selftests/tc-testing: verify that a qdisc can be grafted onto a taprio class

The reason behind commit af7b29b1 ("Revert "net/sched: taprio: make
qdisc_leaf() see the per-netdev-queue pfifo child qdiscs"") was that the
patch it reverted caused a crash when attaching a CBS shaper to one of
the taprio classes. Prevent that from happening again by adding a test
case for it, which now passes correctly in both offload and software
modes.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Pedro Tammela <pctammela@mojatatu.com>
Link: https://lore.kernel.org/r/20230807193324.4128292-12-vladimir.oltean@nxp.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/tc-testing/tc-tests/qdiscs/taprio.json

@@ -206,5 +206,55 @@
             "$TC qdisc del dev $ETH root",
             "echo \"1\" > /sys/bus/netdevsim/del_device"
         ]
+    },
+    {
+        "id": "a7bf",
+        "name": "Graft cbs as child of software taprio",
+        "category": [
+            "qdisc",
+            "taprio",
+            "cbs"
+        ],
+        "plugins": {
+            "requires": "nsPlugin"
+        },
+        "setup": [
+            "echo \"1 1 8\" > /sys/bus/netdevsim/new_device",
+            "$TC qdisc replace dev $ETH handle 8001: parent root stab overhead 24 taprio num_tc 8 map 0 1 2 3 4 5 6 7 queues 1@0 1@1 1@2 1@3 1@4 1@5 1@6 1@7 base-time 0 sched-entry S ff 20000000 clockid CLOCK_TAI"
+        ],
+        "cmdUnderTest": "$TC qdisc replace dev $ETH handle 8002: parent 8001:8 cbs idleslope 20000 sendslope -980000 hicredit 30 locredit -1470",
+        "expExitCode": "0",
+        "verifyCmd": "$TC -d qdisc show dev $ETH",
+        "matchPattern": "qdisc cbs 8002: parent 8001:8 hicredit 30 locredit -1470 sendslope -980000 idleslope 20000 offload 0",
+        "matchCount": "1",
+        "teardown": [
+            "$TC qdisc del dev $ETH root",
+            "echo \"1\" > /sys/bus/netdevsim/del_device"
+        ]
+    },
+    {
+        "id": "6a83",
+        "name": "Graft cbs as child of offloaded taprio",
+        "category": [
+            "qdisc",
+            "taprio",
+            "cbs"
+        ],
+        "plugins": {
+            "requires": "nsPlugin"
+        },
+        "setup": [
+            "echo \"1 1 8\" > /sys/bus/netdevsim/new_device",
+            "$TC qdisc replace dev $ETH handle 8001: parent root stab overhead 24 taprio num_tc 8 map 0 1 2 3 4 5 6 7 queues 1@0 1@1 1@2 1@3 1@4 1@5 1@6 1@7 base-time 0 sched-entry S ff 20000000 flags 0x2"
+        ],
+        "cmdUnderTest": "$TC qdisc replace dev $ETH handle 8002: parent 8001:8 cbs idleslope 20000 sendslope -980000 hicredit 30 locredit -1470",
+        "expExitCode": "0",
+        "verifyCmd": "$TC -d qdisc show dev $ETH",
+        "matchPattern": "qdisc cbs 8002: parent 8001:8 refcnt 2 hicredit 30 locredit -1470 sendslope -980000 idleslope 20000 offload 0",
+        "matchCount": "1",
+        "teardown": [
+            "$TC qdisc del dev $ETH root",
+            "echo \"1\" > /sys/bus/netdevsim/del_device"
+        ]
     }
 ]