Re: [PATCH net-next 1/2] netdevsim: add link speed support

[Hangbin Liu <hangbin.liu@linux.dev>]

On Tue, Sep 01, 2026 at 02:58:17PM +0800, Hangbin Liu wrote:
> On Mon, Aug 31, 2026 at 03:08:18PM -0700, Jakub Kicinski wrote:
> > On Mon, 31 Aug 2026 11:28:10 +0800 Hangbin Liu wrote:
> > > Add ethtool get/set_link_ksettings callbacks to netdevsim so the simulated
> > > link speed and duplex can be queried and configured from userspace.
> > > 
> > > Move NSIM_LINK_SPEED_MAX and NSIM_LINK_SPEED_UNIT from dev.c to netdevsim.h
> > > so they are available to both the devlink rate path and the new ethtool
> > > code. The set callback rejects speeds exceeding NSIM_LINK_SPEED_MAX.
> > > 
> > > The default link speed is set to SPEED_5000 with DUPLEX_FULL, matching the
> > > existing NSIM_LINK_SPEED_MAX definition.
> > 
> > This breaks TDC which uses netdevsim for taprio testing.
> 
> Sigh, I really didn't expect a speed feature could break the tc qdisc
> testing... I will check the reason.

OK, here is the reason. In taprio_set_picos_per_byte(), it init NIC speed to
SPEED_10. If the NIC doesn't support get_link_ksettings, the final calculated
picos_per_byte would be a extremely large number 800000. The later tc taprio
testing `tc qdisc ... sched-entry S 02 300` will always failed in
fill_sched_entry(), as min_duration is also a large number 48000.

After we supports get_link_ksettings for netdevsim, the speed is 5000 and
picos_per_byte will be init to 1600. The later min_duration checking in
fill_sched_entry() is 300 vs 96. So the tc qdisc command always return 0.

To fix the test, I think we should reduce the interval number. e.g.

diff --git a/tools/testing/selftests/tc-testing/tc-tests/qdiscs/taprio.json b/tools/testing/selftests/tc-testing/tc-tests/qdiscs/taprio.json
index cd19d05925e4..c3418bde9ad6 100644
--- a/tools/testing/selftests/tc-testing/tc-tests/qdiscs/taprio.json
+++ b/tools/testing/selftests/tc-testing/tc-tests/qdiscs/taprio.json
@@ -145,7 +145,7 @@
         "setup": [
             "echo \"1 1 8\" > /sys/bus/netdevsim/new_device"
         ],
-        "cmdUnderTest": "$TC qdisc add dev $ETH root handle 1: taprio num_tc 2 queues 1@0 1@1 sched-entry S 01 300 sched-entry S 02 1700 clockid CLOCK_TAI",
+        "cmdUnderTest": "$TC qdisc add dev $ETH root handle 1: taprio num_tc 2 queues 1@0 1@1 sched-entry S 01 30 sched-entry S 02 70 clockid CLOCK_TAI",
         "expExitCode": "2",
         "verifyCmd": "$TC qdisc show dev $ETH",
         "matchPattern": "qdisc taprio 1: root refcnt",

Thanks
Hangbin