": 1
                },{
                    "offset": 5,
                    "count": 1
                },{
                    "offset": 6,
                    "count": 1
                },{
                    "offset": 7,
                    "count": 1
                } ],
            "flags": "0x2",
            "base_time": 200,
            "cycle_time": 100000,
            "cycle_time_extension": 0,
            "schedule": [ {
                    "index": 0,
                    "cmd": "S",
                    "gatemask": "0x80",
                    "interval": 20000
                },{
                    "index": 1,
                    "cmd": "S",
                    "gatemask": "0xa0",
                    "interval": 20000
                },{
                    "index": 2,
                    "cmd": "S",
                    "gatemask": "0x5f",
                    "interval": 60000
                } ],
            "max-sdu": [ 0,0,0,0,0,200,0,0,0,0,0,0,0,0,0,0 ]
        }
    } ]

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David Ahern <dsahern@kernel.org>